Journal of Mechanical Engineering 2012 4

58 (2012) 4

http://www.sv-jme.eu

Strojniški vestnik Journal of Mechanical Engineering

Since 1955

Papers

225

Anton Bergant, Arno Kruisbrink, Francisco Arregui: Dynamic Behaviour of Air Valves in a Large-Scale Pipeline Apparatus

238

Boštjan Gregorc, Andrej Predin, Drago Fabijan, Roman Klasinc: Experimental Analysis of the Impact of Particles on the Cavitating Flow

245

Andrej Kryžanowski, Matjaž Mikoš, Jakob Šušteršič, Velimir Ukrainczyk, Igor Planinc: Testing of Concrete Abrasion Resistance in Hydraulic Structures on the Lower Sava River

255

Mario Krzyk, Roman Klasinc, Matjaž Četina: Two-Dimensional Mathematical Modelling of a Dam-Break Wave in a Narrow Steep Stream

263

Tomaž Šolc, Aneta Stefanovska, Trevor Hoey, Matjaž Mikoš: Application of an Instrumented Tracer in an Abrasion Mill for Rock Abrasion Studies

271

Jasmin Kaljun, Bojan Dolšak: Improving Products’ Ergonomic Value Using Intelligent Decision Support System

281

Mitar Jocanović, Dragoljub Šević, Velibor Karanović, Ivan Beker, Slobodan Dudić: Increased Efficiency of Hydraulic Systems through Reliability Theory and Monitoring of System Operating Parameters

Journal of Mechanical Engineering - Strojniški vestnik

Contents

4 year 2012 volume 58 no.

Strojniški vestnik – Journal of Mechanical Engineering (SV-JME) Aim and Scope The international journal publishes original and (mini)review articles covering the concepts of materials science, mechanics, kinematics, thermodynamics, energy and environment, mechatronics and robotics, fluid mechanics, tribology, cybernetics, industrial engineering and structural analysis.

The journal follows new trends and progress proven practice in the mechanical engineering and also in the closely related sciences as are electrical, civil and process engineering, medicine, microbiology, ecology, agriculture, transport systems, aviation, and others, thus creating a unique forum for interdisciplinary or multidisciplinary dialogue.

The international conferences selected papers are welcome for publishing as a special issue of SV-JME with invited co-editor(s).

Editor in Chief Vincenc Butala University of Ljubljana Faculty of Mechanical Engineering, Slovenia Technical Editor Pika Škraba University of Ljubljana Faculty of Mechanical Engineering, Slovenia Editorial Office University of Ljubljana (UL) Faculty of Mechanical Engineering SV-JME Aškerčeva 6, SI-1000 Ljubljana, Slovenia Phone: 386-(0)1-4771 137 Fax: 386-(0)1-2518 567 E-mail: info@sv-jme.eu http://www.sv-jme.eu

Print Tiskarna Knjigoveznica Radovljica, printed in 480 copies

Founders and Publishers University of Ljubljana (UL) Faculty of Mechanical Engineering, Slovenia University of Maribor (UM) Faculty of Mechanical Engineering, Slovenia Association of Mechanical Engineers of Slovenia

http://www.sv-jme.eu

58 (2012) 4

Chamber of Commerce and Industry of Slovenia Metal Processing Industry Association

Since 1955

Strojniški vestnik Journal of Mechanical Engineering

Cover: Lowering of a large Kaplan runner assembly into the turbine pit in the Zlatoličje powerhouse on Drava river, Slovenia (maximum turbine output: 80 MW, turbine diameter: 5900 mm).

uisbrink, Francisco Arregui: Air Valves in a Large-Scale Pipeline Apparatus

j Predin, Drago Fabijan, Roman Klasinc: of the Impact of Particles on the Cavitating Flow

Image courtesy: Litostroj Power d.o.o., Slovenia

asinc, Matjaž Četina: ematical Modelling of a Dam-Break Wave am

anovska, Trevor Hoey, Matjaž Mikoš: umented Tracer in an Abrasion Mill ies

olšak: gonomic Value Using Intelligent Decision

jub Šević, Velibor Karanović, Ivan Beker, Slobodan Dudić: Hydraulic Systems through Reliability Theory and Operating Parameters

Journal of Mechanical Engineering - Strojniški vestnik

atjaž Mikoš, Jakob Šušteršič, r Planinc: rasion Resistance in Hydraulic Structures on the

year

no. 4 2012 58

volume

International Editorial Board Koshi Adachi, Graduate School of Engineering,Tohoku University, Japan Bikramjit Basu, Indian Institute of Technology, Kanpur, India Anton Bergant, Litostroj Power, Slovenia Franci Čuš, UM, Faculty of Mech. Engineering, Slovenia Narendra B. Dahotre, University of Tennessee, Knoxville, USA Matija Fajdiga, UL, Faculty of Mech. Engineering, Slovenia Imre Felde, Bay Zoltan Inst. for Mater. Sci. and Techn., Hungary Jože Flašker, UM, Faculty of Mech. Engineering, Slovenia Bernard Franković, Faculty of Engineering Rijeka, Croatia Janez Grum, UL, Faculty of Mech. Engineering, Slovenia Imre Horvath, Delft University of Technology, Netherlands Julius Kaplunov, Brunel University, West London, UK Milan Kljajin, J.J. Strossmayer University of Osijek, Croatia Janez Kopač, UL, Faculty of Mech. Engineering, Slovenia Franc Kosel, UL, Faculty of Mech. Engineering, Slovenia Thomas Lübben, University of Bremen, Germany Janez Možina, UL, Faculty of Mech. Engineering, Slovenia Miroslav Plančak, University of Novi Sad, Serbia Brian Prasad, California Institute of Technology, Pasadena, USA Bernd Sauer, University of Kaiserlautern, Germany Brane Širok, UL, Faculty of Mech. Engineering, Slovenia Leopold Škerget, UM, Faculty of Mech. Engineering, Slovenia George E. Totten, Portland State University, USA Nikos C. Tsourveloudis, Technical University of Crete, Greece Toma Udiljak, University of Zagreb, Croatia Arkady Voloshin, Lehigh University, Bethlehem, USA President of Publishing Council Jože Duhovnik UL, Faculty of Mechanical Engineering, Slovenia General information Strojniški vestnik – Journal of Mechanical Engineering is published in 11 issues per year (July and August is a double issue).

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Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4 Contents

Contents Strojniški vestnik - Journal of Mechanical Engineering volume 58, (2012), number 4 Ljubljana, April 2012 ISSN 0039-2480 Published monthly

Editorial 223 Papers Anton Bergant, Arno Kruisbrink, Francisco Arregui: Dynamic Behaviour of Air Valves in a Large-Scale Pipeline Apparatus 225 Boštjan Gregorc, Andrej Predin, Drago Fabijan, Roman Klasinc: Experimental Analysis of the Impact of Particles on the Cavitating Flow 238 Andrej Kryžanowski, Matjaž Mikoš, Jakob Šušteršič, Velimir Ukrainczyk, Igor Planinc: Testing of Concrete Abrasion Resistance in Hydraulic Structures on the Lower Sava River 245 Mario Krzyk, Roman Klasinc, Matjaž Četina: Two-Dimensional Mathematical Modelling of a Dam-Break Wave in a Narrow Steep Stream 255 Tomaž Šolc, Aneta Stefanovska, Trevor Hoey, Matjaž Mikoš: Application of an Instrumented Tracer in an Abrasion Mill for Rock Abrasion Studies 263 Jasmin Kaljun, Bojan Dolšak: Improving Products’ Ergonomic Value Using Intelligent Decision Support System 271 Mitar Jocanović, Dragoljub Šević, Velibor Karanović, Ivan Beker, Slobodan Dudić: Increased Efficiency of Hydraulic Systems through Reliability Theory and Monitoring of System Operating Parameters 281

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4 Editorial

Guest Editorial Special Issue: Hydraulic Engineering Slovenian Association for Hydraulic Research (SDHR) is a voluntary-based association to promote and encourage professional and scientific hydraulic research work in the field of Civil, Mechanical and Environmental Engineering.

It was founded in 1994 and has 89 active members. Its main aims are: to educate association’s members in the field of hydraulic research and accompanying research areas, to prepare and suggest changes of Hydraulic Engineering rules and standards and to organise professional meetings and excursions. As the association is public and open to useful suggestions, it has a good and fruitful collaboration with other professional organisations in the country and abroad. The most important among them are: Slovenian Committee on Large Dams - SLOCOLD, Water Management Association of Slovenia and International Association of Hydraulic Research - IAHR. One of the most important tasks of the association is also to publish professional achievements and news in domestic and international journals in order to present our work to other professionals and interested public.

For that reason it was decided to select some lectures which have been given at SDHR’s meetings since 2004 and publish them in Strojniški vestnik - Journal of Mechanical Engineering. This is a reputable journal with an international exchange and it is indexed in a number of databases. The guest editors of this thematic issue wish to express our gratitude to the editor-in-chief of Strojniški vestnik - Journal of Mechanical Engineering, Prof. Vincenc Butala and to the technical editor of the journal Ms. Pika Škraba, who offered us all necessary professional, logistic and financial support.

Papers based on the following five lectures were selected for publication: (1) Dynamic behaviour of air valves in a large-scale pipeline apparatus (2) Experimental analysis of the impact particles on the cavitating flow (3) Testing of concrete abrasion resistance in hydraulic structures on the lower Sava River (4) Two-dimensional mathematical modelling of a dam-break wave in a narrow steep stream (5) Application of an instrumented tracer in an abrasion mill for rock abrasion studies We would like to tank the reviewers for their concise and fruitful reviews.

Ljubljana, in April 2012 Guest Editors: Anton Bergant Matjaž Četina Matjaž Mikoš

Acknowledgement On grounds of financial cuts, the Publishing council of Strojniški vestnik – Journal of Mechanical Engineering (SV-JME) has abolished the position of Co-Editor in March 2012, thereby consensually relieving of duty the acting co-editor assoc. prof. dr. Borut Buchmeister.

Dr. Borut Buchmeister was the co-editor from October 2009 and gave an important and permanent contribution to the increased reputation of SV-JME journal, evident from the quality of every issue. He was dedicated both to the journal and to the editorial board of SV-JME, always willing to provide assistance with his advice and work.

We thank dr. Borut Buchmeister for his contribution to the development of SV-JME journal, wishing him a lot of success as the Editor-in-Chief of International Journal of Simulation Modelling.

Editor-in-Chief: Vincenc Butala 223

224

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 225-237 DOI:10.5545/sv-jme.2011.032

Paper received: 2011-02-04, paper accepted: 2011-09-14 © 2012 Journal of Mechanical Engineering. All rights reserved.

Dynamic Behaviour of Air Valves in a Large-Scale Pipeline Apparatus Bergant, A. – Kruisbrink, A. – Arregui, F. Anton Bergant1,* – Arno Kruisbrink2 – Francisco Arregui3 Litostroj Power d.o.o., Slovenia University of Nottingham, United Kingdom 3 Universidad Politecnica de Valencia, Spain 1

2

This paper describes an experimental programme on the dynamic behaviour of air valves performed in a large-scale pipeline apparatus.

Dynamic flow tests were performed at large (full) scale, since previous quasi-steady flow tests at small scale did not lead to realistic results.

Investigations in a large-scale pipeline apparatus lead to a better understanding of the physical processes associated with the dynamic performance of air valves. Float type air valves of nominal diameter of 50 and 100 mm were tested in geometrically similar 200 and 500 mm test sections, to allow for the assessment of dynamic scale effects and the development of dimensionless parameter groups and dynamic scale laws. The approach in the determination of the dynamic performance of air valves was to measure their response to flow acceleration/ decelerations, which are imposed upon the valve. In this way, the air valve behaviour following events like system start-up, pump trip and pipe rupture is simulated. Key results of the dynamic flow tests, including air release tests (valve slam) and column separation tests (effect of air valve on surge suppression), are presented and discussed.

Keywords: air valves, large-scale test facility, dynamic flow test, air admission, air release, water hammer and column separation

0 INTRODUCTION Entrapped air is a well-known phenomenon that gives rise to problems in almost any liquid pipeline system with sloped or undulated profiles (e.g. in hilly countries) and (even) flat profiles [1]. It may lead to high uncontrolled peak pressures with a potential risk of pipe rupture as well as considerably increases in energy losses. For this reason air valves are used to control the air flow in and out of the system (e.g. at high points) [2] and [3]. Although in practice the admission of air is not without problems, most of the problems are found during the release of entrapped air, sometimes resulting in pressures, even higher than if the air valves were not installed.

The function of air valves in pipeline systems is twofold: 1) to suppress sub-atmospheric pressures by admitting air rapidly (to avoid cavitation and column separation e.g. after transient events, flow interruptions, draining and system shut-down), and 2) to control the line pressures by releasing air slowly (to avoid high peak pressures e.g. during filling, system start-up). With respect to this dual-action the terms: 1) vacuum breaking valves and 2) air relief valves are commonly used. During the stage of air admission the existing subatmospheric pressures are suppressed.

This event is accompanied with relatively small pressure surges (order of 1 bar or smaller) since the subatmospheric pressures can never be lower than the liquid vapour pressure. During the stage of air release the entrapped air is compressed and accelerated towards the air valve, together with the adjacent fluid column. The air compression may cause high pressure

peaks (air valve slam) [4] to [8]. If all or most of the air is relieved from the system the air valve closes against the (still) accelerating fluid column. The reduction of the liquid velocity to zero (just before valve closure) is accompanied with additional pressure surges (order of 1 to 10 bar or higher), and possibly cavitation due to reflections of pressure waves. Under severe transient flow conditions the air valve may respond to pressure surges by reopening, possibly coupled with chatter (i.e. the repetitive opening and closure of a valve with high frequency) and other resonance effects in the system. In case of air valve failure during pressure transients column separation can occur in a pipeline system [9]. Column separation occurs in a pipe when the pressure drops to the liquid vapour pressure, assuming a negligible amount of free and released gas in the liquid [10] and [11]. This is usually the case in most industrial piping systems. Extremely large pressures may occur when the vapour cavities collapse, with a potential risk of severe engineering implications.

Research on the performance of air valves is traditionally achieved under quasi-steady flow conditions and on a rather small scale only. The lack of full (large) scale experimental data, as well as the dynamic character of the phenomena and complexity in the characterisation of two-phase flows, hinder the research progress in the mathematical modelling of the dynamic behaviour of air valves. Recent comprehensive experimental investigations in largescale test apparatus should lead to new understanding of the physical processes, dynamic scale laws and dynamic performance characterisation of air valves

*Corr. Author’s Address: Litostroj Power d.o.o., Litostrojska 50, 1000 Ljubljana, Slovenia, anton.bergant@litostrojpower.eu

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[12] and [13]. The main objective of the paper is to present the large-scale apparatus, test programme and some key results of dynamic flow tests performed at Deltares, Delft, The Netherlands [6], [9], [12] and [14]. Further, some measured and computational results are compared and discussed. The measurement data are stored on CD and available to readers by the corresponding author of this paper. The presentation of newly developed theoretical models is beyond the scope of this paper [6] and [14].

1 EXPERIMENTAL APPARATUS The dynamic behaviour of air valves has been tested under controlled, dynamic flow conditions in a unique large scale test facility. To allow for the assessment of dynamic scale effects and the practical application of results, the tests were performed at full (large and medium) scale. The experimental work consisted of the following four types of tests: 1) steady flow tests with air, 2) dynamic tests with air release, 3) dynamic tests with air admission, and 4) dynamic tests with air admission and air release. During the dynamic tests the air valve was located at a high point in the test section. A number of dynamic tests were preceded by tests with closed air valve, in order to determine its effect on the line pressures. Float type air valves [1], with either metall ball or plastic cylinder floats, of 50 and 100 mm valve nominal diameter were tested.

1.1 Steady Flow Tests with Air The steady flow tests with air were performed to determine the steady flow characteristics of the air valves. The relation between the air flow rate and pressure drop across the valve was measured at different (fixed) air valve float positions. The test set-up is shown in Fig. 1. A large, compressed air,

reservoir (volume 70.7 m3) with a maximum pressure of 22.5 bar, fed air into a 200 mm test section, with the air valve mounted at the end.

Two vortex flowmeters (Foxboro, USA) of diameter 25 and 200 mm were installed and used depending on the flow range. The data acquisition system recorded pressures at the valve and flowmeters, Dp across air valve, water temperature at flowmeter, and vortex frequency of the flowmeter.

The sampling frequency for each recorded signal was fs = 20 Hz (0.05 s between samples). The duration of the measurements was 30 (most of the tests), 20 or 15 s depending on the flow rate. Each test started after a steady air flow was established. The air valve characteristics were measured at valve openings of 100 (fully opened), 75, 50 and 25%, by fixing the float position. The air valves were tested in two flow directions, to enable air admission and air release. For this purpose, the valve was mounted in two opposite directions. All experiments have shown a high degree of repeatability of the measured pressures and flow velocities.

1.2 Dynamic Tests with Air Release and Dynamic Tests with Air Admission A modified test rig for check valves (Fig. 2) was used to test the dynamic performance of industrial size, float type air valves, during air release or during air admission. Two test configurations, with pipe diameters of 200 and 500 mm, were used for DN50 and DN100 air valves, respectively. The entire 200 and 500 mm test sections were geometrically similar in order to assess scale effects on the dynamic valve behaviour. Basically, the apparatus consists of a large air reservoir, a fast operating on/off valve, a pressurised tank, and a sloped test section. The large air reservoir was used to pressurise the tank (air release tests) or to

Fig. 1. Air loop test rig

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Bergant, A. – Kruisbrink, A. – Arregui, F.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 225-237

depressurise it (air admission tests). The fast operating 300 mm butterfly valve on top of the tank was used to produce rapid transients. In advance of each test the vent valve was used to control the initial water level in the pressurised tank and sloped test section.

At the T-junction on top of the sloped test section, a butterfly valve was installed (Fig. 2), to allow for the simulation of two pipeline configurations (only used for air release tests). When it was closed, the air valve was located at the end of the pipeline. In this case the amount of residual air (air that remains in the system after air valve closure) was relatively small. On the other hand, when it was open, the first closing of the air valve occurred long before the air was completely discharged from the pipe. In that case the scatter in the results increased considerably. The explanation for this scatter can be found in the boundary conditions and the low reproducibility of the amount of residual air.

In the design of the test configuration several variables were identified as critical in the transient events: pressure, flow rate, air-water interface, float movement and temperature. Care must be

taken in the selection of instrumentation (accuracy, frequency response) to be used in water hammer and column separation measurements [15] to [17]. All instrumentation, including pressure transducers and electromagnetic flow meters (200 and 500 mm), were carefully calibrated prior and after the dynamic tests.

The location of pressure transducers (pEMF, pps and pav), six photocells (F1 to F6), displacement transducer (sav), temperature transducer (T) and electromagnetic flow meter (QEMF) is shown in Fig. 3, for both the 200 and 500 mm test sections. The sampling frequency for each recorded signal was fs = 1000 Hz. The dynamic pressure transducers used (Kistler 410 B) had a frequency response as high as 50 kHz. Nevertheless, the main concern was the frequency response of the electromagnetic flow meter available in the laboratory, since there is not an easy procedure for a dynamic calibration of this type of devices.

This special dynamic meter was successfully used in previous studies to characterise the dynamic response of the check valves [17] to [19]. The six photocells were installed along the sloped pipe to detect the airwater interface. A fast response analogue (HBM W

Fig. 2. Modified test rig for check valves with 200 and 500 mm test sections Dynamic Behaviour of Air Valves in a Large-Scale Pipeline Apparatus

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Fig. 3. Test sections of modified test rig for check valves with instrumentation

50) displacement transducer was fitted to the air valve float to detect its vertical position. A temperature transducer was installed at the air valve T-section to measure the fluid temperature.

The test programme was subdivided into (a) air release and (b) air admission tests. (a) Air release tests. The air valve responds to the controlled overpressure above atmospheric pressure, by which a certain amount of entrapped air is accelerated towards the valve. Consequently, the air valve starts to release air. Tests were performed at different entrapped air volumes and at different initial overpressures, resulting in different flow acceleration rates. The average acceleration of the water column measured from the instant it starts moving until it reaches the air valve adopted values from 0.19 up to 0.78 m/s2. In order to accelerate the water column, a large-capacity (70.7 m3) air reservoir was pressurised with a compressor. The overpressures needed to produce the abovementioned accelerations ranged from 0.17 up to 0.28 bar. A dynamic test 228

was initiated by opening the fast acting valve on top of the pressurised tank (Fig. 2). Because of the quick response of the fast operating on/off valve (opening time 0.1 s), the pressurisation was almost instantaneous. Due to the large size of the air reservoir the pressure at the pressurised tank was almost constant during the experiments. As a result the test section was suddenly pressurised to the desired level, the water column was accelerated towards the air valve at the top and the entrapped air in the pipe was relieved through the air valve. The velocity of the water column at the instant that the air valve starts closing ranged from 0.69 up to 1.92 m/s. (b) Air admission tests. The air valve responds to the controlled underpressure (below atmospheric pressure). Consequently, the air valve starts to admit air. In advance of each test the test section (Fig. 2) was completely filled with water and the large air reservoir was depressurised by means of a vacuum pump. A test was initiated by opening the fast acting valve on top of the pressurised tank, which brought the water

Bergant, A. – Kruisbrink, A. – Arregui, F.

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Fig. 4. Test rig for check valves with 200 and 500 mm test sections

column into motion. By controlling the underpressure in the large air reservoir it was possible to conduct tests at different water column accelerations. The underpressures ranged from –0.15 up to –0.42 bar.

1.3 Dynamic Tests with Air Admission and Air Release The test rig for check valves shown in Fig. 4 was used to investigate the effect of air valves on cavitation and column separation. Two types of tests were performed: (a) tests with no air valve (Fig. 4) and (b) tests with air valve (Fig. 5). The air valve was installed immediately upstream of the check valve, to investigate its dynamic response to cavitation and column separation, as induced by the check valve closure. In order to study dynamic scale effects, two

similar sections with industrial size of 200 and 500 mm diameter pipes were used. The apparatus consists of an upstream end pressurised tank, horizontal test section with check valve, downstream end pressurised tank, connected to a large air reservoir, and a control valve. The check valves used were undamped swing types with nominal diameters of 200 and 500 mm and actual discharge (bore) diameters of 154 and 405 mm, respectively. The air valves were double orifice float types with diameters of 50 and 100 mm, respectively; the small orifice was blocked during all tests. The geometrical similarity of the 200 and 500 mm T-junction test insertions and the two air valves was fair.

The test procedure was as follows. The steady state flow conditions (in advance of a dynamic test) were controlled by a control valve (600 mm diameter).

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Fig. 5. Test rig for check valves with 200 and 500 mm test sections with air valve and instruments

The water level in the downstream end pressurised tank was adjusted by the vent valve. From initial steady flow conditions, a transient event was initiated by opening the fast operating on/off valve (diameter 300 mm) on top of the tank. The high-pressure air from the large air reservoir rapidly increased the downstream pressure. Consequently, the flow in the test section was decelerated, the check valve closed after flow reversal, and pressure surges were generated. Column separation occured at the upstream side of the check valve, when the pressure dropped to the vapour pressure. The degree of cavitation was controlled by the flow deceleration imposed upon the check valve. The events of air admission and subsequent air release were followed by an air valve closure with pressure surges.

230

In Fig. 5 the location of the instruments in the 200 and 500 mm test sections is shown. The sampling frequency for each recorded signal was fs = 2000 Hz. The check valve opening (disc position) could not be measured due to design constraints. Pressures pp, pcv,u, pcv,d and pt,d were measured with Kistler 410 B high frequency (50 kHz) piezoelectric pressure transducers.

Pressures pt,u and pcv,u (parallel measurement) were measured by Statham PD high accuracy absolute strain-gauge pressure transducers. All pressure transducers were flush mounted to the inner pipe wall.

The measured results confirmed the fast response of both electromagnetic flow meters. A fast response Hottinger (HBM) W 50 analogue displacement transducer, fitted to the air valve float, was used to measure the vertical float position. The location of

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instruments in the respective test sections without and with air valve was practically the same. Temperature T and air valve opening sav (float position) were not measured during the test series without air valve.

The test programme was subdivided into (a) tests with no air valve in the system and (b) tests with dualacting vacuum breaking/air relief valve positioned at the upstream end of the check valve. (a) Tests without air valve. Water hammer and column separation measurements in the 200 mm test section were carried out with steady state (initial) velocity V0 = 1.02 m/s (about 20% above the critical flow velocity at which the check valve is just fully open) and static pressure in the system of 0.3 MPa. The intensity of transients was controlled by the magnitude of the mean deceleration |dV/dt| of the liquid column during the check valve closure event.

The mean |dV/dt| was in the range from 0.55 to 3.93 m/s2, and the corresponding reverse flow velocity VR from about 0.08 to 0.22 m/s. Some measurements were performed with an additional pressure transducer at the closed-junction 21.56 m upstream of the check valve (see Fig. 5). The idea was to investigate how far the pressure at the assumed upstream boundary of the 200 mm test section (i.e. the tapered junction DN500/ DN200) is constant. The measured results showed that this boundary in first approximation may be modelled as a constant head boundary in numerical analysis. In this case the mean |dV/dt| was in the range from 0.42 to 4.08 m/s2, and the corresponding VR from about 0.05 to 0.21 m/s. Measurements in the 500 mm test section were carried out with V0 = 1.06 m/s (about 20% above the critical flow velocity) and static pressure of 0.3 MPa. The mean |dV/dt| was in the range from 0.63 to 3.70 m/s2, and the corresponding VR from 0.26 to 1.15 m/s. (b) Tests with air valve. The objective of these measurements was to investigate to what extent the air valve suppresses column separation in pipelines.

Dynamic tests in the 200 mm test section were carried out with V0 = 1.02 m/s and static pressure of 0.3 MPa. The |dV/dt| was in the range from 0.81 to 4.24 m/s2, and the corresponding VR from about 0.10 to 0.23 m/s. Dynamic tests in the 500 mm test section were performed with V0 = 1.06 m/s and static pressure of 0.3 MPa. The mean |dV/dt| was in the range from app.

0.60 to 3.70 m/s2, and the corresponding VR from app.

0.26 to 1.15 m/s.

1.4 Uncertainty Analysis The uncertainty of a measurement may be represented by the sum of bias and precision errors. The rootsquare-sum (RSS) uncertainty Ux is expressed as: U x = B 2 + Px2 , (1)

where B and Px are bias and precision errors, respectively [20]. The bias error is the fixed, systematic error, which is usually constant for each measurement. The precision error is the random error which results in data scatter in experiments. Table 1 shows a list of uncertainties Ux in the measurement.

The uncertainties are given either as absolute values (same units as quantity) or as percentage (uncertainty/ quantity×100%) values, where appropriate. The repeatability of identical experimental runs, performed in both the 200 and 500 mm test sections, was investigated. The objective of the repeatability tests is to estimate the data scatter (precision error) due to an inability to reset the system to identical initial conditions. A comparison of results between ‘identical’ runs show that the magnitude and timing of the bulk pressure pulses are repeatable, whereas some high frequency pressure spikes do not exhibit repeatability.

Table 1. Estimated uncertainties in the measurement Quantity Pipe internal diameter Pipe length Piezoelectric pressure transducer Strain-gauge pressure transducer Electromagnetic flow meter DN200 Electromagnetic flow meter DN500 Water temperature Air-valve float position

Uncertainty ±0.1 mm ±0.01 m ±0.7% F.S. ±0.3% F.S. ±2% of rate ±4% of rate ±0.5 oC ±0.5% F.S.

2 CASE STUDY: AIR RELEASE EVENT WITH AIR VALVE SLAM The objective of this case study is to investigate the air release event in an industrial size DN500 sloping pipe test section with installed DN100 cylindrical float type dual-acting vacuum breaking/air relief valve at the top of the pipe (Section 1.2). The test section with air valve and instruments is given in Fig. 3. Basically, the experiment simulates the filling of a pipe partially filled with water, with an air pocket at its end. In the initial stage, the air valve is open and the air pressure inside the pipe is atmospheric. The initial water level in the inclined pipe was 3.80 m above datum level

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(Fig. 3). The corresponding initial volume of the air pocket was 0.334 m3, (with the butterfly valve near the air valve closed). The pressure difference between the large air reservoir and pressurised tank was 21.5 kPa. The temperature measured at the air valve (see Fig. 3) was T = 18.8 oC and remained constant during the test.

The transient event was initiated by opening the fast operating on/off valve on the top of the pressurised tank (Fig. 3). Due to the pressure difference the water column is accelerated towards the air valve. When the water front reaches the air valve, its float moves up, and the valve closure results in a rapid flow velocity change/reduction. This water hammer effect, together with the compression of residual air inside the pipe, produces a large pressure rise (air valve slam). In this stage of the transient the entire pipe is filled with water, except near the air valve, where small air pockets remain entrapped.

A typical result of a release test is shown in Fig. 6 where the time histories of the pressure p*av, the water flow rate QEMF and the air valve float displacement sav are presented. Each signal is depicted at two time scales; the smaller scale at the right side allows for a more detailed observation of the events.

Initial event: The water column starts moving at the instant of initiation (Fig. 6c, ti = 4.11 s) and is accelerated towards the air valve. The photocell at the air valve F6 (Fig. 3) detected the water-air interface at time t = 5.50 s (registered as a change of signal).

There are no significant pressure changes until the instant of pressure rise at time tp = 5.56 s (Fig. 6b).

The time interval [ti, tp] is defined as initial event [6].

During this event rigid column theory is valid.

Air compression event: Subsequently air compression takes place. Three stages may be distinguished here, with fully opened, partly opened

Fig. 6. Air release test in 500 mm test section with air valve slam; a) and b) pressure at the air valve (p*av), c) and d) discharge (QEMF) at electromagnetic flow meter position and e) and f) air valve opening (sav)

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(closing) and fully closed air valve (Figs. 6e and f).

The air valve starts closing at time to = 5.57 s. After the instant of air valve closure at time tc = 5.62 s, the residual of air is further pressurised until the maximum pressure p*av,max = 18.4 bar is reached at time tm = 5.67 s (Figs. 6a and b). At this instant the water column is brought to a rest (Figs. 6c and d; QEMF = 0 m3/s) and then reverses from the air valve. The pressure rise is relatively high, while the duration of this event is relatively short. The time interval [tp, tm] is defined as air compression event [6]. Thermodynamics, aerodynamics and elastic waterhammer theory are valid.

Residual air: The residual air, which remains inside the pipe system after valve closure, which plays a significant role in the water transient magnitude, can be indirectly evaluated with an acceptable uncertainty, by integrating the flow signal. For the evaluation of the residual air mass it is referred to Kruisbrink et al.

[6].

Pressure surges: The flow velocity at the end of the initial event (at time tp) is defined as the terminal velocity (Vp when based on pipe diameter and vp when based on air valve diameter). At this instant the flow rate reaches its maximum value. After this instant the flow is reduced to zero, which results in a pressure rise. According to water hammer theory, the terminal velocity is proportional to the pressure rise and may therefore be used to estimate the pressure surges, induced by the air valve closure.

3 CASE STUDY: WATER HAMMER AND COLUMN SEPARATION WITH AIR VALVE The objective of this case study is to investigate transient event with water hammer and column separation in an industrial size DN500 horizontal pipe test section with installed DN100 cylindrical float type dual-acting vacuum breaking/air relief valve (Section 1.3). The test section with air valve and instruments is given in Fig. 5. A typical result of a column separation test is shown in Fig. 7. The steady state (initial) flow velocity in pipe was V0 = 1.06 m/s and the static line pressure was about 0.3 MPa. The transient event was initiated by opening the fast operating on/off valve on top of the downstream end tank (Figs. 4 and 5) at the time of t = 1.53 s (Fig. 7d). The compressed air from the large air reservoir with a higher set pressure than in the horizontal pipe test section increased the pressure in the downstream end tank for about 1.4 bar. The flow in the test section was decelerated at a rate of about |dV/dt| = 3.6 m/s2 (Fig. 7e). The check valve was closed after the flow

reversal generating column separation at the upstream end of the valve (pressure dropped to the liquid vapour pressure - see Figs. 7a and b) and the pressure rise of 9.2 bar at the downstream end of the valve (Fig. 7c). Initialy a large vapour cavity was created at the valve. Air valve opened with a time delay of 20 milliseconds and it stayed open for 110 milliseconds.

The pressure in the large cavity increased to the atmospheric pressure of 70 milliseconds after pressure dropped to the liquid vapour pressure (effective time to fill the vapour cavity with air). A discrete vaporous cavitation zone (small void fraction) may be observed along the pipeline (Fig. 7a). The maximum absolute pressure due to the collapse of the large gas cavity at the upstream side of the check valve (p*cv,u)max = 10.2 bar occurred about 10 milliseconds after the air valve was completely closed. A closed check valve reopened when the pressure difference across the valve exceeded a threshold value (pcv,u > pcv,d) and this occurred at the time of cavity collapse. Consequently, the partly opened check valve reduced the propagating pressure pulse. The measured temperature at the T-junction with air valve (see Fig. 5) was T = 23.3 oC and it remained constant during the considered transient event. The same holds true for the pressure in the upstream end pressurised tank. This case study clearly shows the delay of the valve opening when pressure drops to liquid vapour pressure and the time shift between air valve closure and pressure rise due to gas cavity collapse. The valve response is not instantaneous as traditionally assumed in the standard theoretical models used for transient analysis.

3.1 Effect of Air Valve on Surge Suppression The effect of the air valve as a surge suppression device is studied. For this purpose tests without and with air valve are compared, as performed in the 500 mm test section (Figs. 4 and 5). In Fig. 8 the results of Fig. 7 are presented, together with results of a test without air valve performed under identical flow conditions. Investigation of pressures (absolute) in the upstream pipe (p*p) and at the check valve (p*cv,u and p*cv,d) shows that in the first instance the air valve hardly has any effect on the sudden pressure surges induced by the check valve closure. This may be attributed to the above-mentioned delay in its response. In the second instance however, the pressure surges are strongly reduced, while a further occurence of cavitation (i.e. at vapour pressure) is avoided.

The air valve acts here as a surge protection device, i.e. it suppresses pressure surges, avoids cavitation (potential risk of pipe collapse) and it attenuates

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Fig. 7. Column separation test in 500 mm test section with air valve; a) pressures along the pipe (p*p), b) and c) at the check valve (p*cv,u and p*cv,d), d) in the downstream end pressurised tank (p*t,d), e) discharge (QEMF) at electromagnetic flow meter position and f) air valve opening (sav)

pressure oscillations (a potential risk of pipe rupture).

The surge protetion device should alter the system characteristics and consequently, the intensity of the flow velocity changes in the system [1], [10] and [21].

3.2 Comparison of Test and Simulation with Simple Air Valve Model Recently an attempt was made to compare and discuss the measured and the computational results for a number of experimental runs in the considered test rig for check valves without (Fig. 4) and with an air valve (Fig. 5) [14]. A set of water hammer and column separation equations was solved using the Godunov’s method [14] and [22]. The numerical model includes constant pressure upstream end reservoir, pipe test section and downstream end boundary. The test section of total length L = 30.76 m from the upstream 234

end pressurised tank to the check valve (downstream end boundary condition) was considered in numerical simulations. The air valve was modelled as a simple source term. The air mass flow through the air valve (in or out) was assumed constant with cushioning at the end of the releasing period. The check valve was modelled as a downstream end boundary condition (i) as a dead end when the check valve was closed and (ii) by applying the measured pressure and flow rate history downstream the check valve when the check valve was opened. The time step in the simulation was 0.0001 s. In Fig. 9 results of a simulation are presented together with the test results of Fig. 7, showing the measured and calculated pressures along the upstream pipe (p*p) and at the check valve (p*cv,u). The agreement between test and simulation is reasonable, bearing in mind that the transient flow, induced by the

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Fig. 8. Column separation tests in 500 mm test section with and without air valve; a) pressures along the pipe (p*p), b) and c) at the check valve (p*cv,u and p*cv,d)

Fig. 9. Column separation test in 500 mm test section with air valve; a) comparison of measured and calculated pressures along upstream pipe (p*p) and b) at the check valve (p*cv,u)

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dynamic behaviour of both the air valve and check valve, is complex.

activity within the Improving Human Potential (IHP) Programme.

4 CONCLUSIONS

6 NOMENCLATURE

The dynamic behaviour of air valves has been tested under controlled, dynamic flow conditions in a unique large scale test facility at Deltares, Delft, the Netherlands. The tests were performed at full (large and medium) scale in geometrically similar test sections, to allow for the assessment of dynamic scale laws and the practical application of results.

The experimental work consisted of the following four types of tests: 1) steady flow tests with air, 2) dynamic tests with air release, 3) dynamic tests with air admission, and 4) dynamic tests with air admission and air release. During the dynamic tests the air valve was located at a high point in the test section. The approach to determine the dynamic behaviour of air valves was to measure their response to different flow accelerations towards (air release) and from the air valve (air admission). In this way, the valve behaviour following events like system start-up, pump trip or pipe rupture is simulated.

In two case studies some key results of the dynamic flow tests are presented and discussed. The air release tests show that the closure of air valves may result in significant pressure peaks (up to the order of 10 bar), possibly followed by air valve slam, while some residual air remains entrapped in the system.

The column separation tests reveal a delay in valve response to sudden underpressures. Although the delay in the valve opening is relatively short (order of 0.01 s), it does not prevent the occurence of cavitation, when the pressure suddenly drops to the liquid vapour pressure. After a delayed air admission however, the cavitation disappears and pressure surges are strongly reduced, although the valve closure also here is accompanied with pressure peaks. The response of the valve is not instantaneous as traditionally assumed in literature on transient flow in liquid-filled pipelines.

Column separation tests with and without air valve show a benifical effect of the air valve as surge supression device. Only when properly designed, dimensioned and installed in the pipe system, the air valve may act as surge protection device.

5 ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the European Commission for their funding of the Transnational Access to Major Research Infrastructure 236

The following symbols are used in this paper: B bias error D pipe diameter, diameter dV/dt mean flow deceleration at check valve F air-water interface position fs sampling frequency L pipe length, length Px precision error p pressure Q discharge sav air valve float position T temperature Ux uncertainty in a measurement V flow velocity Vp(vp) terminal flow velocity VR reverse flow velocity x distance Dp pressure difference across the valve ˅ volume Subscripts: air valve av c instant of air valve closure check valve cv d downstream EMF electromagnetic flow meter i initial m instant of maximum pressure max maximum o start of air valve closure event p pipe; instant of pressure rise sloping pipe ps t pressurised tank u upstream steady state conditions 0 Superscripts: * absolute pressure 7 REFERENCES [1] Thorley, A.R.D. (2004). Fluid transients in pipeline systems, 2nd ed. Professional Publishing Limited, London.

[2] Cabrera, E., Fuertes, V., García-Serra, J., Arregui, F., Gascón, L., Palau, C. (2003). Reviewing air valves selection. Pumps, Electromechanical Devices and Systems Applied to Urban Water Management, Cabrera, E., Cabrera, E.Jr. (eds.), vol. 2, A.A. Balkema Publishers, Lisse, p. 633-640.

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[3] Ramos, H., Borga, A., Bergant, A., Covas, D., Almeida, A.B. (2005). Analysis of surge effects in pipe systems by air release/venting. Portuguese Journal of Water Resources (Revista Recursos Hídricos), vol. 26, no. 2, p. 45-55.

[4] Campbell, A. (1983). The effect of air valves on surge in pipelines. Proceedings of the 4th International Symposium on Pressure Surges, BHRA, p. 89-102.

[5] Lee, T.S. (1999). Air influence on hydraulic transients on fluid system with air valves. Journal of Fluids Engineering, vol. 121, no. 3, p. 646-650, DOI:10.1115/1.2823518.

[6] Kruisbrink, A., Arregui, F., Carlos, M., Bergant, A. (2004). Dynamic performance characterization of air valves. The Practical Application of Surge Analysis for Design and Operation, Murray, S.J. (ed.), vol. I, BHR Group Limited, Cranfield, p. 33-47.

[7] Lingireddy, S., Wood, D.J., Zlocower, N. (2004).

Pressure surges in pipeline systems resulting from air releases. Journal of American Water Works Association, vol. 96, no. 7, p. 88-94.

[8] Fuertes, V.S., Iglesias, P.L., Lopez, P.A., Mora, D. (2009). Air valves sizing and hydraulic transients in pipes due to air release flow. Proceedings of the 33rd IAHR Congress: Water Engineering for a Sustainable Environment, CD-ROM, paper 10463.

[9] Bergant, A., Bournaski, E., Arregui, F., Kruisbrink, A. (2004). Column separation measurements in a largescale experimental apparatus. The Practical Application of Surge Analysis for Design and Operation, Murray, S.J. (ed.), vol. II, BHR Group Limited, Cranfield, p.

589-604.

[10] Wylie, E.B., Streeter, V.L. (1993). Fluid Transients in Systems. Prentice-Hall Inc., Englewood Cliffs.

[11] Bergant, A., Simpson, A.R., Tijsseling, A.S. (2006).

Water hammer with column separation: a historical review. Journal of Fluids and Strucures, vol. 22, no. 2, p. 135-171, DOI:10.1016/j.jfluidstructs.2005.08.008.

[12] Bergant, A., Arregui, F., Cabrera, E., Bournaski, E., Kruisbrink, A., de Silva, A., Thorley, A.R.D. (2007).

Dynamic behaviour of air valves. Transnational access to major research infrastructures - access to

experimental facilities of WL|Delft Hydraulics. Report No. 1412, Litostroj E.I., Ljubljana.

[13] Lemos de Lucca, Y.F., Alcântara de Aquino, G., Dalfré Filho, J.G. (2010). Experimental apparatus to test air trap valves. Proceedings of the 25th IAHR Symposium on Hydraulic Machinery and Systems, Susan-Resiga, R., Muntean, S., Bernad, S.I. (eds.), vol. 2, p. 801-807.

[14] Gale, J., Bergant, A. (2010). Modeling of dynamic response of air valves during pipeline transients.

Proceedings of the 1st European IAHR Congress, CDROM, Paper FMIIb. [15] Graze, H.R., Horlacher, H.B. (1983). Pressure transients following the collapse of vapour cavities. Proceedings of the 6th International Symposium on Hydraulic Transients in Power Stations, IAHR, Gloucester.

[16] Simpson, A.R., Bergant, A. (1994). Developments in pipeline column separation experimentation. Journal of Hydraulic Research, vol. 32, no. 2, p. 183-194, DOI:10.1080/00221689409498722.

[17] Kruisbrink, A. (1997). The dynamic behaviour of check valves. Ph.D. Thesis, City University of London, London.

[18] Kruisbrink, A.C.H., Lavooij, C.S.W., Koetzier, H. (1986). Dynamic behaviour of large non-return valves.

Proceedings of the 5th International Conference on Pressure Surges, BHRA, Hannover, p. 237-244.

[19] Kruisbrink, A.C.H., Thorley, A.R.D. (1994). Dynamic characteristics for damped check valves. Proceedings of the 2nd International Conference on Water Pipeline Systems, BHR Group, Edinburgh, p. 459-476.

[20] Coleman, H.W., Steele, W.G. (1989). Experimentation and uncertainty analysis for engineers. John Wiley and Sons, New York.

[21] Riasi, A., Raisee, M., Nourbakhsh, A. (2010).

Simulation of transient flow in hydroelectric power plants using unsteady friction. Strojniški vestnik Journal of Mechanical Engineering, vol. 56, no. 6, p.

377-384.

[22] Gale, J., Tiselj, I. (2008). Godunov’s method for simulations of fluid-structure interaction in piping systems. Journal of Pressure Vessel Technology, vol.

130, no. 3, p. 031304-1 - 031304-12.

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Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 238-244 DOI:10.5545/sv-jme.2011.062

Paper received: 2011-03-17, paper accepted: 2011-10-21 © 2012 Journal of Mechanical Engineering. All rights reserved.

Experimental Analysis of the Impact of Particles on the Cavitating Flow Gregorc, B. – Predin, A. – Fabijan, D. – Klasinc, R. Boštjan Gregorc1,* – Andrej Predin2 – Drago Fabijan3 – Roman Klasinc4 2 University

1 Dravske

elektrarne Maribor, Slovenia of Maribor, Faculty of Energy Technology, Slovenia 3 Litostroj Power d.o.o., Slovenia 4 Graz University of Technology, Austria

The purpose of this paper is to present an analysis of the impact of solid particles on the development of cavitating flow conditions around a hydrofoil. Experimental studies have been conducted in a cavitation tunnel with three different mixtures of particles and water. We used particle-like properties such as are found in river water, and with increasing mass concentration. We performed measurements of torque and the relative noise in the hydrofoil. The point in the formation of vapour phase on the hydrofoil and the pronounced frequency effect were determined by measuring the relative noise. Based on the analysis the results show that the particles increase the intensity and extent of cavitation.

Keywords: particles, cavitation, noise, measurements

0 INTRODUCTION Operation of water turbines and pumps in the natural environment depends on the seasonal changing of physical and biological properties of water (temperature, viscosity, surface tension, the content of dissolved and non-dissolved air content of impurities).

As the flow of rivers and streams changes, so does the amount of impurities (particles) in water. A stronger local rainfall caused to 30-fold increase of impurities in water depending on underlying conditions. The lifetime of hydraulic machinery is highly dependent on the quality of the media (water). The effect of dirt (mud) in the water in most cases occurs in the form of wear and tear of mechanical parts, and leaks in the connection between rotating and fixed parts. It also causes and increases the risk of cavitation due to impurities and gases under specific conditions (pressure, velocity) [1] and [2]. The impact of cavitation in hydraulic machinery represents a loss of energy with regard to the optimal operating conditions. In the case of the operation of water turbines, the effect of cavitation results in reduced utilization, increased noise and vibration, as well as mechanical damage to the turbine runner surfaces and other exposed areas [3].

Determination of the efficiency changes on prototypes caused by impurities in the water is difficult to achieve from the perspective of performance measurement. Constantly changing various parameters (oscillations of generating power pulsations, pressure, difference in altitude, temperature, air content and mass concentrations of particles) affect the credibility of the results [4] and [5]. The change in turbine 238

efficiency is detected only over time, during which cavitation abrasion damage to surfaces are already present. For these variations of parameters in the case of the prototypes, we studied the influence of the particles in water on the development of cavitation on hydrofoil in the laboratory. Due to controlled parameters at the inlet of the tunnel, we used particles of known diameter, density and mass concentration.

With this we came closer to reality state in river water.

1 MULTI-PHASE FLOW INFLUENCE ON HYDRAULIC MACHINERY OPERATING PROPERTIES Research related to the movement of liquids and particles in the transition through the pump and turbine has been conducted by many authors [6] and [7]. For research, different material particle concentrations have been used, as well as various basic materials surfaces [8]. For the evaluation of developments and implications in the process of developing cavitation and abrasion, authors use CCD cameras - visualization, PIV Technology, methods of weighing and vibrations methods [2] to [9].

Authors of other studies have discovered a number of instabilities, among which the pulse cavitation cloud [1] and [10] is the most frequently addressed. In recent decades, research has been based on the mutual interaction between the liquid and the vapour phases. Mei et al. [6] and Shengcai [9] discovered that cavitation in water occurs earlier due to the presence of particles, in comparison to pure water. The mass concentration ξc of particles, where the largest increase of the vapour phase occurs, is between 8 and 13 kg/m3.

*Corr. Author’s Address: Dravske elektrarne Maribor, Obrežna ulica 170, 2000 Maribor, Slovenia, bostjan.gregorc@dem.si

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 238-244

Hydrodynamic cavitation causes a change of resistance, a change of hydromechanical flow properties, thermal and lighting effects, and erosion at streaming areas. During the operation of hydraulic machinery, the most undesirable cavitation is in the form of a cloud. This causes turbine or pump efficiency decrease and mechanical damage as a consequence of high local pressures when the vapour bubbles condense, i.e. implode. During the turbine and/or pump operation, cavitation is detected as a sharp sound that is not constant. A released pressure wave spreads through the vapour phase when the vapour phase collapses and liquid is drawn from a wide range of frequencies. The sound accompanies the basis of increased vibration in the occurrence of cavitation. Vibration and noise are the consequences of the shock-wave that is spreading in space and striking in the surrounding area. The noise is generated when bubbles collapse and is located in the high frequency band. Capturing cavitation noise is highly dependent on the position of installing the sensor.

Detection of cavitation noise forms the basis to determine the impact of particles on the development of cavitating flow (initial cavitation).

2 HYDRAULIC FORCES ON THE HYDROFOIL With the creation of multi-phase flow in mixed streaming flow, changing forces are observed around hydrofoil. The flow of water and particles causes a change of the lift force Eq. (1) and drag force Eq. (2), and consequently, torque Eq. (3). The forces could be written as:

Fl = CL ⋅ B ⋅ L ⋅ ρ ⋅ v∞2 / 2 , (1)

Fd = CD

Mt = CM

( ) ⋅ ( B ⋅ L ⋅ ρ ⋅ v / 2 ) , (2) ⋅ ( B ⋅ L ⋅ ρ ⋅ v / 2 ) , (3) 2 ∞

2

2 ∞

where C is coefficient, length of the profile is L, the width is B, and ν∞ is the free stream velocity.

Characteristic surface (A = B · L) is equal to the projection of the hydrofoil in a plane which is perpendicular to the free stream velocity. Lift and drag are a result of the distribution of pressure and shear forces. When the condition is pm ≤ pv, and at an appropriate velocity at the suction side of the hydrofoil, a vapour phase is developed. Initial cavitation number is expressed as Eq. (4):

(

)

σ = ( p ∞ − pv ) / ρ ⋅ v∞2 / 2 . (4)

The impact of the vapour phase is reflected in the lift and drag force, which depends on the development of the vapour phase.

Determination of changes in pressure based on the relative motion of particles can only be approximated.

The movement of particles in the body part streaming and changing its direction. Basically, the following can be written:

Qm ρm = Qd ρd + Ql ρl , (5)

where Qm is suspension flow rate, Ql is water flow rate and Qd is flow rate of dispersed particles.

Suppose that a mixture of water with particles, and water without particles have the same pressure and velocity (p0m = p0l, ν0m = ν0l) at the entrance through the plane 0-0, which is perpendicular to the direction of speed. In the area of minimum pressure on the hydrofoil at the plane 1-1, the conditions are different (p1m ≠ p1l). Using an energy approach for moving plane from 0 to a plane 1 the following can be written: p0 m − p1m v02m − v12m + = ρm g 2g (6)

( v02 p − v12p ) + ( v02l − v12l ) p0l − p1l = + .

2g ρl g Assume that velocities ν1p ≈ ν1m ≈ ν1l are in the same mixture because of the distance between the particles, (L / dd) > 1 and low concentrations of dispersed particles. The assumption is the case of natural river water, where the maximum mass concentration in most cases does not exceed ξc = 1 kg/m3. For this reason, we ignore the interactions between particles. Considering this fact, the velocity differences in the Eq. (6) disappear. Eq. (6) can be written using p0 – p1 = Δp, as:

∆pm ∆pl ρ = ⇒ ∆pm = m ∆pl . (7) ρm g ρl g ρl

The ratio of densities of mixture and pure water influence the pressure change of the mixture.

By increasing the concentration, the influence on the development of vapour phase condition (pm – pv(T) = Δp) is achieved.

3 EXPERIMENT We have performed tests on a small cavitation tunnel.

The tunnel was designed to study the development of cavitation in various forms of hydraulic hydrofoils.

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Fig. 1. Cavitation tunnel sketch

It is a closed type and allows mass flow up to Qi = 22 kg/s in the tunnel (Fig. 1). The cavitation number change is performed by the controlled application of pressure ps in the system. Flow rate is measured using an ultrasonic flowmeter.

the hydrofoil (16 and 20°). The water temperature in the system was 20±1 ˚C. By reducing pressure in the system, the decrease in the cavitation number is influenced, thus increasing the developed vapour phase. Measurement of lift and drag forces (Mt) was conducted without addressing the relative friction in the bearings (roller bearings). Visual monitoring of initial cavitation was performed on a metal hydrofoil block (Fig. 2) located in the middle part of the input hydrofoil surface (nozzle is square: 4×2×6 mm). The block was located 12 mm from the inlet edge of the hydrofoil.

Fig. 2. Geometry of cavitation tunnel and testing hydrofoil The length of the blade profile is L0 =104 mm and the width is B = 64 mm (Fig. 2), overall length (l = 17×L0) of the cavitation plane is part of the tunnel (64.5×70 mm). The hydrofoil was observed through the Plexiglas on the two perpendicular sides. The change in lift and drag forces were measured via changes in a torsion lever attached to the shaft on a fixed hydrofoil and a dynamometer accuracy of 0.1% (Ahlborn K-25). The link shaft and the dynamometer placed with cantilever handle with a length of 94.5 mm. The pressure in the channel was measured before and after the hydrofoil. The measurements were performed at different flow velocities (average) in the tunnel (2.6, 2.9, 3.3 and 3.6 m/s). Measurements were conducted at two different angle settings of 240

Fig. 3. A case measurement Mt of the profiled testing hydrofoil

Torque is shown as Mt/Mt0, where the ratio represents the initial state without cavitation Mt0 – ∆p = 0 bar (average in time Δt = 10 s) and Mt which depends on the reduction of pressure ps. The boundary between the influence of cavitation and non-developed area of cavitation can be clearly observed (see Fig. 3).

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Cavitation noise has been measured with the instrument (Cirrus Research plc - CR: 800B) mounted above the Plexiglass and isolated from its surroundings with Styrofoam. The sensor was placed away from the occurrence of vapour phase on the hydrofoil from 11 to 13 mm. Cavitation noise was measured in various frequency ranges (from 25 to 16 kHz), and at various cavitation numbers. Measurements were started with pure water (tap water), and continued with three different mass fractions of particles. The pressure in the channel was measured before and after the hydrofoil.

Estimated measurement errors for pure water (Mt/Mt0) and for three mass fractions are evident in Table 1.

The part of Fig. 6 shows that the largest change in momentum can also be seen in the maximum mass fraction (ξ) of particles in the water. The experimental data is shown as a relative change of Mt/Mt0 in the reduction of the dependence on pressure ∆p (lower cavitation number). The change in Mt/Mt0 for ∆p = 0.15 is lower in case of pure water, compared to the mixture of particles and water. The increase of Mt/ Mt0, caused by additional particles, is the largest in the range of ∆p = 0.2 to 0.3 bar, where the difference is 11%. Mt/Mt0 begins to reduce with the increase of ∆p (∆p = 0.32 bar), due to the intensity of the vapour in the suction side of hydrofoil. The relative mass fraction between 0.001 and 0.0016 shows the same trend of increasing a relative change of Mt/Mt0.

Table 1. Particle mass fraction used in the experiment and measurement errors – Mt/Mt0 Mass fraction

ξ [-]

Pure water Fraction 1 Fraction 2 Fraction 3

ξ=0 ξ = 0.001 ξ = 0.0016 ξ = 0.0032

Estimated measurement errors STDEV (Δp = 0 bar) 2.1 2.9 3.1 3.6

For measuring the impact on the development of cavitation, we used particles of material FR 240/F. The density of particles is ρd = 1700 kg/m3, (diameter of the used particles is between 25 < dd < 35 µm – the mean diameter of particles is 30 µm); they are insoluble in water, inert and do not oxidise. Particle density ρd and the mean diameter of particles dd were chosen because of similar qualities of particles in the rivers in our geographic area. All measurements were performed at the same conditions (mass flow, the reference pressure in the system, temperature).

Fig. 4. The impact of mass fraction of particles to change Mt/Mt0 (vsu = 3.6 m/s,α = 16°)

3.1 Results and Discussion Experimental measurements are shown as a relative comparison between the particle-free water and state water with particles.

The results of the occurrence of cavitation - σ on hydrofoil (pure water) at experimental measurements are comparable to results of other authors [7] and [10].

Fig. 4 shows an increase in torque (ratio) Mt/ Mt0 by the addition mass fractions of particles in suspension. The difference between Mt/Mt0 of clean water and a suspension of particles is greater in the case of small inclination angles of the hydrofoil (Fig. 5).

Torque increases above the difference pressure of ∆p = 0.15 bar (Fig. 5).

Fig. 5. The impact of mass fraction of particles to change Mt/Mt0 (vsu = 2.6 m/s,α = 20°)

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Fig. 7 shows the variation of the formation of vapour phase of the mass fraction of particles (measurement of the relative noise). We see that a vapour phase occurs at relatively the same flow conditions (∆p, vsu). Particles in suspension increased the intensity of occurrence of cavitation.

Fig. 6. Experimental comparison of torque and ∆p on hydrofoil, using pure water and suspension with particle (vsu = 3.3 m/s, α = 20°)

Cavitation number is changed by 3.6% in relatively the same relative intensity noise (Fig. 8).

The increase in the vapour phase is estimated by comparing the results of measurements of water free of particles.

Fig. 8. Changes in the initial cavitation number - σ (intensity) on the development of vapour phase (vsu = 2.6 m/s, α = 16˚)

Fig. 9. The impact of mass fraction of particles on the change of relative intensity noise (vsu = 3.3 m/s, α = 20˚)

Fig. 7. Dependence of cavitation formation of the mass concentration of particles in suspension (σsu = 2.6 m/s, α = 16˚)

By increasing the relative noise (cavitation) by 15%, changes in torque on the hydrofoil are not detected. In the case of pure water the difference in the change in noise is 7% (∆p = 0.06, ξ = 0.0032).

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By increasing the vapour cloud (reduced cavitation number) and the mass fraction of particles decreases the relative noise (Fig. 9). We believe that the reduction of the relative noise, impact of increased volume of vapour phase, which reduces the speed of sound. By analyzing the standard deviations the maximum deviation of the relative noise in the area of initial cavitation is found (Fig. 10). By increasing the vapour phase, the standard deviation decreases.

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In all cases (mass concentrations) in the area of maximal developed vapour phase in a suspension the standard deviation is minimal.

Fig. 10. Standard deviation of noise as a function of changes in mass concentrations and the reference pressure (vsu = 2.9 m/s, α = 16˚)

Fig. 11. The impact of particles on the frequency range of cavitation noise

Fig. 11 shows a comparison between the situation where no cavitation and the situation with developed cavitation (without particles and with the particles).

Particles markedly increase the intensity of sound at a frequency of 315 Hz, and the area between 4 and 12 kHz. We believe that the particles cause intense tearing vapour clouds at a frequency of 315 Hz. Similar experimental results of relative intensity noise are also recognized by other authors [1] and [10].

4 CONCLUSIONS In the cavitation tunnel, we explored the effect of particles in the water on the change of the relative ratio of torque (Mt/Mt0) and the relative intensity of cavitation - noise on a hydrofoil for three different mass fractions of particles in water. In examining the

impact of particles on lift-drag forces and noise, we compared the measured relative values of clean water free of particles. From the measured values, we have drawn the following conclusions: • Change in torque (Mt/Mt0) depends on the development of vapour phase and the attachment hydrofoil. Development of vapour phase before the centre mounting reduces torque.

• Particles increase the torque on the hydrofoil.

By increasing the mass fraction ξ of particles in suspension increases the torque. Increased change in torque between the state of the particles in suspension and pure water is measured at a higher pressure difference (lower cavitation number).

• With the increasing angle of hydrofoil (20˚) and suspension velocity in the tunnel, the differences are due to added particles on the lower torque Mt/Mt0.

• Vapour phase occurs at relatively the same cavitation number in case of pure water and the added particles. Particles in suspension increased the intensity of occurrence of cavitation. By increasing the relative noise (cavitation) by 15%, changes in torque on the hydrofoil are not detected.

• Standard deviation of the relative noise is the largest in the area of initial cavitation.

By increasing the vapour phase, the standard deviation decreases.

• The formation of cavitation on the hydrofoil is rapidly detected by increased noise in the range of the occurrence of vapour phase. Noise increases in developed cavitation by 35%, in comparison to the state without cavitation. By measuring the relative noise rapidly the emergence of a vapour phase is detected, as in the case of torque measurement (Mt/Mt0).

• The values of maximum amplitude of noise in frequency areas are reached in the range - 1 kHz. Particles markedly increase the intensity of sound at a frequency of 315 Hz, and the area between 4 and 12 kHz. 5 NOMENCLATURE p0 barometric pressure system pressure ps v characteristic velocity ρ density mean diameter dd T temperature mass concentration ξc ξ mass fraction

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α angle of hydrofoil σ cavitation number initial cavitation number σint flow rate Qi drag coefficient C D Mt torque Mt/Mt0 ratio of torque STDEV standard deviation standard deviation of noise leq length of the profile L0 Subscripts l water phase d dispersed phase m mixture su suspension 6 REFERENCES [1] Brennen, C. (1995). Cavitation and bubble dynamics.

Oxford University Press, Oxford.

[2] Širok, B., Dular, M., Stoffel, B., Novak, M., Hočevar, M., Ludwig, G., Bachert, B. (2002). The influence of cavitation structures on the erosion of a symmetrical hydrofoil in a cavitation tunnel. Strojniški vestnik Journal of Mechanical Engineering, vol. 7, p. 368-378.

[3] Wang, D., Atlar, M., Sampson, R. (2006). An experimental investigation on cavitation, noise, and slipstream characteristics of ocean stream turbine.

University of Newcastle upon Tyne, Newcastle upon Tyne.

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[4] Xavier, E., Egusquiza, E., Farthat, M., Avellan, F., Coussirat, M. (2006). Detection of cavitation in hydraulic turbines. Mechanical Systems and Signal Processing, vol. 20, p. 983-1007, DOI:10.1016/j. ymssp.2004.08.006.

[5] Osterman, A., Dular, M., Hočevar, M., Širok, B. (2010).

Infrared thermography of cavitation thermal effects in water. Strojniški vestnik - Journal of Mechanical Engineering, vol. 56, no. 9, p. 527-534.

[6] Mei, Z.Y., Wu, Y.L. (1996). Review of research on abrasion and cavitation of silt-laden flow through hydraulic turbine runner in China. Proceedings of the 19th IAHR, Section of Hydraulic Machinery and Cavitation, p. 641-650.

[7] Shengcai, L. (2003). Cavitation enhancement in silt erosion: Obstacles & way forward, 5th International Symposium of Cavitation, Osaka.

[8] Duan, C.G., Karelin, V.Y. (2002). Abrasive erosion & corrosion of hydraulic machinery. International Research Center on Hydraulic Machinery, vol. 2, Imperial College Press, London.

[9] Shengcai, L. (2006). Cavitation enhancement of silt erosion - An envisaged micro model. Wear, vol. 260, no. 9-10, vol. 31, p. 1145-1150.

[10] Širok, B., Dular, M., Stoffel, B. (2006). Cavitation, i2, Ljubljana. (in Slovenian) [11] Keller, A.P., Rott, H.K. (1999). Scale effects on tip vortex cavitation inception. Proceedings of 3rd ASME/JSME Joint Fluids Engineering Conference, International Symposium On Cavitation Inception, San Francisco.

Gregorc, B. – Predin, A. – Fabijan, D. – Klasinc, R.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 245-254 DOI:10.5545/sv-jme.2010.217

Paper received: 2010-10-20, paper accepted: 2012-03-14 © 2012 Journal of Mechanical Engineering. All rights reserved.

Testing of Concrete Abrasion Resistance in Hydraulic Structures on the Lower Sava River Kryžanowski, A. – Mikoš, M. – Šušteršič, J. – Ukrainczyk, V. – Planinc, I. Andrej Kryžanowski1,* – Matjaž Mikoš1 – Jakob Šušteršič2 – Velimir Ukrainczyk3 – Igor Planinc1 1University

of Ljubljana, Faculty of Civil and Geodetic Engineering, Slovenia 2IRMA Institute, Slovenia 3Mostprojekt, Croatia

The paper deals with the issues of resistance of concrete linings to long-term abrasion loading caused by waterborne particles, particularly for the proposed hydro power plants on the Sava River in Slovenia. The main purpose of the research work was to define the possibility of forecasting the process of concrete lining wear on the Sava River dam structures based on the standard procedures of abrasion resistance testing. Abrasion resistance of concrete has been researched in accordance with the standard ASTM C 1138 and Böhme (DIN 52108) methods.

The research work was based on a comparison between laboratory results and measurements of abrasion resistance of concrete under natural conditions by performing test plots in the stilling basin of the Vrhovo HPP. Concrete composites with different mechanical properties have been analysed within the research programme. The analysis showed a qualitative similarity of the level of concrete abrasion between laboratory simulations and measurements in the field, as well as suitability of the ASTM C 1138 laboratory method for the assessment of abrasion resistance of concretes in the spillway of the HPP chain on the Lower Sava River.

Keywords: abrasion resistance, concrete, abrasive wear, laboratory experiments, field experiments

0 INTRODUCTION An energy exploitation project that involves 6 run-ofriver cascading HPPs is underway on the lower Sava River, for which concession has already been granted and the project is under construction at present. One of the key problems inherent to the operation of energy generation facilities on the Sava River is the erosion process on the dam structures and riparian structures, as a consequence of adverse effects of water flow. Advancing erosion processes can, under extreme circumstances, produce damages to an extent causing conditions for the emergence of uncontrolled hydrodynamic phenomena with impacts reaching outside the direct area of the dam structure. The spread of uncontrolled processes means a constant, while hidden, threat for damage to occur on downstream structures and, consequently, the potential reduction of operational safety and the rise of operational costs for remedy of problems [1].

The term abrasion in hydraulic structures is used for the process of disintegration of exposed concrete surfaces, resulting from loads arising from sediment transport [1] and [2]. The rate of disintegration of the concrete surface largely depends on the transport capacity of water and the ways of transport of solid matter [3] and [4]. When designing concretes in hydraulic structures it should be emphasised that there is no general criterion for defining abrasion resistance. Usually, abrasion resistance of concretes is assessed based on a set of parameters that define the single mechanical properties of concretes, such as: compressive strength, tensile strength, aggregate

strength, use of special cements, modulus of elasticity, water/cement (w/c) ratio, surface polishing, concrete cure, cement additives (fly ash, fibres), connected with investigating methods that more or less realistically simulate abrasive processes [5] to [10]. The problem of studying abrasion resistance of concretes arises from the inability to create proper hydraulic laboratory conditions for the fully developed abrasive action.

In general, two groups of methods are used in the research of abrasion resistance of concretes.

The first group of methods uses procedures involving uniform abrasion of the test surface with the abrasive body, which is in relative movement. These methods are most frequently used for determination of abrasion resistance of concrete surfaces (industrial floors, roads, walk able surfaces etc.). Among these methods, the standardized method utilising the Böhme (DIN 52108) principle is most comprehensively applied, where the surface of the concrete specimen is exposed to a rotating grinding disk [11]. To accelerate the wear process, an abrading agent (corundum, silicon carbide etc.) is applied to the disk or the test is performed in the presence of a water medium. A similar principle for establishing abrasion resistance of horizontal concrete surfaces is prescribed by the ASTM C 779/C-779M standard, providing three test procedures simulating the action of the abrasive medium against the surface of the concrete specimen using: (a) revolving disks; (b) dressing wheels; (c) ball bearings. The advantage of abrasive methods is that they mostly follow standard procedures, which enables good comparison between experimental results [12]. The disadvantage, however, lies in the

*Corr. Author’s Address: University of Ljubljana, Faculty of Civil and Geodetic Engineering, Hajdrihova 28, 1000 Ljubljana, Slovenia, andrej.kryzanowski@fgg.uni-lj.si

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fact that they do not simulate the actual conditions of abrasion in hydraulic structures, thus representing a limitation to their application [1] and [13]. In order to accelerate the wear process a method with high water pressure jet was developed [2]. The above mentioned method simulates cavitation or impact erosion in similar manner as in [14] and [15].

The second group of methods that enable the modelling of tribology mechanisms of the water current with bed load come closest to the conditions present in the natural environment [1] and [13]. In order to test the abrasion resistance of concretes several methods were developed illustrating the action of the water current carrying bed load: Šetina [16] has studied abrasion resistance of concrete with water current and an abrasive device (siliceous sand) in a circular flume Liu [17] has reported on the development of a test method, where the concrete test specimen is in a cylindrical container exposed to the abrasive action of steel balls. The method was standardized by the procedure prescribed by ASTM C 1138 [18]. Bania [19] has reported on the development and studies of a test method, which is composed of a fixed steel drum, which is partly filled with a mixture of water and aggregate, and a rotating drive shaft with fixed tested concrete samples.

Common to the methods investigating abrasion resistance is that they provide only qualitative comparisons between the tested specimens, based on a proportional loss of mass or input of the abrasive medium during the investigation. The validation of results and applicability of the methods for forecasting the behaviour of concretes in natural conditions can only be achieved by performing the test under the conditions similar to those in the actual operation environment of the designed structure, including the monitoring of all relevant hydraulic and hydrological parameters [5] and [20]. Testing abrasion resistance of specimens under natural circumstances is highly difficult due to the specific conditions of execution mentioned and the related costs. In the literature, there has been a single case documented, which is that of the Runcahez site in Switzerland, where the abrasion resistance of concretes was compared following the method proposed by Bania, using the measurements in the test fields of the main outlet of the dam. In the conclusion of the study, a correlation of the results of measurements in the nature with those in the laboratory was established [13]. When studying the adequacy of abrasion-resistant concretes in building the HPP chain on the Sava River a similar investigation was made, using test fields in the Vrhovo HPP spillway, while the correspondence of the results was checked against the 246

ASTM C 1138 and Böhme test methods. One of the key goals of this study was to establish the possibility of forecasting the development of concrete wear on the Sava River dam structures based on the results of testing abrasion resistance using the standard procedures [1].

1 PROBLEM DOMAIN The Sava river has a typical torrential character with considerable hydrological extremes that are reflected in the fluctuation of discharge (from 40 to 3,000 m3/s) with highly marked bed-load discharge [21]: the usual annual bed-load discharge is 66,000 m3 per year (mostly limestone), with extremes exceeding 260,000 m3 per year and suspended load discharge reaching up to 100,000 m3 per year. Based on sieve analyses and 20-year observations of sediment transport, the arithmetic mean grain size diameter is dm = 30.9 mm and at 90% passing value d90 = 72.5 mm. Based on the calculated transport capacity of the Sava River after the construction of the Vrhovo HPP, as the first hydro power plant in the chain, it became evident that the majority of the sediment would be retained in the reservoir. The washing of the sediment from the reservoir would become possible only when the Sava river flow would exceed 630 m3/s. The transport capacity of the river rises proportionally to its flow and reaches natural conditions, prior to the damming, at 2,500 m3/s, representing a 20-year return period.

Based on the monitoring of abrasive action in the existing dam structures it can be established that sediment loaded discharge represents a latent danger when trying to ensure operation security of objects and functionality of evacuation structures [22].

A typical dam structure on the lower Sava River is composed of a powerhouse and 5 spillways, transversal to the river flow. Each spillway is equipped with a segmental gate with a flap. To the level of the installed discharge Q = 500 m3/s HPP operates in the run-of-river flow regime, and the evacuation of flood water is through flap gates (up to 365 m3/s on Vrhovo HPP) or by outflow under the segmental gate. Under normal operating conditions, based on the calculated transport capacity of the Sava river on the evacuation structures of the dams, the flow of sediment in the form of suspension or bed load with a grain diameter of d = 1 mm or less is expected. During extreme flow of the Sava River when the discharge exceeds 1,200 m3/s, the natural flow regime is established through the underspill below the gates and de-levelling of the water level. The natural flow regime through the dam section is achieved by full opening of the

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gates, which is achieved at 2,500 m3/s. Under such conditions the transport of coarse grains is expected.

Because of such operation regime, the exposed parts of spillways, where abrasive erosion is to be expected, are covered with abrasion-resistant concrete lining [5] and [22]. In designing abrasion resistant concretes it was necessary to solve a number of questions related to the proper concrete composition to be able to determine the appropriate research methods, the application in the field and, last but not least, to check the material resistance during the exploitation. As part of intervention/remedial measures on the spillways of the Vrhovo HPP the possibility of performing a research project was provided, studying the adequacy of technologies producing abrasion resistant concrete linings and the methods for testing abrasion resistance of materials. The research project enabled the introduction of test plots in natural conditions trying to realize, among others, the following goals [1]: (i) To enable quantification of results of the laboratory method investigating abrasion resistance compared to measurements in the natural environment. (ii) To enable forecasting of behaviour of material during exploitation.

2 ABRASION EROSION The general tribological structure of wear of concrete surfaces in hydraulic structures can be described as follows: (i) the basic body being motionless, is the concrete surface of the construction, and (ii) the body acting upon the concrete surface, being in motion, is represented by the water current running past the concrete surface. The term abrasion erosion in hydraulic structures is used to describe the wear of the solid surface being the result of action of the water current past the surface with associated hydrodynamic phenomena and solid particles (sediment) transported with the water current. The emergence of abrasive erosion on concrete surfaces of hydraulic structures can be caused by action of deterioration processes (abrasion) due to: â&#x20AC;˘ friction forces during sliding and rolling of sediment grains transported by the water current in contact with the concrete surfaces, â&#x20AC;˘ impact forces of sediment grains transported by the water current upon impact with the concrete surface and pressure pulsations in the surrounding water body.

Accordingly, the abrasion erosion of concrete can be divided into several phases. In the initial phase, the process of abrasion is caused by sediment transport.

The damage to concrete structures thus results from polishing/milling due to rolling or sliding of sediments (solid particles) against the surface. By increased transport capacity, the small particles start to move in suspension, and large solid particles move by way of bouncing. In this phase, the abrasion process depends on bed load transport or suspended matter. In addition to the grinding action against concrete surfaces, damage due to impacts of solid particles against the surface may be observed. By increased transport capacity the size and quantity of bouncing particles increase significantly, and, simultaneously, pressure pulsations in the water increase. This contributes to the intensity of the abrasion process. The emerging damages on concrete surfaces are related to the increase of the size of solid particles and the intensity of impacts against the bottom, where at first initial damages occur, which represent, with progressing processes, the core of progressive spreading of the damage in the direction of the water current [4] and [20].

Damages to concrete surfaces are the result of hydrodynamic processes of the water flow and abrasive action caused by waterborne particles striking against the base. The impacts of the water flow in the process of failure can be expressed by shear stresses acting on the surface as a consequence of water action, normal stresses, and pressure within the body of the hydro structure. Waterborne particles act against the surface due to their characteristic movement: rotation, translational motions, saltation, or a combination of these. Abrasive action of waterborne particles causes the development of fine cracks on the surface and within the concrete structure. The cracks develop due to the exceeded limited tensile stresses in concrete.

Tensile and compressive forces accelerate the development of cracks, weaken the structure (material fatigue strength) and destroy the internal bonds in the structure, while the water flow starts to wash away the particles of cement binder and aggregate. The degradation process continues in the interior of the structure revealing visible damages. The deterioration process is even faster in turbulent waters due to the dynamic phenomena (pressure pulsation, vibrations in the structure and strokes of particulate matter against the foundation). The failure of concrete structure occurs both in the cement and in the aggregate, mostly along the interface between the cement binder and the aggregate, where repeated dynamic processes widen the initial cracks and weaken the contact. The damages caused by the abrasive water action are characterized by roughness, irregularly corroded surface with hollows and swellings in the basic structure. Abrasion

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erosion on hydro structures typically occur on exposed parts of evacuation structures, such as: spillways, outlet channels, by-pass channels, stilling basins, drainage outfalls and culverts [20].

A number of experimental methods which more or less realistically illustrate the abrasion process are used in practice. The purpose of all test methods is to obtain the relevant parameters within the real time to assess the quality of the material. Consideration is to be given to the fact that due to the complex problem solving, only a comparative evaluation of the results is possible. Therefore, it is reasonable to use such an experimental method that will give the real data in a short period of time and enable comparison of the results with the results obtained from other sources at the same time. When choosing an adequate method the following starting points were considered: • The method of testing abrasion resistance must illustrate the expected conditions of transport of bed load and suspension along the evacuation structures on the hydro structures of the Sava river as faithfully as possible.

• The testing procedure of abrasion resistance must be standardised, thus providing the repeatability of the test and the possibility of comparison of the results of measurements with those from the literature.

• The performance of the test must be economical – the purchase of testing equipment and the duration of the test are of significance, which must be comparable to the standard methods for testing the quality of hardened concrete.

• To ensure the comparability of results of measurements those methods testing abrasion resistance that have already been used for the demonstration of quality of building the Lower Sava river HPPs need to be included.

These starting points provided the grounds for choosing two standard methods for testing abrasion resistance in our research.

2.1 The Grinding Method after Böhme The test of resistance of concrete specimens to grinding was performed following the procedure prescribed by standard DIN 52108. The test was performed on standard specimens (cubes with 7.1cm sides) at 90-day age, using three samples per composition. The specimen was clamped and fixed to the mounting holding it against a rotating grinding disk that provided a uniform grinding of its surface.

The specimen was weighed before the test and then each time after a set of 4 cycles (one cycle consisted of 248

22 revolutions of the grinding disk) at 0.1 g accuracy.

An abrading agent (corundum, quartz sand etc.) can be applied, in dry or wet state, thus accelerating the rate of deterioration. The prevailing deterioration process is abrasion through grinding; typically occurring in roads, walk able surfaces etc. The result of the test is the wear of concrete after 16 cycles, which is determined by the loss of volume of the specimen in cm3 in an area of 50 cm2.

Indeed, the method does not simulate the real conditions of the abrasion process on hydraulic structures, but its applicability lies in the fact that this is a standardised procedure that is generally used in the research of abrasion resistance. Until recently, the demonstration of abrasion resistance to grinding after Böhme in Slovenia had been the only standardised method of testing abrasion resistance and therefore the only possible qualitative comparison with the results of measurements of abrasion resistance performed in the past. Based on the comparative analysis the quality class of the tested samples can be predicted and therewith the starting points for qualitative comparisons of results of abrasion resistance of concretes with other chosen test methods.

2.2 Abrasive Erosion Method after ASTM C1138 The abrasion resistance test was performed in accordance with standard ASTM C 1138 method at 90- and 900-day ages, on cylinders of Ø30/10 cm, taking one sample per composition. The test method is designed to duplicate the abrasive action of waterborne particles in the stilling basins. Circulating water moves the steel grinding balls on the surface of a concrete sample, producing the desired abrasion effects. The water velocity and agitation effect are not sufficient to lift the steel balls off the surface of the concrete sample to cause any significant impact action against the surface. The test method can only be used to determine the relative resistance of the material to the abrasion action of waterborne particles.

The standard procedure of the investigation provides for the measurement of the wear of specimen surface at 12-hour intervals; the total investigation time is 72 hours. The result of the test is the average depth of wear expressed by the average volume of wear on the surface of the specimen in the duration of the test.

Of all the methods used, this method provides the most adequate illustration of the real conditions during the transport of bed load along the concrete surfaces. The applicability of the method is that it is standardised, making it possible to provide a qualitative comparison of the results of measurements

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of previous studies and the data from the literature. In Slovenia, the method was first used for experimental purposes, simultaneously to the start of building of the Vrhovo HPP. This is why there are relatively large data sets on the measurements performed, enabling relatively good qualitative comparisons of the adequacy of concrete compositions in relation to achieving abrasion resistance. The disadvantage of the method is the prolonged duration of the test, and in addition to the limited equipment available, this is restricting the potential large-scale investigations.

Therefore, the preparation of specimens is adapted to the frame available for the performance of the test, thereby optimising the scope and duration of testing. The advantage of the method, however, is that the procedure is selective enough to provide representative results for qualitative assessment of abrasion resistance.

3 MEASUREMENTS IN NATURAL CONDITIONS In 2001, a total of 9 test plots, dimensions of 2.5/2.5 m, thickness of 0.1 m at a distance of 1.0 m (Fig. 1), were built at the bottom of the Vrhovo HPP spilling basin.

Out of these, 6 fields were equipped with concretes of laboratory compositions, and the rest were fitted with commercial high-strength concretes. Concrete mixtures were prepared by using Portland cement with 15% slag, gravel aggregate (mostly limestone), grading 0 to 8 (16) mm, and super plasticizer.

Six samples of different concrete composition were intended for test purposes (Table 1): The C1 composition is adopted as control composition, which is basically the same as the composition of abrasion resistant concrete built in the spillways of the Vrhovo

HPP. In C2 composition and all further modifications the nominal maximum gravel of 8mm was adopted.

The C2 composite with smaller modifications was used with concretes on the spillway of the Boštanj HPP. With the PMC1 composition, representing the initial composition for all further modifications, the mineral additive and super plasticizer were replaced by polymeric binder: (i) in the PMC2 composition the proportion of the finest fraction (0 to 4 mm) was replaced by rubber aggregate and the proportion of polypropylene fibres was doubled; (ii) in the PMC3 composition the polypropylene monofilament fibres were added; (iii) in the PMC4 composition the proportion of the polymeric binder was halved. The value of the w/c ratio in the composites did not vary considerably.

The properties of the hardened concrete were proven with the standard investigation methods. The average values of investigation results of the hardened concrete are given in Table 2: (i) Compressive strength and density were performed at the ages of 3, 7, 28 and 90 days, respectively, on cubicles of dimensions of 15 cm, by taking three samples of each composition; (ii) The static modulus of elasticity of the concrete was defined at the 90-day age on prisms of 10/10/40 cm, taking one sample per each composition; (iii) Abrasive resistance test was performed at 90- and 900-day ages, on cylinders of Ø30/10 cm, taking one sample per composition.

The concrete composites were transported to the construction site in the form of prefabricated dry mixtures. The preparation of concretes of the test plot was performed following the same procedure as for laboratory concretes. The concretes were built in manually, using a vibration pin, and afterwards

Table 1. Overview of concrete compositions (for 1 m3 of concrete) CEMENT W/C SUPERPLASTICIZER MINERAL SUPPLEMENT: SiO2>90%; Al2O3 0.5~1%; CaO 0.5~1%; Na2O 0.5~1% POLYMER - DRY PORTION STEEL FIBERS: L=16 mm, Ø 0.5 mm PP FIBERS: L=10 mm, Ø 30~40 μm PP MONOFILAMENT FIBERS: L=30 mm, Ø 0.5 mm 0 to 4 mm GRAVEL AGGREGATE 4 to 8 mm 8 to 16 mm RUBBER AGGREGATE 0 to 4 mm

% mass of cem.

C1 440 0.391 0.9

C2 450 0.414 0.9

% mass of cem.

5

5

[kg/m3]

% mass of cem.

PMC1 450 0.416

PMC2 450 0.416

PMC3 450 0.416

PMC4 450 0.412

10

10

10

5

0.05 0.60 70 30

0.05

% of vol.

0.5

0.5

0.5

% of vol. % of vol.

0.05

0.05

0.05

0.10

48 15 37

69 31

70 30

56.9 33.6

% of vol. % of vol.

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69.4 30.6

9.5

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they were manually finished. In the next 14 days an intensive wet curing with additional covering of the test plots with PVC foil was carried out. Before the overtopping of the stilling basin a levelling of all irregularities in contact points with concrete base was conducted together with a surveying campaign measuring the surface area of test plots with an accuracy ±10-4 m, and elevation was determined based on the existing bench-mark network on the dam. In order to exclude the influence of boundaries (dimension of 0.5 m), the campaign was performed only in the central part of test plots (dimensions of 1.5/1.5 m), in a raster of 30×30 cm, and in a total of 36 measuring stations. After the surveying campaign, the test plots in the stilling basin were filled with water and operation of the spillway was blocked until the required 90-day age of the test concrete.

Fig. 1. Location of test plots and the control profiles of base concrete in the stilling basin of the Vrhovo HPP spillway

Table 2. Results of investigations of hardened concrete

C1 C2 PMC1 PMC2 PMC3 PMC4

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Compressive strength SIST EN 12390-3 [MPa] 28 days 90 days 62.33 67.17 73.09 79.17 51.12 54.05 22.45 23.81 46.90 49.06 54.79 58.4

Modulus of elasticity DIN 1048-5 [GPa] 90 days 31.17 35.43 26.43 16.37 22.36 25.75

Wear ASTM C 1138 [mm] 90 days 900 days 1.79 0.98 1.64 1.16 2.09 2.42 0.61 0.60 1.79 1.92 2.81 1.84

The program of monitoring operational characteristics in the test plot plots commenced with setting-up of the spillway in function in February 2002, and ended in August 2004. The records on operational manoeuvres, discharges over spillway and on the Sava river flow were held based on hourly records from the operational journal of the hydroelectric power plant. The transport of sediments in the dam cross-section was not directly monitored, but an assessment was performed based on the known discharges of the Sava river during the operational manoeuvres and sediment discharge curve in the dam cross-section, which was obtained from years of measuring turbidity and sediment transport in the Vrhovo storage reservoir [21]. During the entire investigation period we registered a total of 804 operation hours with the operation of the spillway, while the condition for sediment transport was met in a total of 297 operation hours. During the time, the transport of over 1,937 t of sediments was recorded in the spillway area at a total discharge of 204.73 hm3 of water, which was on average ~0.01% of mass flow of water and solid particles over the spillway.

The geodetic surveying of wear of test plot surfaces was performed in measuring stations that were height-referenced to the bench marks on the dam, following the same procedure and using the same equipment as in the campaign performed after the introduction of test plots. Special attention was required for the implementation and interpretation of the measurements. The average of the wear depth of the test plots varied from 2×10-4 m in the test plot C2 to 8×10-4 m in the test plot PMC4. The measuring equipment enabled the measuring precision of ±10-4 m. For the interpretation of the measurements we therefore excluded all the stations where no changes had been registered or where extra values compared to the starting point had been registered, which gave rise to the doubt about the credibility of the measurement.

Based on the conclusions of the modelling studies it was assumed that, due to the changing velocity of the water current, the wear of concrete surfaces in the stilling basin cannot be uniform [22].

The estimate of the scale of the wear of the existing base concrete in the stilling basin was performed based on the measurements of the size of wear of representative grains of the aggregate d = 64 mm of the existing base concrete, that is, in the belt between the pillar and the test plots in a total of 21 control profiles. In each control profile we measured the diameter of representative grains on the surface of 500 cm2 with raster grid of 2 cm. It was found that the characteristic diameter of the largest grain in the

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profile of the plots (K1, K2, K3) was 5 mm, in the profile of plots (C1, C2, PMC1) 9 mm and the profile of plots (PMC2, PMC3, PMC4) 11 mm (Figs. 1 and 2). The measurements have shown that the wear of the base concrete increases linearly downstream, that is, from the assumed starting point right under the chute. The wear in the central line of the plots (C1, C2, PMC1) increases by a factor of 3.3 as to the initial value, and in the central line of the plots (PMC2, PMC3, PMC4) by a factor of 5.7, respectively. In processing of results, the values obtained for test plots were multiplied with the corrective factors, depending on the location, so that the measurement results were converted to a common denominator.

Fig. 2. The level of wear of the existing base concrete in the control profiles, in a distance from the chute of: (I) 1 m; (II) 7 m; (III) 13 m

depth of wear [mm]

3.00

ASTM C 1138 - 90 days Boehme - 90 days ASTM C 1138 - 900 days field

2.50 2.00 1.50 1.00 0.50 0.00

C1

C2

PMC1

PMC2

PMC3

PMC4

Fig. 3. Comparison of wear between concrete compositions after ASTM C 1138 and Böhme and measurements in the test plots

Fig. 3 shows the measurement results of abrasion resistance following the procedures after the Böhme method at the specimen ages of 90 days and the ASTM C 1138 method at the specimen ages of 90 and 900 days, respectively, and the corrected results of abrasion resistance in the test plots, after

approximately 2.5 years of operation. The following findings were made: • According to the depth of wear obtained after both standard test procedures and through comparison of the results of measuring abrasion resistance from the literature, it can be established that all the investigated concrete compositions can be classified as concretes highly resistant to the abrasive action of the water current, which also provides a validation of the proper composition for abrasion resistant concretes on the Lower Sava River dams.

• It can be drawn from the comparison that there is a similarity between the wear of the test concretes subject to laboratory conditions and those in the natural environment.

• According to the control C1 composition, a considerable improvement of resistance to wear in composition with the added rubber aggregate PMC2 was achieved.

• It became evident that abrasion resistance changes with the age of specimens: at the 900day age, the achieved resistance in the PMC2 composition was still higher than the control composition; however, the improvement of resistance with age with the C1 composition is considerably higher than in PMC2 composition.

The abrasion resistance increased with age also in the C2 and PMC4 compositions, but it decreased in compositions with polymeric binders (PMC1 and PMC3); in all compositions it remained smaller than the control composition.

In the research work an influence of various parameters on resistance of concrete was studied in detail (e.g. mechanical properties of concrete such as: compressive strength, tensile-flexural strength, split, etc.). It was shown that the usefulness of abrasion resistance methods was independent of parameters considered above. As a result, these conclusions are not explicitly included in this paper however they are presented in reference [1].

4 ANALYSIS OF RESULTS The comparison of results between the laboratory measurements and measurements in test plots showed the similarity between the wear samples of both investigations. The findings were checked with the regression analysis where the measurements on the test fields were analysed with laboratory measurements.

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4.1 The Böhme Method

4.3 Prediction of the Concrete Wear Process

When analysing the results obtained after the Böhme method the correlation with the measurements in the field (R2 = 0.15) could not be confirmed; the comparison of results of composition of concretes with polymeric binder gave a better correspondence of results (R2 = 0.93), which is probably due to the similar mechanical properties of the concretes. Also, the coherence of the results obtained by the Böhme and ASTM procedures could not be confirmed (R2 = 0.37). Similar findings are reported by other researchers [2] and [7].

When testing abrasion resistance of concretes on the spillways on the Lower Sava HPPs, using the Böhme procedure, the applicability of the testing was revealed, however, the comparability of measurements in the laboratory and in the field was valid only for those compositions of concrete that had similar mechanical properties. Regarding further studies of abrasive resistance of concretes on the structures on the Sava River, the testing after Böhme should remain included, while it would be recommended to include the results of wear after single duration cycles in order to recognize the potential influence of testing duration to the correlation of the results with the measurements in natural conditions.

In the final part we analysed the possibility of predicting the processes of concrete wear in the test fields on the basis of operational data and the estimated bed load through the spillway, by taking into consideration the results of testing abrasion resistance using the ASTM C 1138 method. To make an estimate of the prediction, the following data on the spillway operation were available: operation duration with bed load discharge and the volume of sediment being transported through the spillway during the time of investigation. Based on the time of operation and the sediment volume the mass discharge was determined, i.e. the sediment volume passing through the spillway in a unit of time.

In the calculation it was assumed that sediment travels with the speed of the water current in the stilling basin, which was determined based on the model research, on average being around 20 m/s. It was also considered that the concentration of sediment in the water current is the same along the width of the flow and that the uniformly distributed sediments are in translatory motion with the water current along the concrete surface in the stilling basin. The specific energy is the released kinetic energy of the moving mass of sediment on the concrete surface in the stilling basin of the spillway. A similar estimate of the kinetic energy released was made for the test cylindrical container following the ASTM C 1138 method, where the balls move on the bottom with an average speed of 1.8 m/s. The relationship of the specific energy of the moving sediment in the test fields and the moving balls in the test container is approximately 1:10 (Table 3).

Based on the relationship between the released specific energy we can make a rough estimate that 297 operational hours in the test fields in the time of the research correspond to approximately 30 hours of duration of investigation of abrasion resistance according to the ASTM C 1138 test method. The estimate corresponds well with the analysis of the results of laboratory tests and measurements in test fields, where the best correspondence with the measurements in natural conditions was obtained after 24 hours (at the 90-day age of specimens) and after 36-hour duration of the laboratory experiment (at the 900-day age of specimens), respectively. Considering the fact that the age of concretes in the test fields during the duration of experiment was between 90 and 900 days, the wear after 30 hours of the test under laboratory conditions is the exact value to be expected after performing the testing after ASTM C1138 at a

4.2 ASTM Results The comparisons with the results of laboratory measurements in the entire duration of investigation of 72 hours and measurements in test plots have shown that based on the coefficient of correlation, at the 90-day age (R2 = 0.37), it is not possible to confirm the dependence between the measurements; at the 900-day age of specimens (R2 = 0.83) a fairly good correlation between the measurements is confirmed. By taking into consideration the results of laboratory measurements of wear in the single cycles of the duration of the investigation it was found for the 90-day specimens that the best correlation was that between the measurements after 24 hours into the investigation, when a rather good correlation was identified (R2 = 0.79). For the 900-day specimens the best correlation between the measurements was found after 36 hours of investigation duration, when an excellent correlation was obtained (R2 = 0.98). The comparison of the results of the investigations for the durations of investigations analysed is seen in Fig. 4, where only the deviation in the PMC1 composition is evident, while in other compositions the correlation of results is satisfactory.

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comparable age of specimens, considering the natural conditions. Accordingly, the expected values of wear of specimens of comparable ages with those in the natural conditions are within the values corresponding to the wear after 30 hours of testing with the upper boundary at the 90-day age of specimens and the lower boundary at the 900-day age of specimens (Fig. 4).

Table 3. Estimation of mass discharge and released kinetic energy Test fields ASTM C 1138

Mass flow of particles [kg/s] Specific energy [J/m2] 3.13 2.08 1.03 23.5

Fig. 4. Comparison of wear between the ASTM C 1138 results, at 90-day age (after 24 and 72-h duration of investigation) with measurements in the test plots

In order to confirm the estimate provided above it would be necessary to repeat the measurements of wear in the test fields, analyse the operational characteristics and monitor the hydrologic parameters during the execution as part of regular operational monitoring. For a more precise quantification of sediments it would be necessary to set up permanent monitoring of concentration of suspension in the reservoir and downstream of the dam and perform continuous measurements of movement of gravel zones in the reservoir. The continuous monitoring of sediment would help to narrowly define the volume of sediment transport and the way of transport through the dam - in the form of bed load or in suspension.

The setting up of a permanent monitoring of dynamics of sediment transport in the reservoir area is important from the aspect of flood safety (clogging of the reservoir) as well as providing the possibility of a more accurate estimation of abrasive action in the spillways of energy generation facilities on the Sava River.

If the repeated measurements would once again show the correlation between the mass discharge of sediment in the time of operation of spillways on the hydropower plant and the mass discharge in the test container used in the ASTM C 1138 test method under laboratory conditions, this would additionally confirm the adequacy of selecting the method of testing abrasion resistance using the ASTM C 1138 method for prediction of abrasion processes of concrete surfaces on evacuation structures of the Lower Sava River hydropower plants.

5 CONCLUSIONS In the paper we presented the suitability of the laboratory methods for the assessment of abrasion resistance of concretes in hydraulic structures on the Lower Sava River by performing a comparison between laboratory measurements and measurements in the natural environment. The following has been established: • Good correlation between the results of investigation of abrasive resistance according to the ASTM C 1138 procedure and measurements in the natural environment for concretes of 900day age.

• The testing of abrasion resistance after the Böhme procedure revealed the applicability of the test, taking into account certain limitations, since the comparability of the results of abrasive resistance is possible only with those concretes that have similar mechanical properties. Owing to the ease-of-use and short duration of the test, it is recommended that the test remains to be part of further research of abrasion resistance of concretes on the Lower Sava River structures, while, however, it might be worth considering, including the results of single cycles of testing duration, as is the case with the ASTM method.

• In the analysis of abrasion resistance according to the ASTM C 1138 procedure and interpretation of measurement results, important parameters are the duration of investigation and the age of the specimens during the investigation.

• Suitability of the ASTM C 1138 laboratory method for the assessment of abrasion resistance of concretes in spillway of the HPP chain on the Lower Sava River.

• The possibility of forecasting the dynamics of concrete wear on dam structures on the Lower Sava River has been indicated based on the ASTM C 1138 test. The thesis should be supported by an extensive programme of monitoring the dynamics

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of wear of concrete surfaces in dam structures, operational characteristics and hydrologic parameters.

6 REFERENCES [1] Kryžanowski, A. (2009). Abrasion Resistance of Concrete on Hydraulic. Structures Ph.D. thesis.

University of Ljubljana, Ljubljana.

[2] Kryžanowski, A. (1991). Analyses of concrete resistance to deterioration due to water action.

M.Sc. thesis. University of Ljubljana, Ljubljana. (in Slovenian) [3] Mikoš, M. (1993). Fluvial abrasion of gravel sediments., Acta Hydrotechnica, vol. 11, no. 10, University of Ljubljana.

[4] Kim, J.K. (2004). Controls over bedrock channel incision. Ph.D. thesis, University of Glasgow, Glasgow.

[5] Kryžanowski, A., Mikoš, M., Šušteršič, J., Planinc, I. (2009). Abrasion Resistance of Concrete in Hydraulic Structures. American Concrete Institute (ACI) Material Journal, vol. 106, no. 4, p. 349-356.

[6] Horszczaruk, H. (2004). The model of abrasive wear of concrete in hydraulic structures. Wear, no. 256, p. 787796, DOI:10.1016/S0043-1648(03)00525-8.

[7] Horszczaruk, H. (2005). Abrasion resistance of highstrength concrete in hydraulic structures. Wear, no. 259, p. 62-69, DOI:10.1016/j.wear.2005.02.079.

[8] Horszczaruk, H. (2008). Mathematical model of abrasive wear of high performance concrete. Wear, no.

264, p. 113-118, DOI:10.1016/j.wear.2006.12.008.

[9] Horszczaruk, H. (2009). Hydro-abrasive erosion of high performance fiber-reinforced concrete. Wear, no.

267, p. 110-115, DOI:10.1016/j.wear.2008.11.010.

[10] Hu, X.G., Momber, A.W., Yin, Y., Wang, H., Cui, D.M. (2004). High-speed hydrodynamic wear of steel-fibre reinforced hydraulic concrete. Wear, no. 257, p. 441450, DOI:10.1016/j.wear.2004.01.019.

[11] DIN 52108. (2010). Wear test using the grinding wheel according to Böhme.

[12] ASTM C 779/C-779M. (2005). Standard test method for abrasion resistance of horizontal concrete surfaces.

[13] Jakobs, F., Winkler, K., Hunkeler, F., Volkart, P. (2001). Betonabrasion im Wasserbau, Miteilungen der Versuchsanstalt für Wasserbau, Hydrologie und

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Glaziologie, no, 168. Eidgenössische Technische Hochschule (ETH), Zürich.

[14] Aquaro, D. (2010) Impact of solid particulate on brittle materials. Strojniški vestnik - Journal of Mechanical Engineering, vol. 56 no. 4, p. 275-283.

[15] Borkowski, P. (2010). a novel technique for spatial objects shaping with a high-pressure abrasive water jet.

Strojniški vestnik - Journal of Mechanical Engineering, vol. 56 no. 5, p. 287-294.

[16] Šetina, B. (1969). Technical report: Determination abrasion resistance of various materials exposed to abrasion erosion. Hydraulic Laboratory, Ljubljana. (in Slovenian) [17] Liu, T.C. (1981). Abrasion Resistance of Concrete. ACI Journal, vol. 78, no. 5, p. 341-350.

[18] ASTM C 1138M.-05 (2006). Standard Test Method for Abrasion Resistance of Concrete (Underwater Method).

Annual Book of ASTM Standards, vol. 04.02, American Society for Testing and Materials, West Conshohocken, p. 621-624.

[19] Bania, A. (1989). Bestimmung des Abriebs und der Erosion von Betonen mittels eines GesteinsstoffWassergemisches, Ph.D. thesis, TH Wismar.

[20] Kryžanowski, A., Šušteršič, J. (2003). Performance of concrete exposed to long-term underwater abrasion loading, 21st Congress of International Commission on Large Dams ICOLD Montreal, Proceedings, Q.82-R.13, p. 207-218.

[21] Colarič, O. (1985). Technical report No. 831: Estimation of river bed and water level rise in the Vrhovo storage reservoir. Hydraulic Laboratory, Ljubljana. (in Slovenian) [22] Šušteršič, J., Kryžanowski, A., Planinc, I., Zajc, A., Dobnikar, V., Leskovar, I., Ercegovič, R. (2004).

Technical report: Performance of concrete exposed to underwater abrasion loading. Institute for research in materials and applications , Ljubljana. (in Slovenian): [21] Mlačnik, J. (1999). Technical report No. 806: Measurement of turbidity and sediment transport in the Vrhovo storage reservoir. Institute for hydraulic research, Ljubljana. (in Slovenian) [22] Mlačnik, J. (2004). Technical report No. 857: Hydraulic model research of Boštanj HPP. Institute for hydraulic research, Ljubljana. (in Slovenian)

Kryžanowski, A. – Mikoš, M. – Šušteršič, J. – Ukrainczyk, V. – Planinc, I.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 255-262 DOI:10.5545/sv-jme.2010.216

Paper received: 2010-10-20, paper accepted: 2012-03-16 © 2012 Journal of Mechanical Engineering. All rights reserved.

Two-Dimensional Mathematical Modelling of a Dam-Break Wave in a Narrow Steep Stream

Krzyk, M. – Klasinc, R. – Četina, M. Mario Krzyk1 – Roman Klasinc2 – Matjaž Četina3 1,3 University of Ljubljana, Faculty of Civil and Geodetic Engineering, Slovenia 2 Graz University of Technology, Institute of Hydraulic Eng. and Water Resources Management, Austria The paper deals with hydraulic aspects of a wave, emerging as a result of a potential dam break of the upper storage reservoir of the pumpedstorage hydropower plant Kolarjev vrh. A two-dimensional depth-averaged mathematical approach was used. The upper storage reservoir and its dam failure were modelled with the mathematical model PCFLOW2D, which is based on the Cartesian coordinate numerical mesh.

The results of PCFLOW2D were used as the upper boundary condition for the mathematical model PCFLOW2D-ORTHOCURVE, based on the orthogonal curvilinear numerical mesh. The model PCFLOW2D-ORTHOCURVE provided a tool for the analysis of flood wave flow in a steep, narrow and geometrically diversified stream channel. The classic Manning’s equation fails to give good results for streams with steep bed slopes and therefore, a different equation should be used. The application of the Rickenmann’s equation was chosen, presented in a form similar to Manning’s equation. For the purpose of the example given here, the equation was somewhat simplified and adapted to the data available. The roughness coefficient used at each calculation cell depended on the slope of that cell. The results of numerical calculations were compared to measurements carried out on a physical model in the scale of 1 : 200. Regarding the complexity of the flow phenomenon a rather good correlation of maximum depth was established: only at one gauge the difference in water depth was up to 27% while at the other four it was 7% of water depth on average.

Keywords: dam-break wave, steep curved channels, two-dimensional mathematical model, orthogonal curvilinear coordinates, roughness coefficient, model PCFLOW2D-ORTHOCURVE

0 INTRODUCTION Mathematical modelling has taken a leading role in solving certain practical problems and is used more frequently than physical models. Nevertheless, physical modelling remains irreplaceable for solving many problems. One of these is establishing hydrodynamic flow characteristics with complex geometry features, where capturing all geometrical details is too difficult for mathematical modelling.

Moreover, many problems can only be solved with a simultaneous use of both, physical and mathematical models. Such is also the case of a potential dam failure at the Kolar’s peak (Kolarjev vrh, NE Slovenia), where measurement results of unsteady flow on the physical model provided the basis for verifying the mathematical model PCFLOW2D-ORTHOCURVE. As the upper storage reservoir of pumped hydro power plant (HPP) Kolarjev vrh is planned as artificial accumulation built by dykes, the reason of dyke damaging could also be uncontrolled water hammer [1] and [2].

The planned installed power of the pumpedstorage HPP Kolarjev vrh is 300 MW, with the height difference of up to 714.7 m, depending on the water levels in both storage reservoirs. The installed turbine flow would be 51 m3/s and the pumps would have a discharge of 37 m3/s. The upper storage reservoir would be connected to the powerhouse with a pressure

penstock 2660 m long and 3.0 to 3.5 m in diameter. As the lower storage reservoir of the pumped-storage HPP Kolarjev vrh, the existing storage reservoir of HPP Fala on the Drava River would be used. At the Water Management Institute in Ljubljana, a physical model of the upper storage reservoir was made, together with the valley lying to the south in the scale of 1 : 200 [3]. The view of the storage reservoir and modelled valley (upper narrow part) with measurement gauges is shown in Fig.1.

The upper storage reservoir would be located on the flattened Kolarjev vrh. The level of the reservoir bottom would be at 975.0 m a. s. l., with maximum dimensions of 350×750 m. The storage reservoir would comprise an area of 160,000 m2. The bank slope on the inner side of the storage reservoir would be approximately 1 : 2, and on the outer side 1 : 1.5.

The crest elevation would be 996.5 m a. s. l., 1.5 m above the highest water level in the upper reservoir (995.0 m a. s. l.) and would have a width of 5 m. The water volume in the reservoir would be approximately 2.8×106 m3. An axonometric view of the upper storage reservoir that was built in the physical model is presented in Fig. 2, together with the position of gauges that recorded water level oscillations. The measurement results at these gauges were used for the verification of the mathematical model PCFLOW2D, which was used for hydrodynamic modelling of flow in the upper reservoir.

*Corr. Author’s Address: University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, SI-1000 Ljubljana, mkrzyk@fgg.uni-lj.si

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Selnica lying approximately 700 m below is 6 km, of which 4500 m are represented by a fairly narrow valley that passes into the wide valley of the Drava River. To verify the results of the mathematical model PCFLOW2D-ORTHOCURVE, data relating to the narrow part of the valley were used.

Fig. 2. Axonometric view of the upper storage reservoir with positions of measuring gauges

Fig. 1. Photo of physical model of south valley with installed gauges

A topographic analysis of terrain below the upper storage reservoir of HPP Kolarjev vrh has shown that during a dam break of the upper storage reservoir, the potential discharge spillways are the three valleys gravitating to the Drava River. Since the physical model was made only to facilitate the verification of results of the mathematical model, only the shortest, south valley of the Logar’s Creek (Logarjev potok) was built. The part of the dam gravitating to the valley is approximately 1 km long. The length of the waterway spilling from the reservoir to the town of

In the physical model, the analysis of the dambreak wave caused by an instantaneous break of part of the dam was performed. Several possible widths of a dam breach were considered, namely 48, 99, 237 and 433 m. The length of the flow under study was 4620 m, and the head difference between the reservoir bottom (975 m a. s. l.) and the more flat part of the valley was 646 m. The average channel slope was 14%. In order to monitor the motion of the wave and its characteristics on the physical model, six gauges were set up in the upper part of the valley and six in the lower part of the valley. For the purpose of this study, only the results of the first five gauges are of interest, because other gauges were positioned outside the area covered by the mathematical model.

The measurements gave the propagation time of the wave front to each gauge and water depth oscillation

Fig. 3. Longitudinal profile of the narrow part of the south valley with the position of gauges

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at each gauge. The position of the first five gauges is shown in the longitudinal profile of the narrow part of the valley in Fig. 3. The data on water levels indicate a longitudinally non-uniform course of the wave and significant changeability of the flow between supercritical, critical and subcritical flow. The wave reached the lower part of the valley extremely fast, in three to four minutes.

1 MATHEMATICAL MODEL OF UPPER STORAGE RESERVOIR For the analysis of a possible dam break wave arising from the upper storage reservoir of the HPP Kolarjev vrh, two mathematical models were prepared. The first covered the area of the reservoir and the other covered the dam break flow through the south valley to its widening. Hence, with the model of the storage reservoir and its dam-break, we acquired data on the dependence of discharge versus time at the dam site, which was used as the upper boundary condition for the second mathematical model of the downstream valley. Because of the shape of the reservoir, with no considerable difference between its width and length, the mathematical model PCFLOW2D was used for flow modelling. PCFLOW2D is based on the orthogonal Cartesian numerical mesh. For modelling

∂t

∂ ( hv ) ∂t

+

+

− ghn 2

( ) = − gh ∂h − gh ∂z

− ghn 2

∂x

∂ ( huv ) ∂x ∂ ( hk ) ∂t

∂t

The mathematical model PCFLOW2D uses the orthogonal Cartesian coordinate system. The continuity (Eq. (1)) and momentum equations (Eqs. (2) and (3)) describing a two-dimensional unsteady depth-averaged flow, are written in the conservative form. The last two terms on the right hand side of Eqs. (4) and (5) express the influence of turbulent viscosity, which is determined by the k - ε turbulence model. In some cases, constant values of υef can also be used.

( ) + ∂ ( huv ) = − gh ∂h − gh ∂z

∂ hu 2

∂ ( hε )

1.1 Basic Equations

∂h ∂ ( hu ) ∂ ( hv ) + = 0, + ∂t ∂x ∂y

∂ ( hu )

the flow through the valley of the Logarjev potok, with a vast disproportion between the flow length and width with several considerable bends, the mathematical model PCFLOW2D-ORTHOCURVE was used, based on the orthogonal curvilinear numerical mesh.

As mentioned above, four different widths of a dam breach were considered in the physical model.

Earlier, authors analysed the flow downstream in three consecutive bends with simplified geometry during dam breach of a width of 237 m with a twodimensional (2D) mathematical model [4]. The same width was chosen for the computations of the dam break wave with the models PCFLOW2D and PCFLOW2D-ORTHOCURVE.

+

+

∂y

+

∂x

∂ hv 2 ∂y

∂y

∂ ( huk ) ∂x

∂ ( huε ) ∂x

+

+

∂ ( hvk ) ∂y

∂ ( hvε ) ∂y

=

b

∂x

b

∂y

u u 2 + v2 h

4

3

v u 2 + v2 h

4

3

(1) +

∂  ∂u  ∂  ∂u  +  hυef hυef ,   ∂x  ∂x  ∂y  ∂y 

+

(2)

∂  ∂v  ∂  ∂v  +  hυef hυef  , (3)   ∂x  ∂x  ∂y  ∂y 

∂  υef ∂k  ∂  υef ∂k  h + h  + hG − cD hε + hPkv , ∂x  σ k ∂x  ∂y  σ k ∂y 

(4)

∂  υef ∂ε  ∂  υef ∂ε  ε ε2 h + hPεv ,  h  +  h  + c1 hG − c2 ∂x  σ ε ∂x  ∂y  σ ε ∂y  k k

(5)

=

where t is time, h is water depth, u and v are velocity components in the x and y directions, zb is the bottom level, n is the Manning’s friction coefficient, g is acceleration due to gravity, υef is the effective coefficient of viscosity, k is the turbulent kinetic energy per unit of mass, ε is the dissipation rate of the turbulent kinetic energy per unit of mass, c1, c2, σk and σe are constants of the turbulent model, and Pkv and Pεv are production and dissipative terms due to bottom roughness.

1.2 Boundary and Initial Conditions with Computational Details Boundary conditions used in the mathematical model were as follows: flow velocities through reservoir dykes were zero and at the site of the dam-break, critical flow was assumed at each cell. The sum of discharges through drainage cells represented the total outflow in the profile of the destroyed part of the

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dyke. As the initial condition in the reservoir, water level at Z = 995 m a.s.l. was assumed.

The size of the numerical mesh was Δx = Δy = 4 m and the time step was Δt = 0.5 s. On the basis of measurements conducted on the physical model of the downstream valley, it was estimated that it would be appropriate to monitor the time of wave propagation across the valley up to about 400 s after the demolition of the dyke. Therefore, maximum computation time of 420 s was chosen.

1.3 Calibration and Results of the Mathematical Model of the Flow in the Reservoir During the calibration process of the mathematical model of the flow in the upper reservoir, we tried to find the appropriate bottom friction coefficient.

For each measuring gauge presented in Fig. 2, the nearest calculation point was found and its water

surface oscillation was noted. It was then compared to measured water levels on gauges of the physical model. The period of the first 160 s after the dyke was broken was analysed. This was also the duration of measurements recorded at gauges on the physical model.

On the basis of comparison of the computed and measured water depth oscillations on all six gauges in the area of the upper reservoir, we determined the uniform Manning’s coefficient of the bottom and slopes of the dyke to be n = 0.024 s/m1/3. As an example, the results of comparison for the gauge S1 are presented in Fig. 4. In Fig. 5, the resulting outflow from the reservoir is shown. This Q – t curve was then used as the upper boundary condition for the mathematical model of the wave propagation in the downstream valley, which is discussed in Chapter 3.

Fig. 4. Measured and calculated oscillation of water depth in the upper reservoir at gauge 1 (S1)

Fig. 5. The computed Q-t curve at the outflow from the reservoir

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continuity Eq. (6) and momentum equations (Eqs. (7) and (8)) describing a two-dimensional unsteady depthaveraged flow are written in the conservative form.

2 MATHEMATICAL MODEL OF THE DOWNSTREAM VALLEY 2.1 Basic Equations The model PCFLOW2D-ORTHOCURVE uses an orthogonal curvilinear coordinate system. The

(

∂huξ

∂t

( )

+

(

∂ huξ 2 mξ ∂ξ

) + ∂ ( hu v ) + 2 hu v ξ η

mη ∂η

∂mξ

ξ η

mξ mη ∂η

u uξ 2 + vη 2 ∂zb ∂h 1 2 ξ = − gh − gh − ghn + 4 mξ ∂ξ mξ ∂ξ mξ mη h 3

∂hvη

)

∂ hvη huξ ∂mη hvη ∂mξ ∂h ∂ huξ + + + = 0, + ∂t mξ ∂ξ mη ∂η mξ mη ∂ξ mξ mη ∂η

∂t

+

(

∂ huξ vη mξ ∂ξ

 ∂   ∂ξ

) + ∂ ( hvη 2 ) + 2 huξ vη ∂mη + mη ∂η

mξ mη ∂ξ

v uξ 2 + vη 2 ∂zb ∂h 1 2 η = − gh − gh − ghn + 4 mη ∂η mη ∂η mξ mη h 3

where t is time, h is water depth, uξ and νη are the velocity components along (ξ) and normal (η) to the flow direction, mξ and mη are Lamé’s coefficients, zb is the bottom level, n is the Manning’s friction coefficient, g is acceleration due to gravity and υef is the effective coefficient of viscosity.

Since the investigated case was one involving a highly unsteady flow with rapid velocity and depth changes, we estimated that an adequate distribution of υef had no significant influence on the results. Thus, a constant value of υef = 0.01 m2/s was used, based on our previous experience with unsteady flow.

2.2 Numerical Method Both sets of coupled partial differential equations (1) to (5) and (6) to (8) are solved by the finite volume numerical scheme proposed by [5]. The main characteristics of the method are staggered control volumes, the so-called “hybrid” scheme and an iterative procedure of depth corrections (SIMPLE).

A fully implicit scheme is used for time integration providing stable and accurate solutions even at relatively high Courant numbers (up to about 10). It is possible to simulate both subcritical and supercritical flows ([5] and [6]).

(

+ uξ 2 − vη 2

 ∂   ∂ξ

) mh

η

∂mη mξ ∂ξ

 mη ∂uξ  ∂  hυef +  mξ ∂ξ  ∂η 

(v

η

2

− uξ 2

) mh

ξ

∂mξ mη ∂η

 mη ∂vη  ∂  hυef +  mξ ∂ξ  ∂η 

(6)

=  mξ ∂uξ  hυef  mη ∂η 

  ,  

(7)

=  mξ ∂vη  hυef  mη ∂η 

  ,  

(8)

The “hybrid” scheme is a combination of the upwind and central difference scheme (application of the scheme depends of the value of the cell Peclet’s number). The first order upwind scheme assures simplicity and robustness [5] and it remains stable even with very complex geometry, relatively coarse numerical grid and complicated boundary conditions.

However, it can sometimes involve a certain amount of the so-called »numerical diffusion«. The problem is more serious in the case of solving transport equations of scalar quantities but can sometimes render questionable hydrodynamic results as well (e.g. when simulating a high velocity river flow with lateral inflow and recirculation zones). The way to avoid the problem of numerical diffusion is to use higher order numerical schemes [7] or/and denser numerical grids. The latter is suggested in this article and can be achieved by introducing curvilinear coordinate systems which are able to fit the irregular boundaries of the computational domain. Orthogonal or nonorthogonal curvilinear meshes can be applied. Details about the equations in curvilinear coordinate systems and description of the discretisation method can be found in [4] and [8].

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2.3 Computer Codes The source computer codes PCFLOW2D for Cartesian and PCFLOW2D-ORTHOCURVE for orthogonal curvilinear numerical meshes are written in Fortran77 and run on personal computers, workstations or large mainframe systems. For the preparation of input topographic data (pre-processing) and graphical presentation of the final results of the model (postprocessing) the AutoCAD and Quick Surf graphic packages are used [8].

In mathematical models, the following boundary conditions can be taken into account at arbitrarily chosen cells of the computational domain: a) Solid boundaries (with zero normal velocities); b) Inflows of rivers (time-dependent discharges or velocities can be prescribed); c) Time-dependent water depths or surface elevations; d) Critical flow; e) An equation of a weir; f) Wind stress can be prescribed at the water surface by giving the wind speed and the wind friction coefficient.

3 SIMULATIONS OF FLOW IN THE DOWNSTREAM VALLEY 3.1 Numerical Mesh, Initial and Boundary Conditions Terrain of downstream south valley was defined with orthogonal curvilinear numerical mesh with 9625 cells. The resolution of the grid was variable from 4×11 m up to 33×33 m.

In the initial stage of calculation, the downstream valley is dry. Some suggestions how to treat the problem of wetting and drying in numerical solutions are given in [9]. To meet this condition in our proposed mathematical model, low water depth was given at the beginning of calculations. We chose a minimum level of 5 cm and all cells with water depth lower than 5 cm were considered dry with flow velocity equal to 0.

During the first ten iterations inside each time step, all the cells of the computational domain were included into the computation process. If the water depth did not exceed 5 cm, the appropriate cell was assumed to be inactive with zero velocities and initial water depth. Lower initial depths resulted in instabilities in the calculation, probably due to the high terrain gradient. Based on the actual conditions, where the wave would propagate with a depth of 15 to 20 m, the chosen initial depth of 5 cm was considered negligible and thus acceptable. The minimum depth was also lower than the expected accuracy of calculations.

The inflow and outflow boundary conditions had to be defined separately. The inflow curve Q – t (Fig. 5) was adopted in the model, which was the result 260

of previous calculations of the flow in the reservoir during partial dam break. In the initial time, the discharge was 46,947 m3/s, which was determined theoretically using momentum equation for the example of instantaneous dam break in the conditions of maximum water level in the reservoir. In the lower (outflow) boundary of the model, the critical depth was defined as a boundary condition. Due to the high channel slope, however, this boundary condition did not have any impact on the upstream flow.

3.2 Determination of Friction Coefficients The friction coefficient of the downstream valley was determined by considering the basic laws governing flow in steep channels. One of the basic observations was that the value of Manning’s coefficient was not a constant and that it depended on several parameters.

These were the hydraulic radius, area of flow crosssection, flow depth, width of free water surface, characteristic grain of a channel bed, channel slope and discharge. In determining the friction coefficient, all the mentioned parameters were not used, since they were mostly not known. By using the Rickenmann’s equation ([10] and [11]) and several simplifications, the dependence between the Manning’s friction coefficient (n) and bed slope (I) was acquired:

1 = C / Iα. n

(9)

Based on the chosen mean value of the friction coefficient for the discussed area of the watercourse and value of exponent α, the values of the friction coefficient (n) were distributed to all cells of the computation domain.

3.3 Analysis of Results Calculated with the Variable Friction Coefficient A comparison between the measured water depth versus time curves at different gauges and the calculated depths was made during the calibration of the mathematical model, where the value of exponent α was 0.2 (slightly above the value of Rickenmann) for different values of the mean friction coefficient n (0.04, 0.045 and 0.05 s/m1/3). The comparison for the last, downstream gauge 5 is shown in Fig. 6. For the average value of the Manning’s friction coefficient n = 0.05 s/m1/3, a very good correlation of maximum depth was established, as well as the propagation time of the wave to the measuring site, even if the path of the calculated wave did not coincide perfectly with the measured one. As it can be seen from Fig. 6, the

Krzyk, M. – Klasinc, R. – Četina, M.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 255-262

Fig. 6. Measured and calculated water depth oscillations at gauge 5 with variable friction coefficients for α = 0.2 and different average values of the Manning’s friction coefficient

Fig. 7. Axonometric view of the wave 178 s after the dam break (approach of the wave front to gauge 5)

measured arrival time of the wave front to the gauge 5 is 174 s and the computed one 178 s (2% relative error). The same level of accuracy is achived in other gauges. Fig. 7 provides an axonometric view of the water surface along the channel at the moment of the wave approach to gauge 5.

4 CONCLUSIONS In the study of the propagation of the dam-break wave in the Logarjev potok stream and by using the Rickenmann approach to determine the friction coefficient in several gauges, we acquired very good results in relation to water level, which for different mean values of friction coefficient n

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and exponent α coincided well with the measured depths. The calculated wave front is somewhat too steep, which resulted from an inadequate simulation of actual occurrences in the model. Even though an instantaneous dam break was planned, this could not be carried out due to manual raising of the gate.

That is why initial flows in the physical model were somewhat lower than the result of modelling of an instantaneous dam break. Very good results were obtained in relation to the wave propagation velocity.

A rather good correlation of maximum depth was established – at one gauge the difference in water depth was up to 27% and at the other four it was 7% of water depth on average. In all cases, unexpectedly low friction coefficients were used. The reason lies in the large depth of the flood wave in comparison to the roughness of the river bottom. Hence, the studied case cannot be considered as a classic case of torrential flow with a relatively small depth and a much higher friction coefficient.

[5] [6]

[7]

[8]

[9]

REFERENCES [1] Riasi, A., Raisee, M., Nourbakhsh, A. (2010).

Simulation of transient flow in hydroelectric power plants using unsteady friction. Strojniški vestnik Journal of Mechanical Engineering, vol. 56, no. 6, p.

377-384.

[2] Karadžić, U., Bergant, A., Vukoslavčević. P. (2009). A novel pelton turbine model for water hammer analysis, Strojniški vestnik - Journal of Mechanical Engineering vol. 55, no. 6, p. 369-380.

[3] Legiša, D., Rajar, R. (1980). Hydraulic model of the wave due to possible dam-break of the Kolarjev vrh

262

[4]

reservoir (HPP Kozjak), Report, Water management institute and Faculty of Civil and Geodetic Engineering, Ljubljana. (in Slovene) Rajar, R., Četina, M. (1983). Two-Dimensional DamBreak Flow in Steep Curved Channels. Proceedings of the 20th IAHR Congress, Vol. II, p. 571-579.

Patankar, S.V. (1980). Numerical Heat Transfer and Fluid Flow. McGraw-Hill Book Company, New York. Četina, M., Krzyk, M. (2003). Two-Dimensional Modelling of Debris-Flow Movement in Log pod Mangartom as an Example of a Non-Newtonian Fluid, Strojniški vestnik - Journal of Mechanical Engineering vol. 49, no. 3, p. 161-172.

Ferrari, A., Fraccarollo, L., Dumbser, M., Toro E.F., Armanini, A. (2010). Three-Dimensional Flow Evolution after a Dambreak. Journal of Fluid Mechanics, vol. 663, p. 456-477, DOI:10.1017/ S0022112010003599.

Krzyk, M. (2004). Two-dimensional mathematical modelling of flow in steep streams, Ph.D. Thesis, Univesity of Ljubljana, Faculty of Civil and Geodetic Engineering. (in Slovene) Gallardo, M., Parés, C., Castro, M. (2007). On a wellbalanced high-order finite volume scheme for shallow water equations with topography and dry areas. Journal of Computational Physics, vol. 227, no. 1, p. 574-601, DOI:10.1016/j.jcp.2007.08.007.

[10] Rickenmann, D. (1996). Fliessgeschwindigkeit in Wildbächen und Gebrigsflüssen. Wasser, energie, luft – eau, énergie, air, Baden, 88. Jhg., H. 11/12, p. 298-304.

[11] Rickenmann, D. (2000). Dynamics of sediments and water in alpine catchments – processes and prediction. Scientific report WSL, Birmensdorf.

Krzyk, M. – Klasinc, R. – Četina, M.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 263-270 DOI:10.5545/sv-jme.2010.215

Paper received: 2010-10-20, paper accepted: 2011-12-16 © 2012 Journal of Mechanical Engineering. All rights reserved.

Application of an Instrumented Tracer in an Abrasion Mill for Rock Abrasion Studies

Šolc, T. – Stefanovska, A. – Hoey, T. – Mikoš, M. Tomaž Šolc1 – Aneta Stefanovska2 – Trevor Hoey3 – Matjaž Mikoš4,* 1 Jožef Stefan Institute, Slovenia 2 Lancaster University, Department of Physics, United Kingdom 3 University of Glasgow, Department of Geographical and Earth Sciences, United Kingdom 4 University of Ljubljana, Faculty of Civil and Geodetic Engineering, Slovenia One of research fields in studying dynamics of gravel-bed rivers is the interaction between sediment particles in motion and incision rates in rock-bottom river reaches. This natural phenomenon of rock abrasion was studied in a laboratory in a Dubree-type abrasion (tumbling) mill with the diameter of 711 mm, using different mixtures of fluvial sediments as abrasive media. A set of rock plates of different lithologies was fixed to the inside mill wall to evaluate rock abrasion by moving sediment particles. The dynamics of the abrasion process of the rock plates was studied by a spherical instrumented tracer with the diameter of 99 mm.

This paper describes our solution to the problem of recognizing and differentiating between impacts of the instrumented tracer with different bodies: sediment particles, rock plates, soft lining of the mill and steel side plates of the mill. For this purpose, the signal analysis of measured 3D accelerations of the instrumented tracer gave sufficient information to recognize the tribological surrounding and sufficiently describe the intensity of the abrasion process (number and amplitudes of contact forces). An effective and computationally inexpensive algorithm for automatic impact recognition and evaluation was developed, based on time domain analysis. Furthermore, the frequency domain analysis gave a method for discriminating different signals. Both mentioned methods allow us to classify all recorded signals into groups based on similarity of measurement conditions.

Keywords: laboratory experiments, tribology, rock abrasion, instrumented tracers

0 INTRODUCTION Understanding sediment dynamics is of great importance in the field of gravel-bed river research.

Advances in computer technology in the last few decades have stimulated the development of sophisticated sediment transport models. One major goal in this field is to develop accurate theoretical models based on physical laws of solid sediment particle interaction to amend or fully replace current semi-theoretical or experimental models based on various regression coefficients. This development is based largely on accurate understanding of sediment particle movement.

One of the research directions in gravel-bed river research is the interaction between sediment transport and river bed evolution that is of interest with regard to climate change [1], during large floods [2], and to estimate incision rates in bedrock river reaches [3].

There are different controls over bedrock riverbed incision, such as climate, tectonics and dynamics of surface processes [4] that include debris-flow scouring, land sliding, glaciation and fluvial incision.

The latter is studied not only in the field but also under controlled laboratory conditions, such as studying rock abrasion in a laboratory and then transfering the measured laboratory relative values of rock abrasion into the field [5]. For such a transfer, it is important to have at least some idea of field values on erosion

rates; we have shown how to collect field data when analyzing abrasion resistance of concrete on hydraulic structures - a hydropower plant [6].

As a part of this study at Glasgow University, we performed laboratory experiments on rock abrasion of fixed rock plates of different lithologies by moving sediment particles in an abrasion mill using instrumented tracer in order to help understand the process and intensity of measured rock abrasion. The main purpose of the tests presented in this paper was to check the effectiveness of the laboratory rock abrasion experiments rather than to develop a new rock abrasion model based on the number and intensity of particle impacts, i.e. integrating characteristics of the impacting sediment particles and the rock mechanical properties [7].

Using different instrumented tracers in the field of sediment transport research started some decades ago, proceeding a period of passive tracers such as painted sediment particles or magnetic tracers. The first line of further tracer development went into the usage of radio waves, and the second one into a more sophisticated development of instrumented tracers using pressure sensors or accelerometers to study sediment transport [8] and [9], but they were developed also to test rock-fall barriers [10], or to study debris-flow dynamics [11] and [12].

*Corr. Author’s Address: University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova cesta 2, 1001 Ljubljana, Slovenia, matjaz.mikos@fgg.uni-lj.si

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1 INSTRUMENTED TRACER AND ABRASION MILL As a laboratory apparatus to study rock abrasion a Dubree type abrasion mill was selected with the inside diameter of 711 mm, and the inside length (width) of 508 mm, shown in Fig. 1. The mill consisted of a hollow steel cylinder, closed at both ends with steel and having rubber lining on its curved inner surface.

Test material was introduced to the cylinder through an opening in the top. The cylinder was mounted on a stub shaft, and rotated with 30 to 33 revolutions per minute around the horizontal axis.

The abrasion mill was operated for 1 hour in each run.

The travel distance of sediment particles in the mill was calculated by Wentworth’s method, which is to multiply the mill inside circumference and the rotation velocity - this approach gave the travel distance of 4.02 km/h [5]. The test material was weighted with the precision of 0.01 g. The surface wear was studied using a 3-D laser scanner with the vertical precision of 0.047 mm. The rock abrasion rates [km-1] were determined as relative weight losses per km of the travel distance.

This set up closely resembles a Los-Angeles type of abrasion mill, used to assess the quality of crushed aggregates used in construction industry (e.g. in road construction) according to a standard ASTM C535 Standard Test Method for Resistance to Degradation of Large-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine.

The Los Angeles test is a measure of degradation of mineral aggregates of standard grading resulting from a combination of actions including abrasion or attrition, impact, and grinding in a rotating steel drum containing a specified number of steel spheres.

The Los Angeles Abrasion test is widely used as an indicator of the relative quality or competence of coarse mineral aggregates larger than 19 mm (3/4 in.).

The main difference when studying rock abrasion was that no steel spheres were used as a charge but rather different mixtures of real fluvial sediment

(different mass and number of sediment particles as well as of different lithologies, see Table 1) were introduced into the mill.

a)

rubber padding

instrumented tracer b) Fig. 1. a) The abrasion mill used in the rock abrasion experiments and b) a vertical cross section through it

Both cases, the original Los Angeles mill tests and our rock abrasion experiments are dry and not wet abrasion tests and in this respect only to some extent resemble field conditions during sediment transport.

Table 1. Experimental conditions of various sediment sizes Sediment mixture 1 2 3 4 5 6 7 8

264

Initial mean weight and standard deviation [g] 48.15 / 4.83 63.14 / 5.05 83.44 / 8.68 106.84 / 6.28 192.72 / 49.17 250.40 / 13.28 334.81 / 25.28 456.12 / 32.30

Number of sediment particles 42 33 24 19 11 8 6 5

Mean equivalent diameter [cm] 3.24 3.55 3.89 4.23 5.15 5.62 6.19 6.86

Šolc, T. – Stefanovska, A. – Hoey, T. – Mikoš, M.

Initial total weight [g] 2022.49 2083.74 2002.45 2029.93 2119.94 2003.23 2008.85 2280.59

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 263-270

lithologies from Scottish rivers was fixed to inside part of the mill top (see Fig. 2) to evaluate rock abrasion of these plates by moving sediment particles in the mill.

Different combinations of sediment mixtures and rock plates were investigated (Table 2). a) b) c)

Fig. 2. Three rock plates installed on the inside part of the top of the mill (from [5])

A similar but wet and extensive laboratory study on abrasion of fluvial sediments from the Rhine River in Switzerland was conducted in an adapted cylindrical concrete mixer [13]. In order to be able to quantify measured abrasion coefficients, a study on motion of sediment mixtures in a revolving mixer was necessary [14].

The experiments described in this paper were dry rock abrasion experiments in a laboratory apparatus.

Additionally, a set of 3 rock plates of different selected

Fig. 3. SPY-Cobble and its constituent parts: a) opened tracer, b) the closed brass sphere with no epoxy coating, c) the two electronic boards and a battery

The dynamics of the abrasion process were described using an instrumented spherical tracer with the diameter of 99 mm, and mass of 994.6 g, shown in Fig. 3. The tracer was called SPY-Cobble (for Single

Fig. 4. Schematic view of the device Application of an Instrumented Tracer in an Abrasion Mill for Rock Abrasion Studies

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Particle dYnamics) and its schematic view is shown in Fig. 4.

2 RESULTS AND DISCUSSION ON SIGNAL PROCESSING OF MEASURED ACCELERATIONS

Table 2. Combinations of cylinder lining and sediment mixtures used in the mill

The problem to solve was a tribological one: how can we recognize and differentiate between impacts of the instrumented tracer with different bodies: individual moving sediment particles, fixed rock plates, soft rubber lining of the mill and steel side plates of the mill? The starting point were the measured signals (one typical is shown in Fig. 5) during 8 sets of measurements under different conditions (Table 2), each set comprising of 10 runs. Each run was around 2 minutes long, and using the Wentworth’s method, the tracer travelled for around 134 m during each run.

This allows the number of impacts to be computed per km of the travel distance.

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It measures tracer accelerations in three perpendicular axes for at least 120 seconds only due to its internal memory restriction (2 MB Static Random Access Memory) [15]. The tracer realization is not sufficient for deducing its global position from the measurements. Nevertheless, knowing its mass it is straightforward to compute contact forces from measured acceleration data [16]. The tracer has been previously successfully tested under laboratory conditions in turbulent flows in laboratory flume at Free University in Berlin [17].

2.1 Analysis of a Sinusoidal Signal We first focused on a typical sinusoidal signal as shown in Fig. 6 in a set without sediment particles or rock plates. The assumed reason for such a signal was rolling of the tracer along the cylinder surface. Typical time of a period t0 was evaluated to be between 300 and 400 ms. From it the rolling velocity in the mill can be estimated to be between vi = π di / t0 = 80 and

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100 cm/s, where di = 98 mm is the tracer diameter.

This estimated tracer velocity corresponds well to the real circumferential mill velocity in the inner side of the mill wall, computed to be vm = π ((dm – (2×7)) mm n = 109 to 120 cm/s, where dm = 711 mm is the inner diameter of the mill. The thickness of the rubber lining is 7 mm, and n = 30 to 33 min-1 is the mill revolving velocity. The conclusion was made that the tracer rolled within the mill without much slipping due to rather high friction coefficient of the rubber lining.

When trying to describe the abrasion process between the sediment particles and the rock plates in the abrasion mill it is essential to know the number and amplitudes of single impacts between them. This abrasion process was not very effective in our case, since the particles were colliding with each other or with the rubber lining and only for a smaller part of time they hit 3 rock plates causing their abrasion. The full description of this rock abrasion analysis is given elsewhere [5].

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2.2 Analysis in the Time Domain The tracer was used in this rock abrasion study only as an additional tool or as a substitute to describe the abrasion intensity. With this regard it was important to first develop a method for automatic impact recognition in measured signals which then enabled further evaluation of impacts in time domain. The developed method employed two parameters: A threshold M which the vector sum of accelerometer signals must exceed for an impact to be registered and a minimum time between two successive impacts tmin.

The optimization of parameter values for the measured signals gave optimal values of M = 20 (relative units of acceleration) and tmin = 3.53 ms. These values

This approach enabled us to break continuous accelerometer signals into parts, each containing one tracer impact, suitable for further analysis. In order to distinguish between tracer impacts against different materials, we defined two values, which described each recognized impact: α = 1/T [s-1] and β = S/(1000 T) = α (S/1000) [s-1], where S is the local signal maximum (peak amplitude in relative units of acceleration) and T is the time period [s] when the signal is higher than (2/3 S). The impacts of short duration (small T) give large α values and vice-versa.

The intense impacts (high S) give large β values and vice-versa, the impact duration (T and α being the same) If the α values for all automatically detected impacts in all available signals are computed and put into a histogram as shown in Fig. 7, it can be seen that impacts can be subdivided into two groups. The first one comprises impacts with values between α = 100 and 600 s-1, and the other one those with values

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different sizes, and that were of rather short duration.

Such signals are very steep, and the sampling frequency would need to be increased from existing 2832 Hz in order to provide a representative record of such events.

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between α = 900 and 4200 s-1. In the second group two peaks can be recognized: at α = 1600 and 3300 s-1. The total number of impacts is given for each set in Table 3.

From Table 3 it can be seen that the number of impacts with α < 700 s-1 changes only to a minor extent when sediment particles were added to the mill.

This was not true for the impacts with α > 700 s-1 where the number of impacts was much influenced by the sediment mixtures used in the experiment (number of sediment particles in the mill). Therefore, it may be concluded that the impacts with α < 700 s-1 are impacts against the rubber lining, and the second group, with α > 700 s-1, are impacts against sediment particles/rock plates and steel walls of the mill. As expected, the number of impacts against the rubber lining is slightly reduced when sediment particles are added as well as when the rock plates are installed in the mill (compare the sets ”p“, ”pg“ and ”pg5“). The highest number of impacts was achieved with the set ”gr4“.

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The grouping of impacts can easily be seen on a diagram as in Fig. 8 showing the impact amplitudes as a function of the α value for all 80 runs. The value α = 700 s-1 divides both groups, where the first one (impacts against the rubber lining) is limited with a line S = 2/5 α, where S is the impact amplitude. For the impacts having α > 700 s-1 there are no such sharp limits. Two peaks may be noticed: at α = 1600 and 3300 s-1. These two values correspond to the peaks in the histogram (see Fig. 7). The physical reason for these two peaks is not resembled yet and it is not caused by the difference between steel walls and sediment particles.

The large scatter of the impacts in the second group can be explained by the fact that we measured impacts of the tracer with sediment particles of 268

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Fig. 10. The dependence of the impact amplitude S in relative units on the β value, given for low β values (impacts when rolling or sliding) – lower-left corner of the Fig. 9

A grouping of the impacts is seen also in the diagram showing the dependence of the impact amplitudes S on the β value: S = 1000 T β (see Figs.

9 and 10). In Fig. 9, the lower part of the diagram is empty in accordance with the tracer sampling rate of 2832 Hz. In Fig. 10, where this dependence is given for only low β values, two groups can be distinguished, each exhibiting different duration of impact (T). The division between the two groups is the T = 1 ms; the upper-left group with T > 1 ms represents the impacts against the rubber lining, and the lower-right group with T < 1 ms represents all other impacts.

2.3 Analysis in the Frequency Domain All the measured signals were also analysed in the frequency domain using a discreete Fourier

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Two typical histograms are given in Figs. 11 and 12, and their form for different sets can be compared between each other in Fig. 13; the rubber lining does not change the histogram form much (compare signals ”gr11“ and ”pg1“ with signals ”gr51“ and ”pg51“).

The comparison for signals within one set can be seen in Fig. 14.

The form of the histogram curve differs in width and peak position for different sets, but only using frequency analysis is not enough to distinguish between sediment mixtures. However using all relevant parameters of the abrasion process measured by the tracer (number of impacts, the α value, form and amplitude of the histogram of the discrete Fast Fourier Transform) allows reliable classification of an unknown signal to measured sets.

3 CONCLUSIONS We performed a laboratory study on rock abrasion in a laboratory abrasion mill that resembles the Los Angeles abrasion test. Instead of using steel balls as a charge like in the original Los Angeles abrasion tests, we used natural fluvial sediment mixtures of different gradation. Using an instrumented tracer with 3 installed piezoelectric accelerometers for around 2 minutes for each run, and by applying advanced signal processing of measured accelerations in the time domain, it was possible to automatically classify all recorded signals into groups based on the similarity of measurement conditions.

Such an approach makes it possible to describe the efficiency or intensity of the rock abrasion process in similar laboratory apparatus by number and amplitudes of impacts rather than by simply stating the equivalence between the travelled distance in the apparatus computed from the revolving mill velocity and the mill inner diameter, and the distance travelled

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in the field. Nevertheless, a full use of the results presented in this paper with the rock abrasion results presented in [5] still needs to be done. Until then, the major abrasion parameters remain sediment lithology and sediment size. But for any further analysis the results presented in this paper will be of value.

Anyhow, laboratory experiments on rock abrasion are still rather qualitative and are as such able to show differences between the different lithologies rather than quantitative and being able to measure real rock abrasion coefficients to be applied in the field.

Nevertheless, our laboratory attempt has shown that there are at least ways how to refine laboratory abrasion experiments by introducing instrumented tracers into the abrasion apparatus that can help to improve our knowledge about abrasion process in laboratory abrasion set-ups such as in an abrasion mill.

4 ACKNOWLEDGEMENTS Both, the experiments and the analysis, were financed by The Royal Society of London as a part of the project “Automated Tracking of River Sediment Movement“, under the collaboration of University of Glasgow (Dept. of Geographical and Earth Sciences), University Heriot-Watt in Edinburgh (School of the Built Environment) and University of Ljubljana (Faculty of Civil and Geodetic Engineering).

The signal analysis part of the study was prepared by the first author while studying at the Faculty of Electrical Engineering, University of Ljubljana.

5 REFERENCES [1] Bogaart, P.W., van Balen, R.T. (2000). Numerical modeling of the response of alluvial rivers to Quaternary climate change. Global and Planetary Change, vol. 27, no. 1-4, p. 147-163, DOI:10.1016/ S0921-8181(01)00064-9.

[2] Wohl, E. (2007). Review of effects of large floods in resistant-boundary channels. Developments in Earth Surface Processes, vol. 11, p. 181-211, DOI:10.1016/ S0928-2025(07)11125-1.

[3] Sklar, L.S., Dietrich, W.E. (2006). The role of sediment in controlling steady-state bedrock channel slope: Implications of the saltation–abrasion incision model. Geomorphology, vol. 82, no. 1-2, p. 58-83, DOI:10.1016/j.geomorph.2005.08.019.

[4] Seong, Y.B., Owen, L.A., Bishop, M.P., Bush, A., Clendon, P., Copland, L., Finkel, R.C., Kamp, U., Shroder, Jr.J.F. (2008). Rates of fluvial bedrock incision within an actively uplifting orogen: Central Karakoram Mountains, northern Pakistan. Geomorphology,

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vol. 97, no. 3-4, p. 274-286, DOI:10.1016/j. geomorph.2007.08.011.

[5] Kim, J. (2004). Controls over bedrock channel incision.

PhD Thesis, University of Glasgow, Glasgow.

[6] Kryžanowski, A., Mikoš, M., Šušteršič, J., Planinc, I. (2012). Abrasion Resistance of Concrete in Hydraulic Structures on Lower Sava River. Strojniški vestnik – Journal of Mechanical Engineering, vol. 58, no. 4, p.

245-254, DOI: 10.5545/sv-jme.2010.217.

[7] Acquaro, D. (2010). Impact of Solid Particulate on Brittle Materials. Strojniški vestnik – Journal of Mechanical Engineering, vol. 56, no. 4, p. 275-283.

[8] Ergenzinger, P., Schmidt, K.H., Busskamp, R. (1989).

The pebble transmitter system (PETS): First results of a technique for studying coarse material erosion, transport and deposition. Zeitschrift für Geomorphologie Neue Folge, vol. 33, p. 503-508.

[9] Mikoš, M., Krzyk, M., Banovec, P. (1997). European project EROSLOPE and SPY-Cobble. Acta hydrotechnica, vol.15, no. 17, p. 125-136.

[10] Gerber, W., Grassl, H., Böll, A., Ammann, W. (2001). Flexible rockfall barriers – development, standardisation and type-setting in Switzerland.

Proceedings of the International Conference on Landslides: Causes, Impacts, and Countermeasures, p.

515-524.

[11] Hanisch, J., Ergenzinger, P., Bonte, M. (2003).

Dumpling – an “inteligent” boulder for studying internal processes of debris flows. Proceedings of the Third International Conference on Debris-flow Hazards Mitigation: Mechanics, Prediction, and Assessment, p.

843-849.

[12] Arattano, M., Marchi, L. (2008). Systems and sensors for debris-flow monitoring and warning. Sensors, vol.

8, p. 2436-2452, DOI:10.3390/s8042436.

[13] Mikoš, M. (1993). Fluvial abrasion of gravel sediments.

Miteilungen der Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie, no. 123, ETH, Zürich.

[14] Mikoš, M., Jaeggi, M.N.R. (1995). Experiments on motion of sediment mixtures in a tumbling mill to study fluvial abrasion. Journal of Hydraulic Research, vol. 33, no. 6, p. 751-772, DOI:10.1080/00221689509498550..

[15] Mikoš, M., Petrovčič, J., Spazzapan, M. (2001). A method and an apparatus for measuring of movement dynamics elements and forces acting upon single particles in natural environment. Patent no. P-9900222, The Slovenian Intellectual Property Office, Ljubljana.

[16] Spazzapan, M., Petrovčič, J., Mikoš, M. (2004). New tracer for monitoring dynamics of sediment transport in turbulent flows. Acta hydrotechnica, vol. 22, no. 37, p.

135-148.

[17] Mikoš, M., Spazzapan, M. (2005). Laboratory application of a satellite for monitoring dynamics of sediment transport in turbulent flows. Acta hydrotechnica, vol. 23, no. 38, p. 39-55.

Šolc, T. – Stefanovska, A. – Hoey, T. – Mikoš, M.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 271-280 DOI:10.5545/sv-jme.2011.193

Paper received: 2011-11-09, paper accepted: 2012-03-16 © 2012 Journal of Mechanical Engineering. All rights reserved.

Improving Products’ Ergonomic Value Using Intelligent Decision Support System Kaljun, J. – Dolšak, B. Jasmin Kaljun* – Bojan Dolšak University of Maribor, Faculty of Mechanical Engineering, Slovenia

During the process of defining suitable design solutions, a designer has to consider a wide range of influential factors. Ergonomics certainly belongs to more complex ones. Less experienced designers could encounter several problems during this design stage. Although some literature can be found about ergonomic design of hand tools, a designer still has to have amassed quite a lot of experience and knowledge in the field of ergonomic, in order to choose and carry out appropriate design and redesign actions. Existing computer tools for ergonomic design are not able to assist a designer with higher level advice within design process. An intelligent decision support system has been developed in order to overcome this bottleneck. This paper presents a knowledge base, containing ergonomic design knowledge specific for hand tools design. A pneumatic hammer handle design is used as a case study to show how ergonomic design knowledge built in the system is used to improve the ergonomic value of the product.

Keywords: ergonomics, intelligent support, hand tools, knowledge acquisition, handle design, pneumatic hammer

0 INTRODUCTION During the design of the product for everyday use, various influential factors have to be considered.

Ergonomic value of the product is certainly one of the issues that need to be addressed. A less experienced designer could meet several problems to find ergonomically appropriate design solution.

Although the existing ergonomic Computer Aided Design (CAD) software that is discussed in Section 1 can provide some assistance in ergonomic design evaluation, the designer still has to possess a substantial experience and knowledge in the field of ergonomics in order to choose and carry out adequate design actions to improve the ergonomic value of the product in reasonable time.

Product ergonomics is an interdisciplinary scientific discipline concerned with understanding interactions among humans and other elements of a system. In this context, the user has a central role in product development process [1]. Product ergonomics applies theory, principles, data and methods to optimize human well-being and overall system performance.

The ergonomic quality of a product can be defined by a match between anthropometric data and formal attributes. However, the quality of ergonomics is not only based on anthropometrics, as the field of human factors has been realizing over the past thirty years [2]. Cognitive and experiential processes play a major role in deciding whether a design is usable, efficient, satisfying, easy to use, or comfortable.

On the other hand, ergonomic solution must not adversely affect other characteristics of the product.

Among others, ergonomics is very much connected to the aesthetic appearance of the product and seeking an

optimal balance is a delicate manner [3]. It is exactly this kind of skill that a good designer needs to have: finding the optimal balance between the two aspects.

In order to support a decision making process when performing this task, a prototype of the intelligent advisory system, which is briefly presented in Section 2, is being developed. In continuation of this paper we will concentrate on ergonomic part of the system.

Ergonomic design knowledge is discussed in Section 3 by presenting important ergonomic design goals and corresponding design recommendations. We have limited our research to certain group of products – hand tools, as representatives of ergonomically significant products that are manipulated with upper extremities like many household appliances, etc.

A case study of knowledge base application is presented in Section 4 and concluding remarks are given in the last section.

1 EXISTING ERGONOMIC CAD TOOLS In the field of ergonomic CAD, the development process has focused on integrated tools based on threedimensional modelling of the product and the human body that enable the use of ergonomic data originating from various sources, when performing ergonomic analysis of the product. This enables a designer to use a single analysis/simulation tool to evaluate and assess clearances, reach, visual requirements, and postural comfort at the earliest stages of design [4]. Moreover, it makes possible for a designer to incorporate important features into designs, thus minimizing the risk of discomfort or even injuries way before a person ever physically encounters the product or working place that is subject of development.

*Corr. Author’s Address: University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia, jasmin.kaljun@uni-mb.si

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Using an interactive interface, designers are able to manipulate both, the human form and the product design [5].

Two different approaches to developing such software tools have been taken [6]. One approach is oriented towards the development of the so called stand-alone ergonomic CAD software with ergonomic assessment capabilities and a built-in tree-dimensional CAD module. The alternative approach leads to the development of compatible ergonomic software based on special modules that enable ergonomic analysis within commercially available CAD systems, which are used to provide three-dimensional modelling and user interface.

Some of the best-known representatives of both groups of ergonomic CAD software are presented in Table 1.

In continuation of this section more detail description emphasizing some application characteristics is given for each group in a separate sub-section.

Table 1. Representatives of ergonomic CAD tools Stand-alone ergonomic CAD tools SAMMIE APOLIN TADAPS Deneb/ERGO ERGOMAN ErgoSPACE

Compatible ergonomic CAD tools SAFEWORK MINTAC ErgoSHAPE HUMAN RAMSIS ANYBODY

1.1 Stand-alone Ergonomic CAD Software Stand-alone ergonomic software is applied independently of the other CAD software used in product development processes. Thus, the user should learn terminology, command structures, and modelling techniques that are usually different from those in commercial CAD systems. Fortunately, many of these systems have the ability to import geometric models as modelled in other CAD tools, where the complexities of the models can be taken in consideration.

The working environment is composed of the product model and the model of a human that is assigned to perform virtual application of the product [7]. After the modelling/import is finished, the parameters for various ergonomic analyses are set, and the analyses are carried out. The evaluation of the analyses results is the next step in the design process, where ergonomic satisfaction of the product needs to be evaluated. If re-modelling of the product is needed, 272

the whole cycle is repeated until the resulting model of the product does not satisfy ergonomic criteria. Even then, certain changes may still be necessary in order to improve other design requirements, as for example, aesthetics and manufacturability.

1.2 Compatible Ergonomic CAD Software Compatible ergonomic software has been designed for access within available commercial CAD systems, as for example [8] and [9]. These tools take the advantage of the designers’ familiarity with the terminology, techniques, and command structures of commonly used CAD programs. The main advantage of these tools is the application of a single geometric model for all phases of the design process, which prevents many problems related to data transfer.

Similarly to the application of stand-alone ergonomic CAD software, the development process starts with a concept design and usually an ergonomically-imperfect design candidate, which represent the starting point for compatible ergonomic CAD tool. The exact three-dimensional virtual model of the product is modelled using a geometric modeller of the commercial CAD software. When the model is finished, the ergonomic tool is run within the commercial CAD system, where the working environment and a human model are prepared. The modelling is followed by assigning proper values to the ergonomic analyses parameters. In the next step, ergonomic analyses are carried out. Immediately after the results of the ergonomic analyses are evaluated, the product can be re-designed and re-modelled in order to correct eventual ergonomic imperfection.

Alternatively, the process can be continued towards other analyses that need to be carried out for the model. The result of such multi-criteria analysis [10], conducted within single software environment is a final design solution of the product that fulfils all ergonomic, mechanical, and other demands and conditions.

2 INTELLIGENT SUPPORT TO ERGONOMIC DESIGN PROCESS Various advanced approaches have been investigated to improve the product development process, such as those reported in [11] and [12]. The need for integration of ergonomics into product design is evident for quite a long time now [13]. However, the need for knowledge-based decision support within ergonomic design process has been defined more recently. It is based on the cognition that conventional ergonomic CAD tools do not meet the expectations

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of design engineers. While they offer a reasonable level of support in various ergonomic analyses [14], they fail to provide any kind of meaningful advice from an engineering point of view in terms of design recommendations leading to better ergonomic value of the product, when appropriate.

In order to overcome this bottleneck and to round off a cognitive cycle [15] for continuous improvement of product’s ergonomics, we are developing a prototype of an intelligent advisory system Oscar, based on expert design knowledge management.

A possible logical frame of ergonomic knowledge management has been proposed by Du et al. at Computer-Aided Industrial Design & Conceptual Design conference in 2009 [16]. In the proposed frame the aesthetic appearance of the product is not considered as an influential parameter, which, however, is the case in our system, as presented in Fig. 1. Oscar is namely composed of two sub-systems that can be applied in two different modes. They can be used independently from each other, or simultaneously and interdependent on the same design project.

In the simultaneous mode, the task of the inference engine is not only to derive and propose recommendations for both, ergonomic and aesthetic design improvements, but also to synchronize and harmonize possible design solutions in order to find the optimal balance between the two aspects of the product being developed.

The intelligent decision support system Oscar is still a research prototype, and as such, a subject of intensive development, especially the more subjective part of the system dealing with engineering aesthetics and aesthetic ergonomics [17]. On the other hand, the ergonomic part of the system, which is based on more objective ergonomic design knowledge built in the knowledge base of the system, is functional and thus, discussed below in more detail.

Basically, the intelligent advisory system is a computer system that emulates the decision-

making ability of a human expert. Such systems are designed to solve complex problems by reasoning about knowledge, like a human expert, and not by following the procedure of a developer as is the case in conventional programming [18].

The structure of the intelligent advisory system is unique, and significantly different from traditional programs. In foundation, the system is divided into three parts. The first part is fixed, independent of the rest of the system, called the inference engine. The second part is variable, called the knowledge base.

The third part, which serves as a communication module is called user interface. When running intelligent advisory system, the inference engine reasons about the knowledge base like a human expert and communicates with user through user interface.

In the intelligent advisory system technology, the knowledge base is expressed with natural language IF-THEN styled rules, called production rules [18].

Usually inference engine runs in the so called conversational mode. This simply means that the inference engine requests initial data from the user at the start. Since problems are usually complex, the system often cannot find a suitable solution using only initial data. Therefore, the system has to find the way to solve the problem by requesting the missing data form the user.

With additional data the system gradually approaches to the solution.

The user feels like a participant in a dialogue led by an expert. To control a dialogue, the inference engine three different techniques: forward chaining, backward chaining and mixed chaining [19]. Forward chaining is a technique where the inference engine searches the solution (goal) by using specified procedure and asking questions. In backward chaining, the inference engine knows the solution and attempts to find production rules to support the solution. In mixed chaining the inference engine has an “idea” of the solution but cannot simply confirm it. Therefore,

Fig. 1. Basic structure of the intelligent advisory system Oscar Improving Products’ Ergonomic Value Using Intelligent Decision Support System

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the inference engine deduces in forward chaining the technique using information from previous user responses before asking the next question. So, quite often the inference engine deduces the answer to the next question before asking it.

When developing Oscar, a development environment named Exsys Corvid [20] has been used.

The Corvid enables developer to build the knowledge base using natural language. User interface is built simultaneously with knowledge base. The Corvid provides developer with configurable inference engine. The prototype is built as java applet to be used online.

3 ERGONOMIC DESIGN KNOWLEDGE The main focus of ergonomic design is compatibility of objects and environments with human factors. It seeks to harmonize functionality of the tasks with capabilities of humans performing them. Ergonomic design knowledge is extensive.

It considers not only anthropometrics and biomechanics, but also cognitive issues. For this reason, we decided to limit our research on ergonomic design of hand tools [21].

Similarly to some other products that are manipulated with upper extremities, static and dynamic anthropometry, biomechanics and anatomy of the human hand (Fig. 2) needs to be taken into consideration [22].

interviewing some experts. In the next step, these goals have been associated with respective design recommendations that ensure a certain goal to be accomplished. Knowledge acquisition was again a combination of literature survey and transfer of human knowledge and experience. In the last step, the collected knowledge was organized and written in the form of production rules to be used by the intelligent system.

Different classes are interconnected with various attributes and their values in head of the rule in order to describe case specific situation, in which recommendations for product design that are listed in the body of the rule should be taken into consideration, as for example: IF Type is Hand tools for specific tasks Contact Element is Grip Trigger Style is Palm push button THEN Ergonomic goal: Reduce tissue compression Design Recommendations: Design contoured handle Use triggers of adequate length (thickness) Design oval/elliptical cross-section of the handle Avoid sharp edges

In the following sub-sections, the most important ergonomic design goals and respective design recommendations related to hand tools are presented.

The rule shown here is an example of how this knowledge is encoded in the system and refers to the case study discussed in the next section of this paper.

3.1 Dimensions and Configurations In order to define appropriate dimensions and configurations to be respected in hand tools design, the anthropometrical data related to human hand are transformed into design recommendations (Table 2).

Table 2. Anthropometrical recommendations

Fig. 2. Human hand

The development of a knowledge base related to ergonomic design of hand tools has been carried out in three steps. First of all, all crucial ergonomic design goals were defined by studying literature and 274

Attribute handle cross - section handle diameter - power grip handle diameter - precision grip handle length (palm side) handle length (palm side) - gloves handle length (finger side) handle length (finger side) - gloves pistol grip handle angle finger clearness

Kaljun, J. – Dolšak, B.

Value round or oval min. Ø 25 to 45 mm min. Ø 7 to 15 mm min. 100 mm min. 114 mm min. 78 mm min. 90 mm 7° to 10° min. 35 mm

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Fig. 3 shows in a graphic form the recommended dimensions for a handle that will be used in the next section as a case study.

It can be seen that the length of the handle allows the use of protective leather gloves, as shown in the next section (Fig. 9).

It is important to select correct handle diameter to provide optimal grip span and consequently optimal grip force. Cylindrically shaped handles are usually not applied to the tool where rotation of the hand around the grip is not expected. In such cases handles have oval or rectangular cross – section with a grip span between 35 to 50 mm, considering the use of protective leather gloves.

Fig. 3. Recommended handle dimensions

• • •

depends on the work piece height. For example, for the horizontal work piece in a femur level, the pistol grip is suitable.

Use deviated handles, which maintain the straight wrist for tools for specific tasks (Fig. 4).

Provide adjustable handles for tools that will be used in several different positions.

Use power tools instead of traditional tools for tasks that require highly repetitive manual motions.

Fig. 4. Design to assure neutral wrist position (when protective gloves are applied, diameter of circles is bigger)

3.2 Maintain Neutral Straight Wrist Position

3.3 Avoid Tissue Compression

All movements of the wrist, especially those cases in extremes of these movements, in connection with repetitive finger actions or prolonged forceful finger exertions, place extensive pressures upon the flexor tendons passing through the carpal tunnel. This may cause inflammation of tendon sheath and pressure upon the median nerve and in consequence even serious injuries. Bending the wrist, while performing the task, which requires repeated rotation or twisting of the forearm, can also stretch and pull the tendon connection at the elbow. Repeated stress at this connection can cause irritation and swelling, leading to the so called tennis elbow. When the wrist is straight, tendons can slide easily through the sheath. It is thus very important to maintain the wrist in neutral straight position.

Design recommendations: • Use pistol grip for tools, which are used on vertical surfaces. The type of the grip used depends on the work piece height.

• Use inline cylindrical grip for tools, which are used on horizontal surfaces. Again, the type

Local pressure upon tissues of a palm or fingers may cause loss of circulation, damage of nerves, leading to tingling of fingers; or damage of tendons or muscles, leading to pain and difficult hand movement. This unsuitable pressure is caused by insufficient handle length (thickness) or hard surfaces on handles.

Recommendations: • Use handles of adequate length (thickness) that span the entire hand.

• Use padding to soften the handle surface.

• Use contoured handles which spread the pressure over a large area (Fig. 5).

Among the various tool handle design characteristics, handle diameter has been studied extensively because it is an important factor to maximize grip strength, minimize stress on the digit flexor tendons, first metacarpal ulnar collateral and carpometacarpal ligaments, and it can also influence force exertion in manual work.

Different independent studies have been carried out to determine optimal handle diameter with maximal grip force [23]. On the basis of this research

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work, an optimal handle diameter to develop the maximal hand grip force is between 25 and 45 mm and is larger for men. In case of cylindrical grip, circumference is therefore between 78 and 141 mm.

Circumference of the presented grip is 140 mm, which is close to upper optimal limit, however due to the prevention of tissue compression on the trigger, the grip has to be wider.

• • • •

•

• •

• Fig. 5. Design to prevent tissue compression

According to [23], average grip force for male when using optimal grip diameter is between 300 and 660 N.

• •

3.4 Reduce the Excessive Forces Exertion of high finger forces, either prolonged or repetitive, can stretch and in turn fray tendons. This kind of damage can make it difficult for the tendon to slide through the tendon sheath, which can lead to further irritation and swelling. Irritation and swelling can lead to restriction of the tendon movement through the sheath, eventually causing the so called trigger finger. Especially a combination of both repetitive and prolonged motions may lead to permanent disorders, and carpal tunnel syndrome.

Excessive forces may also overload muscles and create fatigue and potential for injuries. Highly repetitive tasks which may not use such a great force can also cause irritation. Contact with sharp edges of tools or bending the wrist greatly increases the hazard associated with the use of forceful finger exertions.

Recommendations: • Use power grips (Fig. 6) instead of forceful pinch grips with straight fingers. One can exert more force with the power grip than pinch grip.

• Use the appropriate grip size. To generate the most grasping force, design objects to a size that permits the thumb and forefinger to overlap slightly.

276

•

Reduce resistance of tool activators (triggers, trigger lockers etc.).

Use the alternative non-mechanical triggers (vacuum, optical etc.).

Increase leverage within the tool. Add more fulcrums. Extend lever arm.

Improve tool balance. Reduce tool length. Locate heavy masses such as motor, battery, etc. as close as possible to the wrist.

Where a relatively large force is needed to activate the tool (hydraulic or pneumatic power tools), use trigger levers for more fingers instead of a single point trigger to spread the activating.

Avoid sharp edges on triggers and handles.

Add the second handle located near the front end of the tool to spread exertion between two hands.

In this way the control of heavy tools and tools operating with large torque is also improved.

Increase contact friction on handles by using slip resistant, nonporous and slightly compressible materials.

Use a collar where the force is applied coaxially to the handle. It may reduce the grasping force.

Use the expanding springs to prevent the constant need of opening handles.

Use the power tools instead of traditional tools for tasks which require high excessive motion.

3.5 Ensure Proper Height for the Task Working in a position that implies the elbow raised and maintained above the shoulder height for prolonged periods can trap nerves and blood vessels under the bone and muscle, which leads to numbness and tingling in the hands, and can fatigue the muscles of the shoulder and upper arm. It is therefore, required to design the tool in a way that ensures proper height of the task.

Recommendations: • Design tool handles and other features to fit in the proper working height level.

• Reduce muscle exertion and improve control over the tool.

• For heavier manual work with the heavy power tools, design the tools for use at the hip level, with the tool close to the body and the angle between the upper arm and forearm in range of 90 to 120°.

• For precision work with lighter tools, design the tools for the use at the elbow level and higher.

• Use extended poles for work above the head.

Kaljun, J. – Dolšak, B.

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3.6 Protect Against Vibrations, Heat, Noise

•

Local vibrations may cause circulation disorders, white fingers or other serious cumulative trauma disorders. Enhanced noise may increase fatigue and stress; and may cause problems of hearing. The hand may be affected by vibrations in range of 1.5 to 80 g and 8 to 500 Hz. Recommendations: • Improve the overall tool design taking into account the natural frequencies to decrease vibration distribution from the motor or other source of vibrations into other connected parts and handles.

• Use isolation mounts such as springs and rubber silent blocks between individual parts.

• Use damped tool handles.

• Use damping materials on a handle surface.

• Use heated handles where needed.

• Cover hot parts of the tool such as motors.

• Clean and adjust power tools periodically.

Remark: sound (noise) is a consequence of vibrations, actually vibrations transferred from the tool into an air and then into human ears; therefore the same recommendations can be used analogously for decreasing the level of noise.

• •

Turning a dial to the right increases the speed, torque or power.

Use numbers: 1 (slow), ...., 10 (fast).

Use the right hand to operate the trigger and other controls.

4 CASE STUDY

In order to demonstrate practical application of the ergonomic design knowledge built in the intelligent decision support system Oscar, a case study dealing with redesign of the pneumatic hammer handle has been carried out.

The goal of the project was a redesign of the handle and new design of the casing for the pneumatic hammer presented in Fig. 6. A basic edition of the existing pneumatic hammer was not equipped with body casing; simple rubber tubes were provided instead of body casing on customer request.

3.7 Reduce Static Load Holding the same position for a period of time can cause pain and fatigue. The primary problem is duration. However, negative influence is additionally increased by high force or awkward posture.

Recommendations: • Reduce the weight of the tool.

• Use tool supports.

• Improve tool balance. Reduce tool length. Locate heavy masses such as motor, battery, etc. as close as possible to the wrist.

3.8 Cognitive Ergonomics Cognitive ergonomics deals with a mental interrelationship between the human and the product. The goal to be achieved by using design recommendations is to prevent mental overload and misunderstanding when using certain product – in our case the hand tool.

Recommendations: • Use red colour for switch buttons or for warnings and dangers.

• Use vertical switches with the following meaning: up-ON, down-OFF.

Fig. 6. Existing pneumatic hammer (handle)

Pneumatic hammers are hand tools used for smashing and drilling hard materials. They are mostly used in the field of civil engineering and mining industry. Pneumatic hammers belong to a group of tools powered with compressed air and subgroup shock tools.

A pneumatic hammer is a very useful tool for many purposes. Physical properties of air allow it to accumulate relatively big amounts of energy, which is very important for the shock power of a pneumatic hammer. Shock power is one of more important features of a pneumatic hammer, as it is relatively high regarding to the weight of the tool itself.

The hammer that was chosen for the case study was underdeveloped from both, ergonomic and aesthetic point of view. The manufacturing company was aware of that, and took the opportunity using our innovative technologies to improve the design of their product in order to increase their competitiveness [24] and [25].

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It could be argued that aesthetics is not important for such a tool. Yet, it has an important influence on the user’s cognitive comfort and emotional contentment. However, our primary goal for the case study, presented in this paper, was the improvement of the ergonomic value of the tool with emphasize on the handle design.

Quality and usefulness of advice provided by the intelligent system depends on the quality of input parameters. This is why a lot of attention should to be dedicated to define them. Table 3 presents the most important influential parameters and their values as they were presented to the system.

The first step of the evaluation has been made using a virtual hand of average (50th percentile) male.

In that way the size of the grip and finger space has been tested. Visual inspection of the virtual hand/ handle assembly (Fig. 8) has confirmed dimensional suitability.

Table 3. Most important input parameters Parameter user information place of use application direction application technique protective equipment mech. characteristic critical areas emotional contentment

Value male, 18 to 55 years old, average constructing places, dust mainly horizontal one or both handed, 1 operator protective leather gloves solidness, stability, low weight rear handle, front hand support solidness, reliability, power

Fig. 8. Virtual evaluation

In the next step, the knowledge base built in the system was applied to generate ergonomic design recommendations.

In the case study presented here, redesign recommendations proposed by the intelligent system Oscar can be summarized in three groups: • change the shape and dimensions of the handle, • change the cross-section of the handle, • change the position of the handle.

Considering all the recommendations based on the ergonomic knowledge built in the system and presented in previous section of this paper, including Figs. 4 to 6, a virtual model of the new pneumatic hammer handle was modelled (Fig. 7).

The next step was actual evaluation of physical model. On the basis of the computer model of the handle, a prototype of the handle has been build using rapid prototyping technique 3D object printing. The prototype of the handle made in this way has been used for practical ergonomic evaluation by the users of the existing pneumatic hammers (Fig. 9). Two connectors for the tube with compressed air were designed in order to find a better position in terms of tool balance when the tube is attached to it.

Fig. 7. Virtual model of redesigned handle

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Fig. 9. Evaluation using rapid prototyping Kaljun, J. – Dolšak, B.

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Workers (15 persons from different constructing companies) who are using this tool daily, were asked to assess both handle designs (existing and new) using grades from 1 to 5 in areas: • overall feeling of holding the grip in the hand, • grip strength and stability, • finger space needed for manipulation, • pain or tension in the hand (grade 5 – no pain or tension).

Fig. 10 shows the results of assessment. The results of the evaluation show that the overall ergonomic value of the handle has been significantly improved by providing more space for fingers (also for a hand wearing a protective gloves), better wrist position, and better grip of the handle.

However, due to the prototype limitations, the evaluation of handle has not been performed during the work process itself (use of pneumatic hammer in practice). Future research work will also be oriented towards simulation techniques that will enable virtual evaluation of hand tool – user interaction, using principles of simulating nonlinear materials (i.e. human tissue) using cross-linked simulations [26] and [27].

The knowledge built in the prototype of the intelligent system named Oscar is structured in the form of different classes interconnected with various attributes and their values at the input side, while as the output of the system the user can expect (re)design recommendations leading to achievement of certain design goals that can improve the ergonomic value of the product (hand tool) being developed.

An industrial example was used as a case study to test the correctness, reliability and usefulness of the knowledge base. A rear handle of the pneumatic hammer was redesigned for this purpose using recommendations derived by the system. The redesigned handle demonstrates significant ergonomics improvements.

Following these conclusions, it may be summarised that the intelligent decision support to the ergonomic design process represent added value to the existing ergonomic CAD tools that enable various ergonomic analyses, but fail to provide engineering advice on how to improve the ergonomic value of a design candidate that is the subject of ergonomic evaluation.

6 REFERENCES

Fig. 10. Assessment results

5 CONCLUSIONS Among other design factors, ergonomics play an important and complex part in the product design process. Regarding the complexity, multi-factors coupling and fuzziness of ergonomic knowledge, an intelligent support to ergonomic design in the form of advisory decision support system is proposed.

In this context, the knowledge related to ergonomic design of hand tools has been collected, organised and encoded in the form of production rules, which were found to be the most appropriate formalism due to their transparency and closeness with the human way of decision making process.

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[2] Kroemer, K., Kroemer, H., Kroemer-Elbert, K. (2001).

Ergonomics – How to design for ease and efficiency.

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[3] Liu, Y. (2003). The aesthetic and the ethic dimensions of human factors and design. Ergonomics, vol. 46, no.

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[4] Karkowski, W., Genaidy, A.M., Asfour, S.S. (1990).

Computer-Aided Ergonomics. Taylor & Francis, London.

[5] Porter, J.M., Freer, M.T., Case, K. (1999). Computer Aided Ergonomics. Engineering Designer, vol. 25, no.

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[6] Feyen, R., Liu, Y., Chaffin, D., Jimmerson, G., Joseph, B. (2000). Computer-aided ergonomics: a case study of incorporating ergonomics analyses into workplace design. Applied Ergonomics, vol. 31, no. 3, p. 291-300, DOI:10.1016/S0003-6870(99)00053-8.

[7] Gill, S.A., Ruddle, R.A. (1998). Using virtual humans to solve real ergonomic design problems. Proceedings of IEEE International Conference on Simulation, Conference Publication no.457, p. 223-229.

[8] Launis, M., Lehtela, J. (1992). ErgoSHAPE: A design oriented ergonomic tool for AutoCAD. Mattila, M., Karkowski, W. (eds.), Computer Applications in

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[9] Davidoff, N.A., Freivalds, A. (1993). A graphic model of the human hand using CATIA. International Journal of Industrial Ergonomics, vol. 12, p. 255-264, DOI:10.1016/0169-8141(93)90095-U. [10] Hammadi, M., Choley, J.Y., Penas, O., Louati, J., Rivière, A., Haddar, M. (2011). Layout optimization of power modules using a sequentially coupled approach.

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[12] Kremljak, Z., Polajnar, A., Buchmeister, B. (2005).

A heuristic model for the development of production capabilities. Strojniški vestnik – Journal of Mechanical Engineering, vol. 51, no. 11, p. 674-691.

[13] Nagamachi, M. (1995). Kansei engineering: a new ergonomic consumer oriented technology for product development. International Journal of Industrial Ergonomics, vol. 15, p. 13-24, DOI:10.1016/01698141(95)90025-X. [14] Butters, L.M., Dixon, R.T. (1998). Ergonomics in consumer product evaluation: an evolving process.

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[15] Gielingh, W. (2008). Cognitive product development: A method for continuous improvement of products and processes. Strojniški vestnik - Journal of Mechanical Engineering, vol. 54, no. 6, p. 385-397.

[16] Du, S., Wu, Q., Wang, Y., Yi, Z. (2009). Study of Method for Computer Aided Ergonomics Knowledge Management and Design Aiming at Product Design.

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[18] Bratko, I. (2011). Prolog programming for artificial intelligence. Addison-Wesley, Harlow.

[19] Chein, M., Mugnier, M.L. (2009). Graph-based knowledge representation: Computational foundations of conceptual graphs. Springer, London.

[20] Awad, E. M. (2003). Building knowledge automation expert systems: With Exsys CORVID, EXSYS Inc, Albuquerque.

[21] Kuijt-Evers, L.F.M., Groenesteijn, L., De Looze, M.P., Vink, P. (2004). Identifying factors of comfort in using hand tools. Applied Ergonomics, vol. 35, 453-458, DOI:10.1016/j.apergo.2004.04.001.

[22] Cacha, C.A. (1999). Ergonomics and Safety in Hand Tool Design. Lewis Publishers, Boca Raton.

[23] Kong, Y., Lowe, B. D. (2005). Optimal cylindrical handle diameter for grip force tasks. International Journal of Industrial Ergonomics, vol. 35, no. 6, p.

495-507, DOI:10.1016/j.ergon.2004.11.003.

[24] Palčič, I., Buchmeister, B., Polajnar, A. (2010). Analysis of innovation concepts in Slovenian manufacturing companies. Strojniški vestnik – Journal of Mechanical Engineering, vol. 56, no. 12, p. 803-810.

[25] Fandiño Pita, J., Wang, Q. (2010). A simulation approach to facilitate manufacturing system design.

International Journal of Simulation Modelling, vol. 9, no. 3, p. 152-164, DOI:10.2507/IJSIMM09(3)4.162.

[26] Hackenschmidt, R., Alber-Laukant, B., Rieg, F. (2011).

Simulating nonlinear materials under centrifugal forces by using intelligent cross-linked simulations. Strojniški vestnik – Journal of Mechanical Engineering, vol. 57, no. 7-8, p. 531-538, DOI:10.5545/sv-jme.2011.013.

[27] Toma, M., Njilie, F. E. A., Ghajari, M., Galvanetto, U. (2010). Assessing motorcycle crash-related head injuries using finite element simulations. International Journal of Simulation Modelling, vol. 9, no. 3, p. 143151, DOI:10.2507/IJSIMM09(3)3.164.

Kaljun, J. – Dolšak, B.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 281-288 DOI:10.5545/sv-jme.2011.084

Paper received: 2011-04-18, paper accepted: 2012-02-14 © 2012 Journal of Mechanical Engineering. All rights reserved.

Increased Efficiency of Hydraulic Systems Through Reliability Theory and Monitoring of System Operating Parameters Jocanović, M. – Šević, D. – Karanović, V. – Beker, I. – Dudić, S. Mitar Jocanović* – Dragoljub Šević – Velibor Karanović – Ivan Beker – Slobodan Dudić Faculty of Technical Sciences, University of Novi Sad, Serbia

In the process of hydraulics systems design various software simulation systems are used. However, the increase of efficiency of the designed hydraulic systems can be achieved in two ways: by making design modifications based on reliability theory, on one hane, or based on monitoring of system operating parameters, on the other. In this paper, a case study of the improvement of a designed and implemented hydraulic system is reviewed, using those two approaches. Based on the data collected by monitoring the system operating parameters, and the system reliability analysis, it was possible to increase the efficiency of the hydraulic system either it the initial design stage, or during system realization. The result of such an approach is the hydraulic system which successfully operates 24 hours a day, without failure, which indicates that the proposed method of system analysis and improvement allows significant enhancement of hydraulic system efficiency.

Keywords: hydraulics, hydraulic system reliability, hydraulic system monitoring and efficiency

0 INTRODUCTION Design based on reliability is often very complex since, besides requiring statistical data on the component/system, it also depends on other, often insufficiently reliable factors which influence the considered hydraulic component or system. However, despite present uncertainty, throughout the hydraulic systems design process the analysis of statistical data can represent a very useful method for the increase of system availability and reliability, and, by extension, the effectiveness of hydraulic systems.

Analysis of hydraulic systems operation indicates that reliability of systems and components depends on a large number of various factors, which are often interrelated in a complex manner. These factors are: environmental influence, mechanical properties of used materials, wear, dynamic loads, material fatigue, time spent in regular exploitation, as well as the recommendations on maintenance and servicing of system/component. In their research, Fitch and Hong [1] show the influence of various factors on the reliability of components in the hydraulic system and they explicitly show this in their work. Savić et al. [2] are engaged in experimental research on the impact of variability of operating parameters (pressure, temperature, fluid viscosity) to the pressure drop in pipe systems of the hydraulic systems.

Operating conditions have a significant role in the reliability of a hydraulic component or system, which has been discussed in various sources dealing with statistics and reliability [3] and [4]. They directly lower the designed reliability. In his book, Savić et al. [3] defines certain operating parameters that affect e safe operation of hydraulic systems and components. Jocanović [4] studies the impact of

changes of working parameters in hydraulic system (change of the values of pressure, temperature and viscosity of hydraulic fluid to the number and size of solid particles that pass through the gap in the hydraulic component) and their impact on the reliability of components in the system. Herakovic [5] theoretically analyses the occurrence of opposed forces depending of specific flow rates of hydraulic valves and experimentally shows the influence of operating parameters (pressure and flow changes) to the reliable operation of hydraulic valves. Birolini [6] and Ivanović et al. [7] determine the basic formula for calculating the reliability of technical systems used in writing this paper. Moreover, reliability of a system can be increased exclusively during the designing stage, while at every other lifecycle stage the reliability tends only towards diminishment. If a particular component is replaced by a more reliable one, then one speaks about designing an improved system, i.e., such a system is considered a completely different system from the original one. Books and papers listed above are used in the theoretical analysis of reliability and experimental resolution of issues related to the influence of working parameters on the operation of hydraulic components and systems in general, and in the process of monitoring operation of hydraulic systems.

1 INITIAL DATA FOR RELIABILITY ANALYSIS OF A HYDRAULIC SYSTEM In order to review the process of hydraulic system design, and show a proper way of selecting hydraulic components, an existing machine with lower reliability record has been chosen for this experiment. Detection of critical system components has been conducted

*Corr. Author’s Address: University of Novi Sad, Faculty of Technical Sciences, Trg D. Obradovića 6, 21000 Novi Sad, Serbia, mitarj@uns.ac.rs

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by monitoring the operating parameters (pressure, temperature - kept within optimal range of 40 to 60 ºC, and flow) which are presented later in the paper.

The analysis of operation of hydraulic system has shown that, reliability-wise, all the components have been serially connected (in case of a failure of any component, the whole system stops). The hydraulic system is shown in Fig. 1. It is composed of three sub-systems: accumulation, extrusion, and filling subsystem, which operate in alternation.

Based on the data given by Fitch [1], who has defined the expected (most probable) failure rates for each hydraulic system component; it is possible to define the expected reliability of each of the subsystems within a considered hydraulic system. The values of failure rate for the components of the example system are presented in Table 1, as given by [1].

of random failures (system reliability can be described with exponential distribution law) which holds that [6] and [7]:

Analysis of reliability has been conducted under the assumption that the system is operating in the period

27.4

(1)

where R is system reliability, λ system failure rate (for the exponential distribution function, failure rate is constant or nearly constant value), t system operational time (in Nomenclature Section symbols used in this paper are presented).

Based on the data from the literature [1], it is possible to calculate expected failure rate, λ for every component in a hydraulic system. Using thus calculated λ, the expected reliability is derived for all three sub-systems (accumulation, extrusion, and filling sub-system) that comprise the considered hydraulic system. Using thus calculated reliability for all three sub-systems, it is possible to calculate the value for expected reliability, and mean operational time of the entire hydraulic system. For the serially arranged system components, reliability is calculated according to [6] and [7]:

2 RELIABILITY ANALYSIS OF SYSTEM IN OPERATION

STORAGE FACILITY

R = e − λ⋅t ,

FEEDER FACILITY

EXSTRUDE FACILITY 27.2

27.1

16 27.6

27.5

27.3 13 M2

U

I

12.3

12.2

12.1

20.1 U

26.1

I

26.2

U

I

26.3

26.4

20.2

10.2 11

15 14.1

14.2

9

10.1

10.3 23 17.1

17.2

21

28 M1

7 1

3.1

2 U

18

3.2

4.1

4.2

22

I

19.1

8

5

6

24

19.2

25

Fig. 1. Block diagram of the hydraulic system used for reliability-analysis-based component design

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Table 1. Reliability of constituent components within the considered hydraulic system [1] Position

Hydraulic Component (Failure Rate – marks)

(1)

(2)

(1), (5)

Gear pump (λ1, λ5)

2 to 18 6 to 13

9

Check valve (λ9, λ17, λ18)

0.112 to 10.2

6.5

1.41 to 8.13

5.88

Electro – hydraulic valve (λ11, λ15, λ23, λ26)

2.27 to 19.7

11

Gas – charged accumulator (λ13)

0.35 to 7.5

6.8

16.8 to 56

30

Ball valve (λ16, λ28)

1.11 to 7.6

4.6

Check valve – flow restrictor (λ20)

0.112 to 10.2

6.5

Filter (λ22)

0.01 to 1.62

0.79

Axial piston pump (λ3)

(3.1), (3.2) (9), (17.1), (17.2), (18)

Relief valve (λ10)

(10.1), (10.2), (10.3)

Pressure switch (λ12)

(12.1), (12.2), (12.3) 13

Proportional valve (λ14)

(14.1), (14.2) (16), (28) (20.1), (20.2) 22

Hydraulic reservoir (λ25)

25

Relief valve – with relieve of load valve (λ30)

(30.1), (30.2)

Electro – hydraulic valve (directional control) (λ31)

(31.1), (31.2)

n

R ( t ) = R1 ( t ) ⋅ R2 ( t ) Rn −1 ( t ) ⋅ Rn ( t ) = ∏ Ri ( t ) . (2) i =1

Substituting Eq. (1) into Eq. (2), yields [6] and

R (t ) = e

⋅e

⋅⋅⋅ e

− λ n −1 ⋅t

0.48 to 2.52

⋅e

− λ n ⋅t

=

 n  − ∑ λi ⋅t e  i =0  .

(3)

Eq. (3) is used for the calculation of reliability for all three sub-systems in particular, by substituting each failure rate value, λi, with the corresponding value from Table 1.

The time interval, t, is taken from the range of values: 1, 2, 3, 6, 9, 12, 18, 24, 30, and 36 months of operation, that is, 730, 1460, 2190, 4380, 6570, 8760, 13140, 17520, 21900, and 26280 hours for each particular sub-system.

Mean times of hydraulic system components listed in the fourth column of Table 1 are taken for failure rates (λi).

The calculated expected reliabilities for each of the sub-systems, for the time interval of t = 730 hours (1 month) are stated below: a) Filling sub-system R(730) = 0.957, b) Extrusion sub-system R(730) = 0.781,

1.5 0.008

Gear pump with frequency regulation (λ29)

(29.1), (29.2)

− λ 2 ⋅t

1

Hydraulic cilinders (λ27) redesigned hydraulic system

(27.1), (27.2), (27.3), (27.4), (27.5), (27.6),

− λ1 ⋅t

20

Radiator for cooling (λ21)

21

10

10

Pressure compensated relief valve (λ19)

(19.1), (19.2)

[7]:

13

Electrical motor (λ2, λ4, λ6)

(2), (4.1), (4.2), (6)

(11), (15), (23), (26.1), (26.2), (26.3), (26.4)

Failure Rate (×10-6) Range of Value Mean (3) (4)

13 0.224 to 14.1

5.7

1.81 to 7.22

4.6

c) Accumulation sub-system R(730) = 0.976.

The same is applied to the calculation of other time intervals.

The values above indicate that the system’s designer has made a mistake when selecting the system components since one of the sub-systems has got incomparably lower reliability, thus significantly degrading the potential performance of the whole system. In other words, the other two sub-systems feature high quality and costly components which boost system price without significantly improving its performance.

The charts of change of each component’s reliability for every time interval are shown in Figs.

2 to 4.

Total reliability of the hydraulic system has been calculated and charted (Fig. 5) for the considered filling, extruding, and accumulating sub-systems.

Analysis of the charts leads to the conclusion that the sub-system for extrusion represents the least reliable sub-assembly within the hydraulic system (Figs. 3 and 5), i.e. the components with failure rates λ3, λ14, and λ19, exhibit highest unreliability.

Increased Efficiency of Hydraulic Systems Through Reliability Theory and Monitoring of System Operating Parameters

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Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 281-288

Fig. 2. The change in reliability of hydraulic components of the filling sub-system

unreliable components - in this case, the axial piston pump with proportional pressure relief valve, and the proportional distributing valve. The replacement of these components would allow: • increment of extrusion sub-system’s reliability, • satisfying the system’s functional and efficiency requirements.

The expected mean operational time of the system from Fig. 1 is calculated according to following formula, [6] and [7]: Tw =

1 , λ

(4)

and, for the total system failure rate of λ = 4.41×10–4, it equals Tw = 1/ λ = 2268.96.

Thus derived mean operational time represents a theoretical value which indicates that, disregarding the adverse effects of human and process influence, the re-designed hydraulic system should have an average operational time of more than 90 days.

Fig. 3. The change in reliability of hydraulic components of the extruding sub-system

Fig. 4. The change in reliability of hydraulic components of the accumulating sub-system

Fig. 5. Total reliability of the three sub-systems of the considered hydraulic system

Based on these results, the logical conclusion would be to redesign the extrusion sub-system in order to increase its reliability, and to replace 284

3 MONITORING THE SYSTEM IN OPERATION During the operation of the considered hydraulic system, frequent failures have been observed every two hours of operation. The failures observed do not correspond to the calculated mean operational time (Tw ≈ 2269 hours), which indicates a serious fault in the hydraulic system. This extreme deviation from the expected mean operational time occurs due to extrusion system overload, i.e. the axial piston pumps, 3.1 and 3.2, suffer from overheating, as well as the electro motors which drive them – position 4. Extruder cylinders, 27.1 and 27.2, are coupled, which means that when the cylinder 27.1 performs its working stroke, the cylinder 27.2 performs its return stroke, and vice versa. Within the two hour interval the entire hydraulic system suffered from failure which lasted for two hours - as was required by the pump and motors to cool down – which reduced the hydraulic system’s productivity by 50%. It should be noted that the system is designed to operate 24 hours a day, and breaks are allowed just for the simple replacements of particular components or for preventive actions. Based on the results of reliability analysis of the extruding sub-system components - which indicate that the most critical components in the system are piston axial pump, the proportional pressure relief valve, and the proportional distributing valve – pressure and flow monitoring have been performed in the entire extrusion sub-system, in order to establish the parameters which influence disturbances, as well as the pressure drops in the system [2] to [5]. Monitoring and recording of

Jocanović, M. – Šević, D. – Karanović, V. – Beker, I. – Dudić, S.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 281-288

pressure change have been performed at two locations in the hydraulic sub-system of the extruder: after the pump (3.2) at proportional distributing valve (14.2) fluid inlet – measurement spot M1, and after the proportional distributing valve (14.2) at working piston (27.2) fluid inlet – measurement spot M2.

and its pump, is shown in Fig. 8. Based on the data partially presented in this work, it can be concluded that the extruder hydraulic sub-system’s design is flawed.

Fig. 6. Change of operating parameters (p1, p2, Q1 and ∆p1) in the extruder hydraulic system Fig. 8. Pressure p1 peaks to 3421 bar within the 10 ms interval

Fig. 7. Change of pressures p1 and p2 in the extruder hydraulic system within a 20 ms time interval

The recorded working parameters (p1 [bar] – pump pressure, p2 [bar] – cylinder pressure, Q1 [l/min] – flow, and ∆p1 [bar] – pressure drop between pump and cylinder) have been charted and shown in Figs.

6 to 8.The hydraulic system requires a constant fluid feed with relatively small pressure fluctuations during the operation. However, the monitoring of pressure and flow in the extrusion hydraulic sub-system has revealed substantial oscillations of pressure and flow.

These oscillations cause increase in temperature of working fluid, working components (primarily the drive pump), while also causing overheating of the drive electro motors. The pressure peaking to 3421 bar, which exceeds the allowed values for this system

That is, having the primary system requirement in mind, i.e. the necessity for constant flow distribution from the pump to the actuators, and constant operating pressure in all actuators (cylinders 27.1 and 27.2), the design concept has been completely wrong.

Comparing the reliability analyses for the system in hand, as well as the monitoring performed on the system in full operation, the following can be concluded: the axial piston pump in the system provides the required capacity in the extruder hydraulic system, but does not require the capacity regulation by the proportional pressure valve, as shown in Fig. 1.

In addition, the proportional distributing valve has been used in an entirely wrong way, bearing in mind that the extruder sub-system requires maximum fluid quantity to supply the actuator. This leads to a conclusion that instead of proportional distributing valve, a classical electric distribution valve should be used in either fully closed or fully open working position.

An analysis of reliability has shown that the proportional valve is the weakest component not only in the extruder hydraulic sub-system, but also in the entire hydraulic system. The re-design of the original hydraulic system, as well as the replacement of unreliable components by more reliable ones, leads to a hydraulic system of higher reliability and efficiency rate (Fig. 9).

Increased Efficiency of Hydraulic Systems Through Reliability Theory and Monitoring of System Operating Parameters

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4 DESIGN OF A NEW SYSTEM The results obtained through the monitoring of the hydraulic system’s operating parameters have been used in the design of the improved system (Fig. 9), as well as the results of analysis of expected reliability for the considered hydraulic system. The result has revealed the following critical components in the subsystem for extrusion: the axial piston pump (position 3), the proportional pressure relief valve (position 19), and the proportional distributing valve (position 14). These components have been replaced. The axial piston pump (positions 3.1 and 3.2) has been replaced by a gear pump with frequency regulation (positions 29.1 and 29.2); the proportional pressure relief valve (positions 19.1 and 19.2), has been replaced by an internal pressure relief valve (positions 30.1 and 30.2); finally, the proportional distributing valve (positions 14.1 and 14.2) has been replaced by a classical electrohydraulic distribution valve (positions 31.1 and 31.2), Fig. 9. The failure rate values have also been taken from Table 1 for the analysis of expected reliabilities of the considered elements. The analysis of reliability

STORAGE FACILITY

for the newly designed hydraulic system has been performed with an additional assumption that the random variable (operational time) varies according to exponential distribution law (Eq. (1)).

Further analysis of expected reliability of the improved hydraulic system has been performed in the same fashion as the original hydraulic system, using Eqs. (2) and (3). Expected reliability of all components of the three sub-systems has been calculated for the improved system (Fig. 9).

Example: for the t = 730 hours (1 month) time interval, the reliability for all three sub-systems equals: a) Filling sub-system R(730) = 0.957, b) Extrusion sub-system R(730) = 0.955, c) Accumulation sub-system R(730) = 0.976.

Calculation for all other time intervals is performed in the same manner. The charts showing a change in the reliability of each component of the hydraulic system are presented in Figs. 10 to 12, for all time intervals provided by the analysis.

Reliability of the entire hydraulic system is charted in Fig. 13, where all three sub-systems

FEED FACILITY

EXSTRUDE FACILITY

27.4

27.5

27.2

27.1

16 27.6

27.3 13

12.1

U

I

10.2 11

31.1

12.3

12.2

9

15

31.2

18 U

10.1

I

U

I

30.1

30.2

17.1

20

23

17.2

21

7 1

29.1

2 U

4.1

29.2

4.2

22

I

8

5

6

24

25

Fig. 9. Block diagram of the improved hydraulic system

286

Jocanović, M. – Šević, D. – Karanović, V. – Beker, I. – Dudić, S.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 281-288

(filling, extruding, and accumulation) define the total reliability of the hydraulic system from Fig. 9.

The mean operational time for the improved system equals Tw = 1/ λ = 5811.99 h which indicates an increase of 156% in comparison with the original system. After redesigning, the hydraulic system was again put into a 24-hour operating regime.

Fig. 10. The change in reliability of hydraulic components of the re-designed filling sub-system

From the standpoint of reliability engineering, the analysis of improved hydraulic system leads to the conclusion that the extruder remains the most unreliable component, i.e. the least reliable assembly of the hydraulic system (Fig. 13).

Fig. 11. The change in reliability of hydraulic components of the re-designed extruding sub-system

However, compared to the original system, the reliability gain of 17.43% represents a significant improvement.

The replacement of axial piston pump by a gear pump, as well as the replacement of proportional distributing valve with a classical electro-hydraulic valve, has lead to the increase of total reliability of the entire hydraulic system compared to the original solution.

Fig. 12. The change in reliability of hydraulic components of the re-designed accumulating sub-system

Fig. 13. Reliability of all three assemblies of the re-designed hydraulic system

During exploitation, failures due to overheating were completely eliminated (compared to previous situation when they occurred in 2 hour intervals).

Other types of failures have not been observed to this moment. This can be attributed to improved reliability, as well as to the preventive system maintenance procedure, which has significantly reduced the negative influence of humans and working conditions. The only observed down times in the system are due to machine setups to accommodate a change of production type.

5 CONCLUSION Analysis of results obtained by a combined application of the reliability theory and monitoring of operating parameters of the considered hydraulic system has lead us to the following conclusions: • The reliability theory helps identifying the weakest – most critical components in a hydraulic system (which can also be concluded by monitoring the operation of a hydraulic system and analyzing failure data). However, system monitoring alone allows neither determination of deviation between the expected and real reliability of all system components, nor the influence of lower reliability on the reliability of the entire hydraulic system. Reliability analysis estimates reliability of system components, and their ranking and charting, as shown in Figs. 2 to 4 and 10 to 12, • monitoring of operating parameters enables the identification of down-time causes and failures of particular system components (monitoring of operating system parameters, such as the

Increased Efficiency of Hydraulic Systems Through Reliability Theory and Monitoring of System Operating Parameters

287

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, 281-288

pressure, flow, working fluid temperature, key components temperature, etc. reveals the most critically loaded system components, as well as the values of operating parameters, which can lead to component down-times or failures), • adequate replacement of critical elements in a hydraulic system, accompanied by a proper selection of components, not only increases the total system reliability, but also prolongs the mean operational time (i.e., system productivity and efficiency), • continual monitoring of operating parameters in a hydraulic system, i.e. automated control of system operating parameters, makes it possible to predict and influence system reliability.

The general conclusion that can be drawn is that reliability analysis is useful in conjunction with the monitoring of operating parameters of hydraulic system and its components during exploitation, as well as during the designing of a novel hydraulic system.

Unfortunately, designing hydraulic systems based on reliability analysis is still not widespread enough.

This can be attributed to the fact that in many cases the designers lack sufficient knowledge of the features and advantages of reliability analysis in the design process. The monitoring of operating parameters of a hydraulic system in conjunction with reliability analysis allows the critical system components to be singled out and improved in terms of reliability and efficiency, once the hydraulic system is designed and put into operation.

With this in mind, this paper should draw the hydraulic systems designers’ attention towards reliability analysis, which has the potential to increase efficiency of hydraulic systems.

288

6 NOMENCLATURE p pressure [bar] ∆p pressure drop [bar] Q flow [l/min] R system reliability [-] t operational time [h] TW mean operational time [h] λ failure rate [h-1] 7 REFERENCES [1] Fitch, E.C., Hong, I.T. (2004). Hydraulic System

[2]

[3] [4]

[5]

[6] [7]

Design for Service Assurance. BarDyne, Inc., Stillwater.

Savić, V., Knežević, D., Lovrec, D., Jocanović, M., Karanović, V. (2009). Determination of pressure losses in hydraulic pipeline systems by considering temperature and pressure. Strojniški vestnik - Journal of Mechanical Engineering, vol.

55, no. 4, p. 237-243.

Savić, V., Zirojević, Lj. (2003). Oil hydraulics 3.

IKOS, Novi Sad. (in Serbian) Jocanović, M. (2010). Approach to research and define the model for the calculation of flow of solid particles with a mass of mineral oil through the gaps in a function of the constructive operating parameters of hydraulic components, PhD dissertation, University of Novi Sad, Novi Sad. (in Serbian) Herakovič, N. (2009). Flow – force analysis in a hydraulic sliding-spool valve. Strojarstvo, vol. 51, no. 6, p. 555-564.

Birolini, A. (2010). Reliablity Engineering – Theory and Practice, 6th ed.. Springer, Berlin.

Ivanović, G., Stanivuković, D., Beker, I. (2010).

Reliability of technical systems. Faculty of Technical Sciences Publishing, Novi Sad. (in Serbian)

Jocanović, M. – Šević, D. – Karanović, V. – Beker, I. – Dudić, S.

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4 Vsebina

Vsebina Strojniški vestnik - Journal of Mechanical Engineering letnik 58, (2012), številka 4 Ljubljana, april 2012 ISSN 0039-2480 Izhaja mesečno

Uvodnik

SI 47

Razširjeni povzetki člankov Anton Bergant, Arno Kruisbrink, Francisco Arregui: Dinamični odziv zračnih ventilov v preizkusnem cevovodu velikih izmer Boštjan Gregorc, Andrej Predin, Drago Fabijan, Roman Klasinc: Eksperimentalna analiza vpliva delcev na kavitacijski tok Andrej Kryžanowski, Matjaž Mikoš, Jakob Šušteršič, Velimir Ukrainczyk, Igor Planinc: Preiskave abrazijske odpornosti betonov na vodnih zgradbah na spodnji Savi Mario Krzyk, Roman Klasinc, Matjaž Četina: Dvodimenzijsko matematično modeliranje porušitvenega vala v ozki strmi strugi Tomaž Šolc, Aneta Stefanovska, Trevor Hoey, Matjaž Mikoš: Študija obrusa kamnin v abrazijskem mlinu z uporabo opremljenega sledila Jasmin Kaljun, Bojan Dolšak: Povečanje ergonomske vrednosti izdelka z uporabo inteligentnega svetovalnega sistema Mitar Jocanović, Dragoljub Šević, Velibor Karanović, Ivan Beker, Slobodan Dudić: Izboljšanje učinkovitosti hidravličnih sistemov z uporabo teorije zanesljivosti in nadzora obratovalnih parametrov sistema Osebne vesti Magistrsko delo in diplome

SI 49 SI 50 SI 51 SI 52 SI 53 SI 54 SI 55

SI 56

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4 Uvodnik

Gostujoči uvodnik Tematska številka: Hidravlično inženirstvo Slovensko društvo za hidravlične raziskave (SDHR) je prostovoljno združenje, ki spremlja in vzpodbuja strokovno in znanstveno-raziskovalno delo hidravličnih raziskav na področju gradbeništva, strojništva in okoljskega inženirstva. Ustanovljeno je bilo leta 1994 in ima danes 89 aktivnih članov. Zastavljene cilje dosega z uresničevanjem naslednjih nalog: izobraževanje članov društva na področju hidravličnih raziskav in s tem povezanih znanstvenih področij, sodelovanje pri pripravi in dajanje pobud za spremembo predpisov s področja hidrotehnike, sodelovanje pri pripravi ustreznih standardov na področju hidravlike ter organizacija strokovnih srečanj in ekskurzij. Delo društva je javno in odprto za koristne pobude, zato ima razvejano sodelovanje z drugimi sorodnimi domačimi in mednarodnimi organizacijami. Omenimo samo nekatere izmed njih: Slovensko društvo za velike pregrade – SLOCOLD, Društvo vodarjev Slovenije in Mednarodno združenje za hidravlične raziskave – IAHR. Ena od pomembnih nalog društva je tudi publiciranje strokovnih dosežkov, s čimer želimo preko strokovnih in javnih glasil svoje delo in novosti v stroki predstaviti tako domačim in tujim strokovnjakom kot tudi širši javnosti.

Zato smo se odločili, da naredimo izbor strokovnih predavanj, organiziranih s strani SDHR, od leta 2004 naprej.

Izbrali smo Strojniški vestnik - Journal of Mechanical Engineering, ki ima razvejano mednarodno izmenjavo in je indeksiran v številnih bazah podatkov. Pri tem se gostujoči uredniki tako nastale tematske številke želimo posebej zahvaliti glavnemu uredniku Strojniškega vestnika - Journal of Mechanical Engineering prof. dr. Vincencu Butali in tehnični urednici revije ge. Piki Škraba, ki sta nam nudila vso potrebno strokovno, organizacijsko in finančno pomoč.

Za objavo je bilo izbranih pet prispevkov na osnovi naslednjih predavanj: (1) Dinamični odziv zračnih ventilov v preizkusnem cevovodu velikih izmer (2) Eksperimentalna analiza vpliva delcev na kavitacijski tok (3) Preiskave abrazijske odpornosti betonov na vodnih zgradbah na spodnji Savi (4) Dvodimenzijski matematični model porušitvenega vala v ozki strmi strugi (5) Študija obrusa kamnin v abrazijskem mlinu z uporabo opremljenega sledila Zahvaljujemo se tudi recenzentom za njihove odlične recenzije prispevkov.

Ljubljana, april 2012

Gostujoči uredniki: Anton Bergant, Matjaž Četina, Matjaž Mikoš

Zahvala Zaradi finančnega varčevanja je v marcu 2012 Izdajateljski svet Strojniškega vestnika – Journal of Mechanical Engineering (SV-JME) ukinil mesto pomočnika urednika ter posledično sporazumno razrešil dosedanjega pomočnika urednika izr. prof. dr. Boruta Buchmeistra.

Kot pomočnik urednika od oktobra 2009 je dr. Borut Buchmeister dal svoj pomemben in neizbrisen prispevek k povečanju ugleda revije SV-JME, ki se kaže tudi v kakovostnih vsakokratnih številkah. Bil je predan reviji, kakor tudi uredništvu SV-JME ter vedno voljan s svojimi nasveti in delom pomagati.

Zahvaljujemo se dr. Borutu Buchmeistru za njegov doprinos k razvoju revije SV-JME ter mu želimo veliko uspeha kot glavnemu in odgovornemu uredniku revije International Journal of Simulation Modelling.

Glavni in odgovorni urednik: Vincenc Butala SI 47

SI 48

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 49

Prejeto: 2011-02-04, sprejeto: 2011-09-14 © 2012 Strojniški vestnik. Vse pravice pridržane.

Dinamični odziv zračnih ventilov v preizkusnem cevovodu velikih izmer Anton Bergant, A. – Kruisbrink, A. – Arregui, F. Anton Bergant1,* – Arno Kruisbrink2 – Francisco Arregui3 Litostroj Power d.o.o., Slovenija Univerza Nottingham, Velika Britanija 3 Politehniška univerza Valencia, Španija 1

2

Prispevek obravnava dinamični odziv zračnih ventilov v hidravličnih cevnih sistemih. Glavni cilj prispevka je poglobljena analiza rezultatov originalnih meritev dinamičnega odziva zračnega ventila na eksperimentalni postaji velikih izmer v Delftu v okviru EU projekta Dynamic behaviour of air valves.

Preizkusi neustaljenega toka so bili izvedeni v sistemu velikih izmer, ker predhodni preizkusi kvazi-ustaljenega toka v sistemu malih izmer niso dovolj dobro zajeli fizikalnih pojavov pri dinamičnem odzivu zračnega ventila v industrijskem sistemu. Preizkusili smo kinetične zračne ventile s plavačem nazivnega premera 50 in 100 mm v cevovodih premera 200 in 500 mm. Preizkus gradnikov v geometrijsko in hidravlično podobnih sistemih vodi k poglobljenemu razumevanju dinamičnih pogojev podobnosti in iz njih izpeljanih podobnostnih števil.

Eksperimentalni program je zajel štiri tipe preizkusov toka v zračnih ventilih: 1) ustaljen zračni tok, 2) neustaljeno odzračevanje (odvajanje zraka iz cevovoda), 3) neustaljeno zračenje (dovajanje zraka v cevovod), in 4) neustaljeno zračenje in odzračevanje pri vodnem udaru. Med preizkusi so bili zračni ventili nameščeni na geodetsko najvišjem odseku preizkusnega cevovoda. Dinamični odziv ventila smo raziskali pri različnih velikostih pospeška kapljevinske mase v smeri gorvodnega toka (odzračenje cevovoda) in v smeri dolvodnega toka (zračenje cevovoda).

Na ta način smo simulirali tokovne razmere, ki nastopijo v industrijskih sistemih pri zagonu in izklopu črpalke, zapiranju varnostnega zapirala, ali pa pri porušitvi cevovoda.

Podajamo rezultate meritev dveh tipičnih preizkusov: 1) neustaljeno odvajanje zraka iz cevovoda in 2) neustaljeno dovajanje in odvajanje zraka iz cevovoda pri vodnem udaru s prekinitvijo kapljevinskega stebra.

Raziskave dinamičnega odziva zračnega ventila pri nekontroliranem odvajanju zraka iz cevovoda so pokazale, da lahko zaprtje zračnega ventila povzroči ekstremne nadtlake v sistemu (reda velikosti 10 barov) in ujetje večje količine zraka v cevovodu. Nadtlak lahko močno poškoduje plavač ventila (udarna obremenitev), ujeti zrak pa povzroči nihanje kapljevinskega stebra v cevovodu in povečane energijske izgube med nadaljnjim obratovanjem sistema.

Raziskave odziva zračnega ventila pri preizkusih vodnega udara s prekinitvijo kapljevinskega stebra so pokazale, da se ventil odpre z zakasnitvijo glede na padec tlaka v sistemu. Čeprav je zakasnitev odprtja ventila relativno kratka (reda velikosti 0,01 s), zračni ventil ne more v celoti preprečiti pojava prehodne parne kavitacije v cevovodu. Na mestu pretrganja kapljevinskega stebra smo zaznali diskretno parno kavitacijo (volumen po celotnem pretočnem preseku), vzdolž cevovoda pa kontinuirani parni kavitacijski tok (mehurčkasti tok). Faza zakasnitve odprtja ventila je pomembna v cevovodih velikih izmer, ki niso dimenzionirani na podtlak. Po odprtju ventila se prehodna parna kavitacija zaduši, tlačni sunki pri zaprtju ventila pa se znatno znižajo.

Dinamični odziv ventila ni trenuten kot je postavljeno v standardnih računskih modelih v literaturi, kjer se ventili modelirajo s statičnimi pretočnimi karakteristikami. Ta postavka velja le pri blagih spremembah hidrodinamičnih veličin. Temu pa ni tako v obravnavanih primerih pretrganja kapljevinskega stebra in v zaključni fazi odzračevanja sistema.

Preizkusi z in brez zračnega ventila v sistemu pokažejo, da ustrezno zasnovan, dimenzioniran in vgrajen ventil deluje kot blažilec vodnega udara v sistemu.

Izsledke raziskav bomo uporabili pri razvoju novih zračnih ventilov, postavitvi naprednih teoretičnih modelov ventila in njihovi verifikaciji.

Ključne besede: zračni ventili, preizkuševališče velikih izmer, preizkus neustaljenega toka, dovajanje zraka, odvajanje zraka, vodni udar in prekinitev stebra

*Naslov avtorja za dopisovanje: Litostroj Power d.o.o., Litostrojska 50, 1000 Ljubljana, Slovenija, anton.bergant@litostrojpower.eu

SI 49

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 50

Prejeto: 2011-03-17, sprejeto: 2011-10-21 © 2012 Strojniški vestnik. Vse pravice pridržane.

Eksperimentalna analiza vpliva delcev na kavitacijski tok

Gregorc, B. – Predin, A. – Fabijan, D. – Klasinc, R. Boštjan Gregorc1,* – Andrej Predin2 – Drago Fabijan3 – Roman Klasinc4 1 Dravske

elektrarne Maribor, Slovenija v Mariboru, Fakulteta za energetiko, Slovenija 3 Litostroj Power d.o.o., Slovenija 4 Tehnična univerza v Gradcu, Avstrija

2 Univerza

Obratovanje hidravličnih črpalk in turbin z rečno vodo pogosto spremljajo spremembe kakovosti tekočine. Pri uporabi naravnih vodotokov se spreminja masni delež disperznih trnih delcev, kar ima lahko za posledico razvito abrazijo in kavitacijsko erozijo.

Analize vpliva delcev na razvoj kavitacijske erozije in njene posledice so eksperimentalno težje izvedljive, saj so metode zaznavanja nastanka kavitacije v osnovi razvite za tekočine brez delcev. Namen prispevka je analizirati in prikazati vpliv delcev na začetno kavitacijo in na spremembe hidravličnih sil kot posledico kavitacijskega toka.

Meritve smo izvajali na manjšem kavitacijskem tunelu, kjer smo nastanek kavitacije spremljali na osamljenem hidravličnem krilu. Uporabili smo tri različne masne deleže delcev podobnih lastnosti (premer, gostota, masni delež), kot jih najdemo v rečnih vodah. S spremembo tlaka na vstopu smo spreminjali kavitacijsko število.

Uporabljene so bile različne vtočne hitrosti suspenzije ter dva nastavna kota hidravličnega krila.

Vse eksperimentalne meritve so bile izvedene pri temperaturi suspenzije 20 °C. Spremembe vzgonske in uporovne sile na vrtljivo vpetem hidravličnem krilu smo spremljali z merjenjem torzijskega momenta. Nastanek parne faze na iniciatorju kavitacije smo spremljali z meritvijo hrupa, saj so bile ostale metode za ta namen neuporabne (npr. vizualizacija). Vse rezultate meritev smo primerjali z rezultati vode brez delcev.

Iz analize izvedenih meritev je razvidno, da se s povečevanjem masnega deleža delcev povečuje tudi parna faza na sesalni strani krila.

Meritve potrjujejo povečanje torzijskega momenta v vseh primerih uporabljenih vstopnih hitrosti. Analiza vpliva delcev na nastanek kavitacije je pokazala, da se parna faza na krilu oblikuje pri razmeroma enakih vrednostih kavitacijskega števila, pri čimer pa je razvita parna faza v primeru prisotnosti disperznih delcev intenzivnejša.

Ocenjujemo, da je sprememba začetnega kavitacijskega števila pri največjem uporabljenem masnem deležu delcev 3,6%. Prav tako se zaradi delcev poveča standardni odklon v fazi nastanka parne faze, kar potrjuje večjo intenzivnost razvoja parne faze v primeru dodanih delcev. Frekvenčna analiza hrupa je pokazala, da se z dodajanjem delcev poveča amplituda hrupa v območju frekvence 315 Hz. Eksperimentalna analiza je bila opravljena le z enim premerom delcev, prav tako pa je bila omejitev tudi v uporabljenem največjem masnem deležu delcev. Glede na rezultate bi bilo smiselno razširiti eksperimentalne raziskave na področje različnih premerov delcev in povečanih masnih deležev delcev.

Za potrebe teoretičnega zapisa nastanka parne faze bi bilo smiselno tudi eksperimentalno izmeriti razliko hitrosti med suspenzijo z delci in vodo brez delcev v coni nastanka kavitacije.

Na podlagi predstavljenih raziskav je pri določitvi neto pozitivne sesalne višine za črpalke in kavitacijskega števila za turbine smiselno upoštevati povečanje intenzivnosti razvoja parne faze v primeru večjega masnega deleža trdnih delcev v suspenziji.

Ključne besede: delci, kavitacija, hrup, meritve

SI 50

*Naslov avtorja za dopisovanje: Univerza v Mariboru Fakulteta za strojništvo, Smetanova 17, Maribor, Slovenija, bostjan.gregorc@dem.si

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 51

Prejeto: 2010-10-20, sprejeto: 2012-03-14 © 2012 Strojniški vestnik. Vse pravice pridržane.

Preiskave abrazijske odpornosti betonov na vodnih zgradbah na spodnji Savi Andrej Kryžanowski1,* – Matjaž Mikoš1 – Jakob Šušteršič2 – Velimir Ukrainczyk3 – Igor Planinc1 1 Univerza

v Ljubljani, Fakulteta za gradbeništvo in geodezijo, Slovenija za raziskavo materialov in aplikacije - IRMA, Slovenija 3 Mostprojekt, Hrvaška

2 Inštitut

Eden poglavitnih problemov, s katerimi se srečujemo pri vzdrževanju vodnih objektov, je odpornost betonov na izpostavljenih delih konstrukcij glede na rušilne procese, povzročene z vodnim tokom. Problem škodljivega delovanja vodnega toka je večplasten: prvič, povzroči obrabo, poškodbe na površini konstrukcij in s tem posledično materialno škodo s povečanjem obratovalnih stroškov, sanacijskimi ukrepi in izgubljeno proizvodnji v hidroenergetiki; in drugič, z večjim obsegom obrabe in poškodb se spremenijo hidravlični pogoji in s tem se posledično motnje v delovanju vodnega toka širijo tudi zunaj osnovnega obsega, kar lahko končno povzroči nenadzorovane hidravlične procese dolvodno od objekta. Posledice lahko znatno presegajo osnovne stroške za njihovo odpravo in se s širjenjem negativnih vplivov dolvodno tudi skokovito povečujejo. Predmet obravnave v prispevku so poškodbe betonskih površin na vodnih objektih, ki so posledica delovanja procesov abrazije. S pojmom abrazijska erozija označujemo proces razgradnje trdne podlage, ki je posledica delovanja vodnega toka in z vodo nošenih trdnih delcev na trdno podlago. V praksi se je uveljavila vrsta eksperimentalnih metod, ki s potekom preskusa v vodnem mediju in ob prisotnosti abrazivnega sredstva bolj ali manj realistično ponazarjajo proces abrazije. Cilj vseh eksperimentalnih metod je ta, da v realnem eksperimentalnem času pridobimo dovolj relevantne parametre za oceno kakovosti materiala. Najbolj reprezentativne rezultate dobimo s preskusom v naravi, vendar ob pogoju, da so razmere, v katerih poteka raziskava, enake tistim, katerim bo izpostavljen tudi načrtovani objekt.

Značilno za tovrstne preskuse je to, da so dolgotrajni, ker naravnega procesa ni mogoče pospešiti. Interpretacija rezultatov je možna zgolj ob poznavanju hidroloških parametrov (pretok, hitrost, transportne količine, struktura, tekstura, način gibanja delcev). V prispevku obravnavamo problematiko abrazijske odpornosti betonskih oblog na hidroelektrarnah na spodnji Savi. Za izhodišče smo privzeli betonsko sestavo abrazijsko odpornih oblog na HE Vrhovo, ki smo jo z različnimi dodatki (jeklena vlakna, polipropilenska vlakna, mineralni dodatek, polimerna veziva, gumeni agregat…) spremenili z namenom doseči čim boljše mehanske lastnosti preskusnih betonov in ustrezno izvedljivost, kar pomeni, da jih je mogoče vgraditi tudi brez posebnih predpriprav. Fizikalne lastnosti svežega in strjenega betona smo dokazovali s standardiziranimi postopki, na podlagi katerih smo dobili relevantne parametre (tlačna trdnost, natezna/upogibna trdnost, modul elastičnosti, itd.), s katerimi smo posredno dokazovali tudi odpornost na obrabo. Abrazijsko odpornost betonov smo dokazovali po postopku, ki ga predpisuje standard ASTM C 1138. Princip metode je naslednji: betonski preskušanec je v valjasti posodi izpostavljen abrazivnemu delovanju 70 jeklenih krogel različnih dimenzij (od 13 do 25 mm), rinjenih s krožnim vodnim tokom, ki ga s predpisanim številom obratov ustvarja posebno mešalo. Krogle so ves čas raziskave v stiku z betonsko površino.

Po 72 urah, kolikor traja preskus, izračunamo povprečno globino obrabe, ki jo dobimo iz razmerja erodiranega volumna in površine preskušanca. Eden od ključnih ciljev raziskovalnega dela je bila analiza uporabnosti metode po postopku ASTM C 1138 za določitev abrazijske odpornosti betonskih oblog na prelivnih poljih hidroelektrarn na spodnji Savi in uporabnost metode za napovedi degradacijskih procesov na prelivnih poljih hidroelektrarn na Savi. Uporabnost metode smo ocenili s primerjavo meritev v laboratoriju in meritev, opravljenih na poskusnih poljih na prelivnem polju hidroelektrarne Vrhovo. Za ta namen smo v podslapju prelivnega polja vgradili poskusna polja z laboratorijskimi betonskimi sestavami dimenzij 2,5 m, ki smo jih za dobri 2 leti prepustili normalnim obratovalnim razmeram. V času izvajanja raziskave smo spremljali vse relevantne karakteristike obratovalnih razmer (pretoki, delovanje podslapja, obratovalni manevri zapornic, itd.) in na osnovi teh izvedli oceno količine transporta plavin v času obratovanja prelivnega polja. Ključne ugotovitve raziskave so naslednje: (a) potrjeno je dobro ujemanje med rezultati preiskave abrazijske odpornosti po postopku ASTM C 1138 in meritev v naravi, in s to ugotovitvijo je tudi potrjena ustreznost metode za napovedi abrazijskih procesov in dinamike obrabe betonskih površin na jezovnih zgradbah na Savi; (b) beton z dodatkom gumenega agregata je izkazal visoko abrazijsko odpornost, kakor tudi, da se s starostjo betonov odpornost proti obrabi povečuje; (c) za kvantifikacijo napovedi abrazijskih procesov na pregradah bo treba vzpostaviti redno spremljanje pretoka rinjenih plavin na spodnji Savi.

Ključne besede: abrazijska odpornost, obraba, laboratorijski poskusi, poskusi v naravi *Naslov avtorja za dopisovanje: Univerza v Ljubljani, Fakulteta za gradbeništvo in geodezijo, Hajdrihova 28, 1000 Ljubljana, Slovenija, andrej.kryzanowski@fgg.uni-lj.si

SI 51

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 52

Prejeto: 2010-10-20, sprejeto: 2012-03-16 © 2012 Strojniški vestnik. Vse pravice pridržane.

Dvodimenzijsko matematično modeliranje porušitvenega vala v ozki strmi strugi Krzyk, M. – Klasinc, R. – Četina, M. Mario Krzyk1 – Roman Klasinc2 – Matjaž Četina1 1 Univerza v Ljubljani, Fakulteta za gradbeništvo in geodezijo, Slovenija 2 Tehnična univerza v Gradcu, Inštitut za hidrotehniko in upravljanje z vodnimi viri, Avstrija

Posledice porušitev pregrad so največkrat katastrofalne, zato je postalo določanje maksimalnih kot vodne gladine ter hitrosti toka in čela vala obvezni del projektne dokumentacije že v fazi načrtovanja velikih pregrad. Pri obravnavi tokov v strmih gorskih strugah se običajno uporabljajo enodimenzijski matematični modeli, čeprav je polje hitrosti v primerih močno hrapavega dna izrazito tridimenzijsko. Ker se porušitveni val širi po naravnih dolinah, je treba v matematičnem modelu zajeti tudi vplive ukrivljenosti in nepravilne geometrijske oblike struge, kjer v ovinkih prihaja do izrazite razlike v gladini na nasprotnih bregovih.

Pri analizi tokov v strmih ukrivljenih strugah je treba upoštevati posebnosti toka v strugah z velikim padcem dna. Zlasti pomembna je povezava med globino, hitrostjo toka in značilnostmi dna struge, s čimer je določena izguba energije toka. Te vrednosti običajno temeljijo na bolj ali manj empiričnih povezavah. Z upoštevanjem rezultatov novih terenskih raziskav toka v strmih strugah, ki zajemajo tudi primere s padci dna struge, večjimi kot 60%, je bilo razvitih nekaj novih empiričnih enačb. Enačbe večinoma upoštevajo povprečno hitrost toka v odvisnosti od pretoka, padca dna in velikosti značilnega zrna posteljice struge. Obliko nekaterih enačb je možno prilagoditi tako, da postanejo podobne Manningovi enačbi za izračun energijskih izgub. Od novejših pristopov je treba izpostaviti enačbo, ki jo je za vrednotenje srednjih hitrosti toka v strmih gorskih vodotokih ter hudourniških strugah objavil Rickenmann (1994, 1996) in smo jo uporabili tudi pri novem matematičnem modelu.

Razvili, preizkusili in verificirali smo dvodimenzijski matematični model za simulacijo nestalnega toka v strmih geometrijsko nepravilnih strugah PCFLOW2D-ORTHOCURVE. Model je zasnovan na pravokotni krivočrtni numerični mreži. Rezultate modela smo verificirali na primeru fizičnega modela vala, ki nastane zaradi porušitve dela nasipa zgornje akumulacije črpalne elektrarne Kolarjev vrh.

Izdelali smo dva matematična modela. Prvi je zajel območje akumulacije do mesta porušitve nasipa. Zaradi oblike akumulacijskega bazena z ne tako izrazito razliko med širino in dolžino smo pri modeliranju uporabili že obstoječi matematični model PCFLOW2D, ki je zasnovan na pravokotni Kartezijevi numerični mreži. Modela je povezovala krivulja izračunanih pretokov na iztoku iz akumulacije, ki je predstavljala hidrogram Q – t za dolvodno strugo. Tako smo s pomočjo modela akumulacije in rušenja njenega nasipa dobili podatke o časovnem poteku pretoka na mestu nasipa, kar smo nato v drugem modelu uporabili kot zgornji robni pogoj. Z drugim matematičnim modelom smo obravnavali tok porušitvenega vala skozi južno dolino Logarjevega potoka, za katero je značilna velika dolžina v primerjavi s širino in globino toka ter nekateri izraziti ovinki. Uporabili smo nov matematični model PCFLOW2D-ORTHOCURVE na osnovi pravokotne krivočrtne mreže.

Matematični model smo umerili na osnovi meritev globin na fizičnem modelu. Upoštevali smo modificirano Rickenmannovo enačbo ter neenakomerno razporeditev koeficienta trenja. Dosegli smo zelo dobro ujemanje z meritvami, pridobljenimi na fizičnem modelu. Povprečna odstopanja globin so do 15%. Razlike med merjenimi gladinami in rezultati izračuna se s časom manjšajo, tako da smo dosegli najboljše ujemanje na dolvodnem delu modela. Verjetni vzrok za to je precenjeni začetni pretok, kjer smo izhajali iz predpostavke o hipni porušitvi nasipa akumulacije, ki pa je bila na fizičnem modelu počasnejša zaradi ročnega dvigovanja zapornice. Na osnovi razpoložljivih informacij in vpogleda v dosegljivo literaturo je takšen model edini v Sloveniji, v svetovnem merilu pa eden redkih, ki omogoča tako široko uporabo za dvodimenzijsko simuliranje tokovnih razmer v ekstremnih pogojih naravnih strmih strug.

Pri nadaljnjem razvoju matematičnega modela PCFLOW2D-ORTHOCURVE bi bilo treba upoštevati predvsem vpliv premeščanja plavin, do katerega pogosto prihaja v pogojih strmega padca dna. Razvoj lahko teče tudi v smeri ekološkega modeliranja z namenom opravljanja izračunov transporta snovi ter biokemičnih procesov.

Ključne besede: porušitveni val, strma ukrivljena struga, dvodimenzijski matematični model, pravokotne krivočrtne koordinate, koeficient hrapavosti, model PCFLOW2D-ORTHOCURVE

SI 52

*Naslov avtorja za dopisovanje: Univerza v Ljubljani, Fakulteta za gradbeništvo in geodezijo, Jamova 2, 1000 Ljubljana, Slovenija, mkrzyk@fgg.uni-lj.si

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 53

Prejeto: 2010-10-20, Sprejeto: 2011-12-16 © 2012 Strojniški vestnik. Vse pravice pridržane

Študija obrusa kamnin v abrazijskem mlinu z uporabo opremljenega sledila Šolc, T. – Stefanovska, A. – Hoey, T. – Mikoš, M. Tomaž Šolc1 – Aneta Stefanovska2 – Trevor Hoey3 – Matjaž Mikoš4,*

Jožef Stefan, Slovenija v Lancastru, Oddelek za fiziko, Velika Britanija 3 Univerza v Glasgowu, Oddelek za geografske znanosti in znanosti o zemlji, Velika Britanija 4 Univerza v Ljubljani, Fakulteta za gradbeništvo in geodezijo, Slovenija 2 Univerza

1 Inštitut

Zelo zanimivo raziskovalno področje v rečni mehaniki in še posebej dinamiki prodnatih rek je medsebojno vplivanje zrn rečnih sedimentov v gibanju (rinjenih plavin). Omenjene naravne pojave lahko raziskujemo v naravnem okolju (zahtevno) ali pa v dobro kontroliranih laboratorijskih pogojih (enostavneje). Namen tovrstnih raziskav je običajno napovedovanje stopnje dolvodne drobnitve rečnih plavin vzdolž vodotokov (zmanjševanje njihove velikosti in sprememb oblike zrn), stopnje poglabljanja skalnatih odsekov rek in razvoj matematičnih (numeričnih) modelov premeščanja rinjenih plavin, ki temeljijo na opisu kinematike in dinamike posameznih zrn plavin.

Naravni pojav obrusa kamnin (skalnatega rečnega dna) v rečnem okolju smo analizirali v laboratorijskem okolju v posebnem abrazijskem mlinu vrste Dubree s ciljem, da bi ugotovili uspešnost obrusa kamnin v takem aparatu. Raziskovalni mlin je valjaste oblike z notranjim premerom 711 mm in notranjo dolžino 508 mm. Mlin se vrti okrog lastne osi z obodno hitrostjo ~30 obratov na minuto, obod mlina v 1 h prepotuje razdaljo ~4,02 km.

Na notranjem obodu mlina smo vgradili tri ploščice, izdelane iz različnih kamnin, odvzetih v izbranih rekah na Škotskem. Kot abrazijsko sredstvo v mlinu smo uporabili prodnate mešanice rečnih plavin mase ~2 kg in različne zrnavostne sestave; mlin smo vrteli 1 h in vsakič ponovno izmerili maso treh kamninskih ploščic. Spremembo mase ploščic smo določali s tehtanjem in natančnostjo 0,01 g ter z uporabo 3D-laserja.

Dinamiko obrusa kamninskih ploščic v abrazijskem mlinu s prodnato mešanico smo analizirali z uporabo patentiranega opremljenega sledila kroglaste oblike mase 994,6 g in premera 99 mm. Kroglasto sledilo ima razstavljivo in vodotesno kovinsko ohišje ter je opremljeno s triosnim pospeškomerom vrste 4375 Brüel & Kjær, ki meri vršne pospeške do 5000 g. Sledilo ima lastno 9 V napajanje in notranji pomnilnik, ki omogoča zapisovanje meritev pospeška v vseh treh smereh z vzorčevalno frekvenco 2832 Hz v času trajanja do 120 sekund. Sledilo smo že uspešno preizkusili v laboratorijskem žlebu z namenom prepoznavanja kinematike in dinamike prodnatih plavin v turbulentnem vodnem toku.

V prispevku je opisana lastna rešitev problema prepoznavanja in razlikovanja med udarci opremljenega sledila z različnimi telesi v raziskovalnem mlinu: z zrni sedimentne prodnate mešanice, s kamnitimi ploščicami, z mehko notranjo gumeno oblogo raziskovalnega mlina in s togimi stranskimi jeklenimi ploščami raziskovalnega mlina.

Analiza zapisanega signala (tridimenzijskih pospeškov) opremljenega sledila je pokazala, da imamo na razpolago dovolj informacij, da i) prepoznamo tribološko okolico sledila in ii) zadovoljivo opišemo intenziteto pojava obrusa kamnin (število in amplituda sil ob kontaktu sledila). Tako smo razvili uspešen in računalniško hiter algoritem za avtomatsko prepoznavanje trkov sledila in za njihovo analizo, pri čemer smo izvedli analizo signalov v časovni domeni. Ko smo izvedli analizo signalov v frekvenčni domeni, smo razvili tudi metodo za razlikovanje različnih signalov glede na prevladujočo vrsto trka. Z uporabo obeh metod skupaj lahko razvrstimo vse zapisane signale (tridimenzijske pospeške sledila) v skupine glede na podobnost merilnih pogojev.

V tem prispevku opisana raziskava še ni omogočila prenosa v laboratoriju izmerjenih vrednosti obrusa kamnin v naravo, je pa dobra osnova za medsebojno primerjavo različnih laboratorijskih raziskovalnih aparatov, ki se uporabljajo za raziskovanje obrusa kamnin. Način z uporabo opremljenega sledila je zmožen opisati uspešnost ali intenziteto obrusa kamnin v podobnih laboratorijskih aparatih z določitvijo števila in amplitud trkov; kar je precej bolj natančno od preprostega enačenja prepotovane razdalje v raziskovalnem mlinu, določene s pomočjo obodne hitrosti mlina in notranjega premera mlina, z razdaljo v naravi. Nadaljnje raziskave bodo usmerjene v analizo odvisnosti stopnje obrusa kamnin od števila in intenzitet trkov, kot jih lahko določimo v raziskovalnem abrazijskem mlinu s pomočjo opremljenega sledila.

Ključne besede: laboratorijski poskusi, obrus kamnin, opremljeno sledilo, tribologija

*Naslov avtorja za dopisovanje: Univerza v Ljubljani, Fakulteta za gradbeništvo in geodezijo, Jamova cesta 2, 1001 Ljubljana, Slovenija, matjaz.mikos@fgg.uni-lj.si

SI 53

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 54

Prejeto: 2011-11-09, sprejeto: 2012-03-16 © 2012 Strojniški vestnik. Vse pravice pridržane.

Povečanje ergonomske vrednosti izdelka z uporabo inteligentnega svetovalnega sistema Kaljun, J. – Dolšak, B. Jasmin Kaljun* – Bojan Dolšak Univerza v Mariboru, Fakulteta za strojništvo, Slovenija

Vsak izdelek, ki ga neko podjetje ponudi na trgu, mora zadovoljiti neko potrebo. Izdelek opravlja svojo glavno funkcijo v tehničnem smislu, hkrati pa mora zagotavljati tako uporabo, ki bo uporabniku nudila ugodje. V nasprotnem primeru izdelek na trgu ne bo uspešen, kar so pokazale številne raziskave, nekatere omenjene tudi v tem delu. Na ugodje uporabnika vplivajo različni dejavniki, vsekakor pa sta med njimi tako ergonomija kot tudi dojemanje oblike izdelka oz. njegove lepote, t. j. estetika izdelka.

Predvsem estetika je dejavnik, ki je, vsaj v osnovi, izrazito subjektiven. Ergonomija pa je po drugi strani dejavnik, katerega vplive je razmeroma enostavno razvrščati in meriti. Konstruktorji morajo tako pri svojem delu upoštevati tudi ergonomijo.

Kljub objektivnosti je ergonomija obsežen dejavnik, ki lahko vključuje tako ergonomijo delovnega mesta kot tudi natančno ergonomijo izdelka. Kot pomoč za konstruktorje lahko na tem področju najdemo tako strokovno in znanstveno literaturo kakor tudi računalniška orodja. Pri računalniških orodjih pa lahko opazimo ključno slabost, to je nezmožnost obravnave posameznih izdelkov (računalniška orodja so namreč namenjena zlasti obravnavi ergonomije delovnega mesta). Ob tej slabosti pa računalniška orodja za podporo ergonomiji v osnovi tudi ne omogočajo nudenja pomoči uporabniku.

Pričujoči članek obravnava prav področje računalniške podpore ergonomskemu razvoju izdelkov z uporabo metod umetne inteligence. V uvodu sta predstavljeni osnovni skupini obstoječih računalniških orodij. V nadaljevanju so predstavljene možnosti uporabe umetne inteligence na tem področju s kratko predstavitvijo načina delovanja inteligentnega svetovalnega sistema. Okvirno je predstavljen tudi prototip inteligentnega svetovalnega sistema Oscar, ki je namenjen podpori pri ergonomskem in estetskem razvoju izdelkov.

Ker je poglavitni del inteligentnega svetovalnega sistema ob mehanizmu sklepanja in uporabniškem vmesniku predvsem baza znanja, osrednji del članka opisuje strukturo ergonomskega znanja, zapisanega v sistemu Oscar.

Znanje je razdeljeno v osem skupin, v katerih so združena posamezna priporočila.

V članku je predstavljena tudi praktična uporaba prototipa Oscar pri posodobitvi ročke pnevmatskega kladiva. S predstavitvijo priporočila v bazi znanja je predstavljena tudi nova zasnova ročke. Ročka je končno tudi kvalitativno ovrednotena, pri čemer se izkaže, da je nova ročka v okviru opravljenih praktičnih preizkusov primernejša od obstoječe.

Izsledki raziskav so namenjeni snovalcem novih izdelkov, ki bodo ob uporabi predlaganega modela inteligentne podpore v obliki izdelka lažje zajeli fiziološke karakteristike uporabnika. Predloženi članek ob teoretičnem delu raziskav, katerih rezultat je zbrano in evidentirano znanje s problemskega področja, obravnava še sistematizacijo, formalizacijo in strukturiranje ekspertnega znanja. Na osnovi zbranega znanja pa je predstavljen model inteligentne podpore pri ergonomskem razvoju izdelkov, katerega uporabnost je potrjena s praktičnimi preizkusi.

Ob predpostavki, da bo zaživel v praksi, bo inteligentni svetovalni sistem Oscar vsekakor dobrodošla pomoč pri razvoju izdelkov predvsem v malih in srednjih velikih podjetjih, ki si dragih strokovnjakov s področja ergonomije ne morejo privoščiti.

Ključne besede: ergonomija, inteligentna podpora, ročna orodja, zajemanje znanja, razvoj ročke, pnevmatsko kladivo

SI 54

*Naslov avtorja za dopisovanje: Univerza v Mariboru, Fakulteta za strojništvo, Smetanova 17, 2000 Maribor, Slovenija, jasmin.kaljun@uni-mb.si

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 55

Prejeto: 2011-04-18, sprejeto: 2012-02-14 © 2012 Strojniški vestnik. Vse pravice pridržane

Izboljšanje učinkovitosti hidravličnih sistemov z uporabo teorije zanesljivosti in nadzora obratovalnih parametrov sistema Mitar Jocanović, M. – Šević, D. – Karanović, V. – Beker, I. – Dudić, S. Mitar Jocanović* – Dragoljub Šević – Velibor Karanović – Ivan Beker – Slobodan Dudić Tehniška fakulteta, Univerza v Novem Sadu, Srbija

Namen članka je prikaz izboljšanja učinkovitosti hidravličnega sistema. K izboljševanju učinkovitosti zasnovanih hidravličnih sistemov je mogoče pristopiti na dva načina: s spremembo zasnove sistema na osnovi teorije zanesljivosti ali z nadzorom obratovalnih parametrov sistema. V članku je predstavljena študija primera izboljšanja načrtovanega in izvedenega hidravličnega sistema na osnovi teh dveh pristopov.

Kupljeni hidravlični sistemi imajo včasih napake, ki pa jih je zelo težko odkriti. Konstruktorji hidravličnih sistemov včasih nimajo dovolj znanja, da bi zasnovali hidravlične sisteme brez napak, kar pa se lahko izkaže šele med eksploatacijo sistema. Kdorkoli je prepričan v to, da ima sistem napake, mora imeti veliko znanja in samozavesti, da se loti spreminjanja zasnove in izboljšav takega sistema.

Pri raziskavi je bila uporabljena metodologija spremljanja obratovalnih parametrov, analize zanesljivosti, primerjave med izmerjenimi in izračunanimi podatki, analize razlik, iskanja vzroka nezadovoljive učinkovitosti sistema in spreminjanja zasnove hidravličnega sistema.

Na podlagi opravljene raziskave lahko podamo naslednje rezultate in ugotovitve. Spremenjen hidravlični sistem z ocenjenim povprečnim časom obratovanja med izpadi približno 6000 ur, kar je neprimerljivo več kot pri izvornem sistemu. Rezultat takšnega pristopa je hidravlični sistem, ki obratuje uspešno in brez izpadov 24 ur na dan, kar pomeni, da predlagana metoda analize in izboljšav omogoča pomembno povečanje učinkovitosti hidravličnega sistema. Analiza rezultatov, pridobljenih s kombinirano uporabo teorije zanesljivosti in spremljanja obratovalnih parametrov obravnavanega hidravličnega sistema vodi do naslednjih zaključkov: (1) Teorija zanesljivosti pomaga pri identifikaciji najšibkejših členov, oz. najbolj kritičnih komponent hidravličnega sistema; (2) Spremljanje obratovalnih parametrov omogoča ugotavljanje vzrokov zastojev v obratovanju in izpada posameznih komponent sistema (spremljanje obratovalnih parametrov sistema kot so tlak, pretok, temperatura delovne tekočine, temperatura ključnih komponent itd. razkriva komponente sistema, ki so najbolj obremenjene, kakor tudi vrednosti obratovalnih parametrov, ki lahko povzročijo zastoje v delovanju ali izpad komponent); (3) Ustrezna zamenjava kritičnih elementov hidravličnega sistema, ki jo spremlja ustrezna izbira komponent, podaljša srednji čas obratovanja; (4) Neprekinjen nadzor obratovalnih parametrov hidravličnega sistema oz. avtomatiziran nadzor obratovalnih parametrov sistema omogoča napovedovanje in vplivanje na zanesljivost sistema.

Natančne podatke o zanesljivosti je težko najti. Načrtovalce hidravličnih sistemov bi bilo treba vzpodbuditi, da v proces snovanja hidravličnih sistemov vključijo tudi teorijo zanesljivosti. Teoretični del zagotavljanja učinkovitosti sistema sloni na podatkih o zanesljivosti hidravličnih komponentah, ki jih je objavil Fitch [3] leta 2004, zbrani pa so bili gotovo še mnogo prej. Tehnologija proizvodnje hidravličnih komponent je napredovala od izvedbe teh eksperimentov, zato so te komponente danes bolj zanesljive, kot je navedeno v viru. To pomeni, da izračunana zanesljivost ne ustreza absolutno zanesljivosti današnjih hidravličnih sistemov, dejanska zanesljivost pa je večja od izračunane.

V članku je opisano doseganje sinergičnega učinka med teorijo zanesljivosti in prakso / nadzorom obratovalnih parametrov hidravličnega sistema. Splošen zaključek je, da je analiza zanesljivosti uporabna v povezavi z nadzorom obratovalnih parametrov hidravličnega sistema in njegovih komponent med eksploatacijo, kakor tudi za načrtovanje novih hidravličnih sistemov. Načrtovanje hidravličnih sistemov na osnovi analize zanesljivosti pa žal še ni dovolj razširjeno. Nadzor obratovalnih parametrov hidravličnega sistema v povezavi z analizo zanesljivosti omogoča osredotočenje na kritične komponente sistema ter izboljšanje njihove zanesljivosti in učinkovitosti po načrtovanju in zagonu hidravličnega sistema. Članek naj bi zato usmeril pozornost načrtovalcev hidravličnih sistemov na analizo zanesljivosti, ki ima potencial za povečanje učinkovitosti hidravličnih sistemov.

Ključne besede: hidravlika, zanesljivost hidravličnih sistemov, nadzor in učinkovitost hidravličnih sistemov

*Naslov avtorja za dopisovanje: Univerza v Novem Sadu, Fakulteta za tehnične vede, Trg D. Obradovića 6, 21000 Novi Sad, Srbija, mitarj@uns.ac.rs

SI 55

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 56-58 Osebne objave

Doktorske disertacije, specialistično delo in diplome

DOKTORSKE DISERTACIJE Na Fakulteti za strojništvo Univerze v Ljubljani je z uspehom obranil svojo doktorsko disertacijo: dne 22. marca 2012 Aleš BIZJAK z naslovom: »Optodinamska karakterizacija interakcije kratkotrajnih bliskov Er:YAG laserja z vodo na različnih površinah« (mentor: prof. dr. Janez Možina); V delu predstavljamo optodinamsko metodo, ki z uporabo kratkih laserskih bliskov omogoča zaznavanje prisotnosti vode in merjenje hitrosti kondenzacije vodne pare na kovinski površini ter določanje vlažnosti lesa. V ta namen smo razvili in izdelali Er:YAG laser s preklopom kvalitete, ki oddaja bliske z energijo 16 mJ, s trajanjem 270 ns in s premerom 1,5 mm v TEM00 kvaliteti. Hitrost kondenzacije je možno določati z merjenjem optodinamskih odzivov v odvisnosti od časa kondenzacije vodne pare na predhodno ohlajeni in osušeni kovinski površini.

Meritve smo izvajali pri različnih relativnih zračnih vlažnostih in pri različnih temperaturah substrata.

Z analizo izmerjenih signalov je možno oblikovati model, ki opisuje hitrost kondenzacije vodne pare pri danih pogojih. Meritve optodinamskih odzivov kažejo na linearno povezavo med amplitudo in vlažnostjo lesnih vrst. Na osnovi merjenja časovno odvisnih optodinamskih odzivov v neravnovesnem stanju vlažnosti je možno določiti razmerje prestopnostnih konstant za vodo v treh glavnih anatomskih lesnih smereh. * Na Fakulteti za strojništvo Univerze v Mariboru sta z uspehom obranila svojo doktorsko disertacijo: dne 15. marca 2012 Brigita ALTENBAHER z naslovom: »Biorazgradljivost posameznih komponent pralnih odpadnih vod v bioreaktorju« (mentorica: prof. dr. Sonja ŠOSTAR TURK); Industrijske pralnice so veliki porabniki sveže vode in pralnih sredstev, zaradi česar se čiščenju odpadnih vod namenja veliko pozornosti z ekološkega in ekonomskega vidika. Veljavna zakonodaja in visoki stroški okoljskih dajatev za onesnaženje voda, prisilijo industrijske pralnice k zmanjševanju količine odpadnih voda. V dosedanjih raziskavah je bilo ugotovljeno, da so odpadne vode iz pralnic zelo onesnažene, zaradi česar so potrebne različne metode čiščenja odpadnih vod, ki pa so precej drage. Zato je nujno potrebno zmanjšati onesnaženje odpadne vode SI 56

z uporabo okolju prijaznejših pralnih in razkuževalnih sredstev, optimizirati postopke pranja in nato uvesti primerno metodo čiščenja s katero zmanjšamo količino odpadne vode.

V okviru doktorske disertacije smo uvedli nov, okolju prijazen kemijsko-termični postopek pranja bolnišničnih tekstilij, ki poteka pri nižji temperaturi pranja. Pri pranju smo uporabili novo, ekološko primernejšo pralno sredstvo ter kombinirano belilno, razkuževalno in nevtralizacijsko sredstvo na osnovi perocetne kisline, vodikovega peroksida in ocetne kisline. Z uporabo takšnega razkuževalnega sredstva smo lahko znižali temperaturo pranja iz 90 °C na 40 °C in tako znižali porabo električne energije. Istočasno smo preskušali tudi dezinfekcijski učinek postopka pranja, ki ga pralnice bolnišničnega perila morajo zagotavljati, kakor tudi negativne učinke postopkov pranja na življenjsko dobo perila.

V nadaljevanju raziskve smo optimirali sistem za čiščenje in recikliranje odpadnih voda z uporabo membranskega bioreaktorja (MBR), ki se bo lahko prilagodil različnim vrstam odpadnih vod v pralnicah.

Pri različnih obratovalnih pogojih reaktorja smo preučevali biorezgradljivost posameznih komponent v sintetični pralni odpadni vodi. Na podlagi rezultatov smo določili optimalne pogoje delovanja MBR, ki so bili osnova za čiščenje različno obremenjene pralne odpadne vode. Podali smo možnost ponovne uporabe prečiščene vode v postopku pranja (recikliranje), na podlagi katere bi pralnice zmanjšale količino odpadnih voda in porabo sveže vode.

Rezultat izvedene študije prikazuje možnost uporabe nizko temperaturnega pranja, pri čemer je higiena samega postopka pranja še vedno zagotavlja, njegova uvedba pa ima tudi pomemben vpliv na okolje, saj se močno zmanjša poraba električne energije. V študiji predlagana uvedba čiščenja in recikliranja prečiščene vode, zniža poraba sveže vode, zaradi česar so materialni stroški pralnice znižani; dne 27. marca 2012 Simon KLANČNIK z naslovom: »Model inteligentnega CAD/CAM sistema za programiranje CNC obdelovalnih strojev« (mentor: prof. dr. Jože BALIČ); Sodobni obdelovalni sistemi so visoko avtomatizirani, zahtevajo veliko fleksibilnost in težijo k popolni avtonomnosti. Ker je programiranje obdelovalnih strojev zelo kompleksen proces, ki ga sestavlja več med seboj odvisnih problemov, ga kljub velikim naporom do danes še ni uspelo avtomatizirati.

Pregledane raziskave so pokazale, da so do sedaj

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 56-58

razviti sistemi zelo ozko omejeni in lahko služijo človeku le kot pripomoček pri pripravi postopka obdelave.

V disertaciji predlagamo samodejno programiranje CNC-obdelovalnih strojev s pomočjo umetne inteligence. Razvita inteligenca je sposobna ne le delno, ampak v celoti reševati kompleksen problem samodejnega programiranja obdelovalnih strojev.

Sistem na podlagi CAD modela izdelka samodejno, brez pomoči strokovnjaka, pripravi NC program obdelave, in sicer tako, da je obdelava varna, pravilna, časovno učinkovita in hkrati zadosti določenim tehnološkim zahtevam obdelave. Inteligentni CAD/CAM-sistem za svoje delovanje uporablja skupinsko inteligenco, NSGA-II večkriterijsko optimizacijo in usmerjeno nevronsko mrežo, hkrati pa koristi prednosti ter moč informatizacije in tako s porazdeljeno arhitekturo dosega večjo učinkovitost pri celovitem reševanju tako kompleksnega problema. Sistem je sestavljen iz napovedovalnega in evalvacijskega modula. V napovedovalnem modulu umetna inteligenca predlaga rešitve, ki vsebujejo informacije o trajektorijah rezov, izbranih orodjih in predlaganih rezalnih parametrih. Evalvacijski modul, na podlagi razvitih simulacijskih modelov, oceni predlagane rešitve glede na geometrijski, tehnološki in časovni kriterij ter kriterij učinkovitosti obdelave. V okviru raziskav smo razvili diskreten in tudi zvezen simulacijski model, ki ga razvit inteligenten sistem uporablja pri iskanju optimalne rešitve. Predlagani sistem je v splošnem primeren za različne vrste obdelav, v doktorski nalogi pa se zaradi obsega dela pri testiranjih omejimo zgolj na rezkanje.

Rezultati testiranj so potrdili, da je z uporabo metod umetne inteligence mogoče samodejno programirati obdelovalne stroje.

Matej MOZETIČ z naslovom: »Razvoj rotorja peskalnega stroja« (mentor: prof. dr. Jožef Vižintin, somentor: prof. dr. Branko Širok); dne 22. marca 2012: Jernej BRADEŠKO z naslovom: »Naprava za merjenje in analizo karakteristike elektromagneta« (mentor: prof. dr. Janez Diaci); Roman PERGOVNIK z naslovom: »Hladilno mazalna sredstva pri obdelavi titana in Inconela« (mentor: doc. dr. Franci Pušavec, prof. dr. Janez Kopač); dne 23. marca 2012: Matej PANJAN z naslovom: »Avtomatizacija demonstracijske enote hidrodinamičnega prenosnika moči« (mentor: izr. prof. dr. Mihael Sekavčnik); Luka URBANC z naslovom: »Konzolno dvigalo« (mentor: doc. dr. Boris Jerman); Primož VIDIC z naslovom: »Razvoj laboratorijske centrifuge« (mentor: prof. dr. Jožef Duhovnik); dne 26. marca 2012: Pero GATARIĆ z naslovom: »Obvladovanje procesov kovaškega valjanja« (mentor: prof. dr. Karl Kuzman); Grega JERIN z naslovom: »Trdnostna optimizacija keramičnega ohišja nizkonapetostne talilne varovalke« (mentor: prof. dr. Boris Štok); Klemen KASTELEC z naslovom: »Analiza vplivnih parametrov in optimiranje strežnih ter montažnih procesov« (mentor: izr. prof. dr. Niko Herakovič); Gašper KOKELJ z naslovom: »Simuliranje procesa pletenja z uporabo gradnikov metode končnih elementov« (mentor: prof. dr. Boris Štok).

SPECIALISTIČNO DELO

Na Fakulteti za strojništvo Univerze v Mariboru so pridobili naziv univerzitetni diplomirani inženir strojništva: dne 29. marca 2012: Robert BEZGOVŠEK z naslovom: »Obvladovanje kakovosti procesa izdelave polnjene žice v podjetju Filo d.o.o.« (mentor: izr. prof. dr. Bojan Ačko, somentor: izr. prof. dr. Borut Buchmeister); Marko BORKO z naslovom: »Prostosedi (oblikovanje stolov za klubski prostor)« (mentor: izr. prof. Vojmir Pogačar, somentor: doc. dr. Andrej Skrbinek); Robert DVORŠIČ z naslovom: »Načrtovanje lesno obdelovalnega stroja« (mentor: doc. dr. Uroš Župerl, somentor: prof. dr. Riko Šafarič); Peter MAJERIČ z naslovom: »Naprava za doziranje odpadnega materiala s polžastim

Na Fakulteti za strojništvo Univerze v Ljubljani je z uspehom zagovarjal svoje specialistično delo: dne 4. januarja 2012 Janez KRŽAN z naslovom: »Vzdrževanje logistično transportnih sredstev po stanju« (mentor: prof. dr. Jožef Vižintin).

DIPLOMIRALI SO Na Fakulteti za strojništvo Univerze v Ljubljani so pridobili naziv univerzitetni diplomirani inženir strojništva: dne 21. marca 2012: Matjaž HUMAR z naslovom: »Priprava tehnologije uporovnega bradavičnega varjenja električnih kontaktov« (mentor: prof. dr. Janez Tušek);

*

SI 57

Strojniški vestnik - Journal of Mechanical Engineering 58(2012)4, SI 56-58

transporterjem« (mentor: prof. dr. Srečko Glodež, somentor: prof. dr. Iztok Potrč); Damijan SADEK z naslovom: »Konstruiranje nastavljive ergonomske mize« (mentor: izr. prof. dr. Bojan Dolšak, somentorica: viš. pred. dr. Marina Novak); Jasna ZORKO z naslovom: »Eksperimentalno in numerično modeliranje uparjanja vode v vertikalnem kanalu« (mentor: prof. dr. Leopold Škerget, somentor: Rok Kopun, univ. dipl. inž.). * Na Fakulteti za strojništvo Univerze v Ljubljani so pridobili naziv diplomirani inženir strojništva: dne 7. marca 2012: Miha GODEC z naslovom: »Filtracija vode pri rezanju s kriogenim vodnim curkom« (mentor: prof. dr. Mihael Junkar, somentor: doc. dr. Henri Orbanić); Jure GORŠE z naslovom: »Vpliv temperature na mehanske lastnosti poliestrskih kompozitov« (mentor: izr. prof. dr. Roman Šturm, somentor: prof. dr. Janez Grum); Andrej RANFL z naslovom: »Sodobne tehnologije izrabe lesne biomase« (mentor: izr. prof. dr. Andrej Senegačnik); Gašper RAZPOTNIK z naslovom: »Vpliv deleža steklenih vlaken na mehanske lastnosti poliestrskih kompozitov« (mentor: izr. prof. dr. Roman Šturm);

SI 58

Marko SEVNIK z naslovom: »Razvojno vrednotenje razbremenilnega ventila« (mentor: prof. dr. Marko Nagode); Primož UHAN z naslovom: »Izboljšanje točnosti pozicioniranja vertikalnega obdelovalnega centra« (mentor: prof. dr. Janez Kopač). * Na Fakulteti za strojništvo Univerze v Mariboru sta pridobila naziv diplomirani inženir strojništva: dne 29. marca 2012: Gregor AUER z naslovom: »Optimizacija delovnega procesa s postavitvijo valjčnega transporterja« (mentorica: doc. dr. Nataša Vujica Herzog, somentor: doc. dr. Marjan Leber); Rok MERKAČ z naslovom: »Izdelava prototipnih robotskih prijemal s CNC-stroji« (mentor: prof. dr.

Miran Brezočnik, somentor: izr. prof. dr. Ivan Pahole). * Na Fakulteti za strojništvo Univerze v Mariboru je pridobil naziv diplomirani gospodarski inženir: dne 29. marca 2012: Žiga LESKOVAR z naslovom: »Analiza izbranih postopkov preoblikovanja pločevin« (mentor: izr. prof. dr. Borut Buchmeister, mentorica: doc. dr.

Zdenka Ženko).

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Strojniški vestnik – Journal of Mechanical Engineering (SV-JME) Aim and Scope The international journal publishes original and (mini)review articles covering the concepts of materials science, mechanics, kinematics, thermodynamics, energy and environment, mechatronics and robotics, fluid mechanics, tribology, cybernetics, industrial engineering and structural analysis.

The journal follows new trends and progress proven practice in the mechanical engineering and also in the closely related sciences as are electrical, civil and process engineering, medicine, microbiology, ecology, agriculture, transport systems, aviation, and others, thus creating a unique forum for interdisciplinary or multidisciplinary dialogue.

The international conferences selected papers are welcome for publishing as a special issue of SV-JME with invited co-editor(s).

Editor in Chief Vincenc Butala University of Ljubljana Faculty of Mechanical Engineering, Slovenia Technical Editor Pika Škraba University of Ljubljana Faculty of Mechanical Engineering, Slovenia Editorial Office University of Ljubljana (UL) Faculty of Mechanical Engineering SV-JME Aškerčeva 6, SI-1000 Ljubljana, Slovenia Phone: 386-(0)1-4771 137 Fax: 386-(0)1-2518 567 E-mail: info@sv-jme.eu http://www.sv-jme.eu Print Tiskarna Knjigoveznica Radovljica, printed in 480 copies Founders and Publishers University of Ljubljana (UL) Faculty of Mechanical Engineering, Slovenia University of Maribor (UM) Faculty of Mechanical Engineering, Slovenia Association of Mechanical Engineers of Slovenia

http://www.sv-jme.eu

58 (2012) 4

Chamber of Commerce and Industry of Slovenia Metal Processing Industry Association

Since 1955

Strojniški vestnik Journal of Mechanical Engineering

Cover: Lowering of a large Kaplan runner assembly into the turbine pit in the Zlatoličje powerhouse on Drava river, Slovenia (maximum turbine output: 80 MW, turbine diameter: 5900 mm).

uisbrink, Francisco Arregui: Air Valves in a Large-Scale Pipeline Apparatus

Image courtesy: Litostroj Power d.o.o., Slovenia

j Predin, Drago Fabijan, Roman Klasinc: of the Impact of Particles on the Cavitating Flow

asinc, Matjaž Četina: ematical Modelling of a Dam-Break Wave am

anovska, Trevor Hoey, Matjaž Mikoš: umented Tracer in an Abrasion Mill ies

olšak: gonomic Value Using Intelligent Decision

jub Šević, Velibor Karanović, Ivan Beker, Slobodan Dudić: Hydraulic Systems through Reliability Theory and Operating Parameters

Journal of Mechanical Engineering - Strojniški vestnik

atjaž Mikoš, Jakob Šušteršič, r Planinc: rasion Resistance in Hydraulic Structures on the

year

no. 4 2012 58

volume

International Editorial Board Koshi Adachi, Graduate School of Engineering,Tohoku University, Japan Bikramjit Basu, Indian Institute of Technology, Kanpur, India Anton Bergant, Litostroj Power, Slovenia Franci Čuš, UM, Faculty of Mech. Engineering, Slovenia Narendra B. Dahotre, University of Tennessee, Knoxville, USA Matija Fajdiga, UL, Faculty of Mech. Engineering, Slovenia Imre Felde, Bay Zoltan Inst. for Mater. Sci. and Techn., Hungary Jože Flašker, UM, Faculty of Mech. Engineering, Slovenia Bernard Franković, Faculty of Engineering Rijeka, Croatia Janez Grum, UL, Faculty of Mech. Engineering, Slovenia Imre Horvath, Delft University of Technology, Netherlands Julius Kaplunov, Brunel University, West London, UK Milan Kljajin, J.J. Strossmayer University of Osijek, Croatia Janez Kopač, UL, Faculty of Mech. Engineering, Slovenia Franc Kosel, UL, Faculty of Mech. Engineering, Slovenia Thomas Lübben, University of Bremen, Germany Janez Možina, UL, Faculty of Mech. Engineering, Slovenia Miroslav Plančak, University of Novi Sad, Serbia Brian Prasad, California Institute of Technology, Pasadena, USA Bernd Sauer, University of Kaiserlautern, Germany Brane Širok, UL, Faculty of Mech. Engineering, Slovenia Leopold Škerget, UM, Faculty of Mech. Engineering, Slovenia George E. Totten, Portland State University, USA Nikos C. Tsourveloudis, Technical University of Crete, Greece Toma Udiljak, University of Zagreb, Croatia Arkady Voloshin, Lehigh University, Bethlehem, USA President of Publishing Council Jože Duhovnik UL, Faculty of Mechanical Engineering, Slovenia General information Strojniški vestnik – Journal of Mechanical Engineering is published in 11 issues per year (July and August is a double issue).

Institutional prices include print & online access: institutional subscription price and foreign subscription €100,00 (the price of a single issue is €10,00); general public subscription and student subscription €50,00 (the price of a single issue is €5,00). Prices are exclusive of tax.

Delivery is included in the price. The recipient is responsible for paying any import duties or taxes. Legal title passes to the customer on dispatch by our distributor.

Single issues from current and recent volumes are available at the current single-issue price. To order the journal, please complete the form on our website. For submissions, subscriptions and all other information please visit: http://en.sv-jme.eu/.

You can advertise on the inner and outer side of the back cover of the magazine. The authors of the published papers are invited to send photos or pictures with short explanation for cover content.

We would like to thank the reviewers who have taken part in the peerreview process.

ISSN 0039-2480 © 2011 Strojniški vestnik - Journal of Mechanical Engineering. All rights reserved. SV-JME is indexed / abstracted in: SCI-Expanded, Compendex, Inspec, ProQuest-CSA, SCOPUS, TEMA. The list of the remaining bases, in which SV-JME is indexed, is available on the website. The journal is subsidized by Slovenian Book Agency.

Strojniški vestnik - Journal of Mechanical Engineering is also available on http://www.sv-jme.eu, where you access also to papers’ supplements, such as simulations, etc.

Instructions for Authors All manuscripts must be in English. Pages should be numbered sequentially. The maximum length of contributions is 10 pages. Longer contributions will only be accepted if authors provide justification in a cover letter. Short manuscripts should be less than 4 pages. For full instructions see the Authors Guideline section on the journal’s website: http://en.sv-jme.eu/.

Announcement: The authors are kindly invited to submitt the paper through our web site: http://ojs.sv-jme.eu. The Author is also able to accompany the paper with Supplementary Files in the form of Cover Letter, data sets, research instruments, source texts, etc. The Author is able to track the submission through the editorial process - as well as participate in the copyediting and proofreading of submissions accepted for publication - by logging in, and using the username and password provided.

Please provide a cover letter stating the following information about the submitted paper: 1. Paper title, list of authors and affiliations.

2. The type of your paper: original scientific paper (1.01), review scientific paper (1.02) or short scientific paper (1.03).

3. A declaration that your paper is unpublished work, not considered elsewhere for publication.

4. State the value of the paper or its practical, theoretical and scientific implications. What is new in the paper with respect to the state-of-the-art in the published papers?

5. We kindly ask you to suggest at least two reviewers for your paper and give us their names and contact information (email).

Every manuscript submitted to the SV-JME undergoes the course of the peer-review process.

THE FORMAT OF THE MANUSCRIPT The manuscript should be written in the following format: - A Title, which adequately describes the content of the manuscript.

- An Abstract should not exceed 250 words. The Abstract should state the principal objectives and the scope of the investigation, as well as the methodology employed. It should summarize the results and state the principal conclusions.

- 6 significant key words should follow the abstract to aid indexing.

- An Introduction, which should provide a review of recent literature and sufficient background information to allow the results of the article to be understood and evaluated.

- A Theory or experimental methods used.

- An Experimental section, which should provide details of the experimental set-up and the methods used for obtaining the results.

- A Results section, which should clearly and concisely present the data using figures and tables where appropriate.

- A Discussion section, which should describe the relationships and generalizations shown by the results and discuss the significance of the results making comparisons with previously published work. (It may be appropriate to combine the Results and Discussion sections into a single section to improve the clarity).

- Conclusions, which should present one or more conclusions that have been drawn from the results and subsequent discussion and do not duplicate the Abstract.

- References, which must be cited consecutively in the text using square brackets [1] and collected together in a reference list at the end of the manuscript.

Units - standard SI symbols and abbreviations should be used. Symbols for physical quantities in the text should be written in italics (e.g. v, T, n, etc.).

Symbols for units that consist of letters should be in plain text (e.g. ms-1, K, min, mm, etc.)

Abbreviations should be spelt out in full on first appearance, e.g., variable time geometry (VTG).

Meaning of symbols and units belonging to symbols should be explained in each case or quoted in a special table at the end of the manuscript before References.

Figures must be cited in a consecutive numerical order in the text and referred to in both the text and the caption as Fig. 1, Fig. 2, etc. Figures should be prepared without borders and on white grounding and should be sent separately in their original formats.

Pictures may be saved in resolution good enough for printing in any common format, e.g. BMP, GIF or JPG. However, graphs and line drawings should be prepared as vector images, e.g. CDR, AI. When labeling axes, physical quantities, e.g. t, v, m, etc. should be used whenever possible to minimize the need to label the axes in two languages.

Multi-curve graphs should have individual curves marked with a symbol. The meaning of the symbol should be explained in the figure caption.

Tables should carry separate titles and must be numbered in consecutive numerical order in the text and referred to in both the text and the caption as Table 1, Table 2, etc. In addition to the physical quantity, e.g. t (in italics), units

(normal text), should be added in square brackets. The tables should each have a heading. Tables should not duplicate data found elsewhere in the manuscript.

Acknowledgement of collaboration or preparation assistance may be included before References. Please note the source of funding for the research.

REFERENCES A reference list must be included using the following information as a guide. Only cited text references are included. Each reference is referred to in the text by a number enclosed in a square bracket (i.e., [3] or [2] to [6] for more references). No reference to the author is necessary.

References must be numbered and ordered according to where they are first mentioned in the paper, not alphabetically. All references must be complete and accurate. All non-English or. non-German titles must be translated into English with the added note (in language) at the end of reference. Examples follow.

Journal Papers: Surname 1, Initials, Surname 2, Initials (year). Title. Journal, volume, number, pages, DOI code.

[1] Hackenschmidt, R., Alber-Laukant, B., Rieg, F. (2010). Simulating nonlinear materials under centrifugal forces by using intelligent crosslinked simulations. Strojniški vestnik - Journal of Mechanical Engineering, vol. 57, no. 7-8, p. 531-538, DOI:10.5545/sv-jme.2011.013.

Journal titles should not be abbreviated. Note that journal title is set in italics.

Please add DOI code when available and link it to the web site.

Books: Surname 1, Initials, Surname 2, Initials (year). Title. Publisher, place of publication.

[2] Groover, M.P. (2007). Fundamentals of Modern Manufacturing. John Wiley & Sons, Hoboken.

Note that the title of the book is italicized.

Chapters in Books: Surname 1, Initials, Surname 2, Initials (year). Chapter title. Editor(s) of book, book title. Publisher, place of publication, pages.

[3] Carbone, G., Ceccarelli, M. (2005). Legged robotic systems. Kordić, V., Lazinica, A., Merdan, M. (Eds.), Cutting Edge Robotics. Pro literatur Verlag, Mammendorf, p. 553-576.

Proceedings Papers: Surname 1, Initials, Surname 2, Initials (year). Paper title. Proceedings title, pages.

[4] Štefanić, N., Martinčević-Mikić, S., Tošanović, N. (2009). Applied Lean System in Process Industry. MOTSP 2009 Conference Proceedings, p.

422-427.

Standards: Standard-Code (year). Title. Organisation. Place.

[5] ISO/DIS 16000-6.2:2002. Indoor Air – Part 6: Determination of Volatile Organic Compounds in Indoor and Chamber Air by Active Sampling on TENAX TA Sorbent, Thermal Desorption and Gas Chromatography using MSD/FID. International Organization for Standardization. Geneva. www pages: Surname, Initials or Company name. Title, from http://address, date of access.

[6] Rockwell Automation. Arena, from http://www.arenasimulation.com, accessed on 2009-09-07.

EXTENDED ABSTRACT By the time the paper is accepted for publishing, the authors are requested to send the extended abstract (approx. one A4 page or 3.500 to 4.000 characters). The instructions for writing the extended abstract are published on the web page http://www.sv-jme.eu/ information-for-authors/.

COPYRIGHT Authors submitting a manuscript do so on the understanding that the work has not been published before, is not being considered for publication elsewhere and has been read and approved by all authors. The submission of the manuscript by the authors means that the authors automatically agree to transfer copyright to SV-JME and when the manuscript is accepted for publication. All accepted manuscripts must be accompanied by a Copyright Transfer Agreement, which should be sent to the editor. The work should be original by the authors and not be published elsewhere in any language without the written consent of the publisher.

The proof will be sent to the author showing the final layout of the article. Proof correction must be minimal and fast. Thus it is essential that manuscripts are accurate when submitted.

Authors can track the status of their accepted articles on http://en.svjme.eu/.

PUBLICATION FEE For all articles authors will be asked to pay a publication fee prior to the article appearing in the journal. However, this fee only needs to be paid after the article has been accepted for publishing. The fee is 220.00 EUR (for articles with maximum of 10 pages), 20.00 EUR for each addition page.

Additional costs for a color page is 90.00 EUR.

58 (2012) 4

http://www.sv-jme.eu

Strojniški vestnik Journal of Mechanical Engineering

Since 1955

Papers

225

Anton Bergant, Arno Kruisbrink, Francisco Arregui: Dynamic Behaviour of Air Valves in a Large-Scale Pipeline Apparatus

238

Boštjan Gregorc, Andrej Predin, Drago Fabijan, Roman Klasinc: Experimental Analysis of the Impact of Particles on the Cavitating Flow

245

Andrej Kryžanowski, Matjaž Mikoš, Jakob Šušteršič, Velimir Ukrainczyk, Igor Planinc: Testing of Concrete Abrasion Resistance in Hydraulic Structures on the Lower Sava River

255

Mario Krzyk, Roman Klasinc, Matjaž Četina: Two-Dimensional Mathematical Modelling of a Dam-Break Wave in a Narrow Steep Stream

263

Tomaž Šolc, Aneta Stefanovska, Trevor Hoey, Matjaž Mikoš: Application of an Instrumented Tracer in an Abrasion Mill for Rock Abrasion Studies

271

Jasmin Kaljun, Bojan Dolšak: Improving Products’ Ergonomic Value Using Intelligent Decision Support System

281

Mitar Jocanović, Dragoljub Šević, Velibor Karanović, Ivan Beker, Slobodan Dudić: Increased Efficiency of Hydraulic Systems through Reliability Theory and Monitoring of System Operating Parameters

Journal of Mechanical Engineering - Strojniški vestnik

Contents

4 year 2012 volume 58 no.