Parametric study of factors affecting ventilation in a building by Murtaza Mohammadi

Contents

1. Introduction

1.1. Brief

1.2. Need for natural ventilation (a review)

2. Indoor thermal comfort

2.1. Modelling Basic Geometry

2.2. Modelling Windtower

2.3. Indoor ventilation parametric study

2.3.1. Duct inlet air velocity

2.3.2. Duct external vent area

2.3.3. Duct internal vent area

2.3.4. Indoor air temperature

3. Outdoor Thermal Comfort

3.1. Modelling geometry in site

3.2. Outdoor ventilation parametric study

3.2.1. Site inlet wind velocity

3.2.2. Site inlet wind direction

3.2.3. Direct solar radiation on the ground

3.2.4. Ground surface temperature

3.2.5. Ambient air temperature

4. Conclusion and Recommendations

4.1. Conclusions drawn from the project

4.2. Design recommendations

4.3. Limitations

5. Appendices

6. References

7. List of Figures

Introduction 1.1 Brief Increasing awareness of the need for energy efficient and environmentally friendly approach for building design has renewed emphasis on the integration of natural ventilation devices in buildings [1]. Traditional buildings in the hot dry region of Middle-East have developed mechanisms to catch the natural wind and direct it into the interiors, providing

Figure 1: Office space under consideration, (dimensions in meters)

passive cooling and thermal comfort for the inhabitants. This project has two main objectives; one, it investigates the potential of achieving indoor thermal comfort by varying the design of the wind tower under different climatic condition; and two, it investigates the outdoor thermal comfort by varying the site conditions under

Figure 2: Suggested working height

different climate. Study models, environment

The given climatic data represents that of a

and CFD has been simulated in Ansys work-

hot and humid region, like that of Malaysia.

bench, using DesignModeler, Mesh and Flu-

For the purpose of this study the average in-

ent. Ansys offers a comprehensive tool to

door temperatures is taken as 27Â°C and rela-

model and simulate air flow in a defined

tive humidity as 70%. Since people in an of-

space and offers a graphical means of under-

fice have a sedentary activity, their metabolic

standing the behaviour.

activity is rated at 1 Met, which is equal to

A predefined plan exists for which thermal comfort has to be achieved. The base geometry is in the form of an irregular polygon, largely resembling a rectangle of 9.75m by 7.5m, with niches and voids along the edge and a central courtyard. The typology is an office block with people working in a seated position at their desks.

58.2 W/mÂ˛. The clothing is light and equal to 0.5clo. In most offices occupants are sitting at a computer, which is also releasing heat. To dissipate this heat as well as that of people the air is let out at this height, and thus the user plane (iso-surface) is taken at 750mm. For outdoors the user plane is taken at 1.2m, since the breeze will cool the neck region for maximum comfort.

1.2 Need for natural ventilation (a review) With the current trend in energy consumption it is estimated that we will need 2.5 earth to sustain us [2]. Countries like USA and UAE consume even more. Building sector is one

on the roof of a building which looks like a tower and brings in the fresh air from outside [5]. They are called Baud-Geer in the Persian Gulf area and Malqaf in the Arabic architecture [6-8].

of the major consumer of energy and it is high time that alternate means be adopted to conserve our resources. Space cooling, heating and ventilation consumes the highest percentage of energy in a building. In hot climates this is dominated by space cooling. Countries like Dubai, Egypt, Malaysia, Singapore, India etc have a high cooling demand. Another research [3] found that HVAC consumes more than 60% of total building energy demand. The aim is to produce a thermally comfortable space for the occupants.

Figure 4: A typical wind catcher in the form of a tower

Traditional solutions in vernacular architecture can be adopted or integrated with new technology to make them compatible with modern requirements [9]. In naturally ventilated buildings, occupants may accept higher temperatures than in air-conditioned buildings, and natural wind with a relatively high average velocity is more acceptable than mechanical wind [10]. In addition, de Dear and Brager [11] analysed thermal comfort in naturally ventilated buildings using the ASHRAE RP-884 database. They found that the occupants in naturally ventilated buildings pre-

Figure 3:Commercial Sector Buildings Energy End Use

ferred a wider range of conditions than those

It is interesting to note that traditional archi-

in HVAC buildings, and their indoor thermal

tecture in many hot regions adopted mecha-

comfort temperature ranges more closely re-

nisms to cool their houses naturally. One

flected the outdoor climate patterns.

such feature is the use of natural ventilation. Natural wind, not only provides a good indoor environmental quality but also provides a comfortable, healthy and hygiene indoor climate [4]. The means to achieve this, was by using a device called a wind catcher mounted

Figure 5: Types of traditional windcatchers

Natural ventilation uses the natural forces of wind pressure and stacks effect to aid and direct the movement of air through buildings [12]. Wind incident on a building face will produce a positive pressure on the windward side and a relative negative pressure on the leeward side. This pressure difference on the Figure 6: University of Qatar, Doha

outside as well as the pressure differences

With recent studies in thermal comfort of nat-

inside the building will drive airflow [13].

urally ventilated buildings, an increasing awareness has been generated in the bene-

Heat loss due to air change in a building is calculated using the following equation:

fits of naturally ventilated spaces. This is triggered the use of windcatchers in a more con-

Qv=ρ*V*C*dT……………………………….[1]

textual way, for instance, the University of

where Qv is heat in Watts [J/s], V is ventilation

Qatar, Doha has incorporated a brand new

rate [m³/s], C is specific heat capacity of air

windcatcher to ventilate the indoor spaces. 4

and dT is temperature difference inside and

The major drawback of natural ventilation is

outside [K]. The formula can be further sim-

that, it is dependent on wind velocity and di-

plified to:

rection which can constantly fluctuate, thus

Qv=(N*v*dT)/3…………………………….…[2]

reducing their efficiency. The presence of

(Building Services & Equipment- Volume 1 - 3rd edition. F Hall)

where N is number of air changes per hour in the building and v is the volume of the space. A much more complex relationship exists if humidity is also considered. For each specific relative humidity, a curve is plotted showing

many structures in an urban area change the wind pattern significantly, and thus taller towers have to be constructed to catch wind. Other disadvantage of the system is the limited controllability and slow charge/discharge process which are very important parameters for achieving internal thermal comfort [1].

the relation between air speed and change in

For the first part of the research, the variable

temperature.

factors are indoor temperature, duct vent areas, and inlet velocity. For the second part of the research wind velocity, direction, solar radiation, ground surface temperature and ambient

Figure 7: Cooling effect by air movement

This cooling effect is the key behind naturally ventilated buildings. They help in conserving energy by relying on natural phenomenon, maintain a good ambient air quality and, also encourage people to adapt to a large range of temperatures. Based on the research work and finding of Povl Ole Fanger, a software is available which can calculate the predicted percentage dissatisfied (PPD) given the air temperature, mean radiant temperature, air velocity, relative humidity, clo value and met value. Thermal comfort of occupants is calculated using this software for this report.

Indoor Thermal Comfort

2.2 Modelling Windtower The next step is to plan windtowers to catch

2.1 Modelling Basic Geometry The office plan was sketched in DesignModeler and then the surface was extruded to a height of 2.4mts. The volume so formed is the total internal space, excluding the walls. Since the factor under study is the controlled ventilation through wind tower(s), no win-

and direct wind into the room. Since there is a cost associated with each tower, the optimal solution is the one with the least number of them. Following are some towers, from the design evolution stage, which were inspired by the architectural motifs of prominent styles in Malaysia.

dows or doors are placed which could interfere with the air movement.

Figure 8: Step 1 - Sketching the office footprint and extruding the volume Figure 11: A, B- Bidirectional windcatcher; C, D- Unidirectional windcatcher

The final design consists of two windcatchers in the form of minarets, on the windward side of the block and another on the leeward side to draw out air. To drive wind effectively into the duct below, the windcatchers have a funnel shaped head, and a slender triangular Figure 9: Step 2 - Sketching courtyard, extruding and subtracting the two geometries

tower which reduces in cross sectional area to speed up airflow. Air is let into the room at two heights, near the floor at 300mm and near the ceiling at 2100mm. Air is filtered through a damper before being let inside. This is represented by vertical bars of 10mm thickness and depth of 100mm, spaced at 30mm center to center.

Figure 10: The base geometry, isometric view

Figure 12: Building elevations, (not to scale)

2.3 Indoor Ventilation Parametric Study The table lists the independent variables under investigation in section A, and the dependent variables on which the change was observed, in Section B. For each experiment the indoor air velocity was recorded at four critical points determined by their proximity form the inlet and outlet vents. Figure 13: Location of critical points

0.5

Section A

Section B

Inlet Velocity (m/s)

Air Velocity

1.0

1.5

2.0

External Vent Area (m²) 1.395

1.815

2.515

3.119

Internal Vent Area (m²) 0.745

1.225

Temperature (°C) 26

At four key points Predicted Percentage Dissatisfied For the four key points Area-weighted Average Velocity At the workplane Air velocity Uniformity Index At the work plane

Table 1: Parameters to assess indoor thermal comfort

2.3.1 Inlet Velocity It is interesting to note that the PPD for each

For the first study the vent areas and the tem-

point at any inlet velocity is well less than

perature were held constant, while the veloc-

20%, which is the limit set by ASHRAE. Even

ity was varied from 0.5m/s to 2.0m/s with an

the area weighted average velocity at the

increment of 0.5m/s for each CFD simulation.

work plane has sufficient magnitude for ther-

The external vent area was kept 1.815m², the

mal comfort at any given inlet condition. The

internal vent area at 0.825m², and the indoor

uniformity index on an average is 74.5%.

temperature at 27°C.

Since people have control of the inlet velocity The converged result was recorded and the

through fan regulators, thermal comfort can

corresponding PPD calculated, along with

be achieved at almost any condition.

the uniformity index at the work plane (0.75m). Inlet Air Velocity 0.5 Name Point-a Point-b Point-c Point-d

Work Plane

Velocity Magnitude

1.0 PPD

0.14 0.06 0.01 0.11 Area wt. Avg Velocity

8.50% 12.60% 12.60% 10.10%

0.10

73.38%

Velocity Magnitude

1.5 PPD

0.27 0.19 0.18 0.31 Area wt. Avg Velocity

5.40% 6.80% 7.10% 5.10%

0.24

75.28%

Velocity Magnitude

2.0 PPD

0.60 0.25 0.39 0.20 Area wt. Avg Velocity

6.10% 5.70% 5.00% 6.50%

0.35

74.96%

Velocity Magnitude 0.64 0.33 0.10 0.16 Area wt. Avg Velocity

6.50% 5.10% 10.80% 7.70%

0.43

74.20%

Table 2:Results from experiment one

Velocity at Critical Points 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.5 Point-a

1 Point-b

1.5 Point-c

Point-d

PPD

2 Area-Wt Avg Velocity

Figure 14: Graph of point velocity vs inlet speed

The velocity of points near the inlet appear to have a positive correlation with the inlet velocity, while the points away from the inlet shows an increase and then deceasing magnitude. Overall the average velocity at the work plane also increases with the increase in inlet speed. The uniformity index reaches the highest value at 1m/s and then shows a downward trend. This can be attributed to the increased turbulence due to higher velocity. The region south of the courtyard, which in near the inlet vents, has in general well distributed velocity. While in the northern end, closer to the outlet vent, there are pockets of low air speed and wind shadow regions. The maximum speeds occur near the outlet as air rushes into the vent, creating a suction effect. 2.3.2 External Vent Area For the second part of the study, inlet velocity was held constant at 1m/s, the internal vent area and temperature was the same as before i.e. 0.825mÂ˛ and 27Â°C respectively. The external vent area was changed to face more or less wind. The design of the tower and the ducts was also kept the same, while the windcatcher mounted on top of the tower had different alternatives with different areas. This was achieved by having multiple openings or increased funnel head. The design process considers the various styles of windcatcher existing traditionally. Figure 16 on the next page shows the design implemented. Figure 15: Work plane velocity contour at different inlet speed

On an average there is an increase in velocity at the work plane, with the increase in area of opening. Although the change is gradual and not well marked, it can be seen that there is a fall in average velocity in the last case. Point-a shows a positive correlation and point-c shows a negative correlation with the external vent area. Point-d has an increase in velocity until reaching a certain limit and then decreases, while a fluctuating value is seen at point-b. The relationship is represented in

Figure 16: Design of windcatcher

the graph below. Total External Vent Area (mÂ˛) 1.395 Name Point-a Point-b Point-c Point-d

Work Plane

Velocity Magnitude

1.815

2.515

3.119

PPD

Velocity Magnitude

PPD

Velocity Magnitude

PPD

Velocity Magnitude

PPD

0.18 0.08 0.19 0.09 Area wt. Avg Velocity

7.10% 12.50% 6.80% 11.60%

0.27 0.19 0.18 0.31

5.40% 6.80% 7.10% 5.10%

0.29 0.08 0.15 0.46

5.30% 12.50% 8.10% 5.20%

0.43 0.20 0.10 0.06

5.10% 6.50% 10.80% 12.60%

Area wt. Avg Velocity

0.20

72.04%

0.24

75.28%

0.34

74.19%

0.33

70.48%

Table 3: Point velocities recorded for different external vent areas

Velocity at Critical Points 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 Area 1.395

Area 1.815 Point-a

Point-b

Area 2.515 Point-c

Point-d

Area 3.119 Work Plane

Figure 17: Graph of velocity at a point vs the design type

In general, we see a higher ventilation rate with increasing external vent area, moreover even the building corners and edges are well ventilated and airier. Turbulence has also increased, resulting in eddy formation and velocities beyond 1m/s at some locations. Similar to the previous case observation, the southern half is more ventilated compared to the northern side, where there is are pockets of different velocities, especially in the corners. The presence of a courtyard creates a shadow region on the north side and the outlet creates a suction force, the two factors combined produce intense velocities near the exit. This feature is observed in each case 2.3.3 Internal Vent Area The terminal opening in the duct, which opens into the space, actually determine the characteristics of the air flow, since it can regulate the speed, direction as well as the smoothness of the flow. For this part of the study various internal vent design were considered having different opening characteristic and area. For each case the inlet velocity was set at 1m/s, external vent area at 1.815. The PPD was calculated for each critical point for a temperature of 27Â°C. In case one, the inlet had type A damper installed and outlet had type B. For the second case, the inlet had type B and outlet was type C. For the third case both were installed with type C, and for the final simulation, both inlet and outlet with increased area of type C. Figure 18: Work plane velocity contour for different vent areas

In type A damper, a grille of depth 10cm is put next to the vent opening, with mesh size of10mm. Type B damper is a similar structure but with vertical units only. Apart from reducing area of the vent, it also forces air to move in a straight path which gives rise to laminar flow of air in the room. The readings were taken at critical points along with the average velocity and uniformity index, which is shown in the table below. It is interesting to note that the average velocity (area weighted) has a positive correlation with the vent area, while the uniformity index has a negative. Figure 19: Damper types installed for controlling vent area

Total Internal Vent Area (mÂ˛) 0.610

0.825

1.125

2.250

Name

Velocity Magnitude

PPD

Velocity Magnitude

PPD

Velocity Magnitude

PPD

Velocity Magnitude

PPD

Point-a Point-b Point-c Point-d

0.22 0.18 0.11 0.08

6.10% 7.10% 10.10% 12.50%

0.27 0.19 0.18 0.31

5.40% 6.80% 7.10% 5.10%

0.19 0.26 0.12 0.28

6.80% 5.50% 9.50% 5.30%

0.36 0.17 0.20 0.06

5.00% 7.40% 6.50% 12.60%

Area wt. Avg Velocity

0.19

71.25%

0.24

75.28%

0.22

68.96%

0.26

65.36%

Work Plane

Table 4: Point velocity recorded for different internal vent areas:

Velocity at Critical Points

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.610

0.825 Point-a

Point-b

1.125 Point-c

Point-d

2.250 Work Plane

Figure 20: Graph of point velocities vs internal vent area

This can be better understood by the velocity contours plotted at use plane. At lower area, the velocity is generally low and evenly spread. Increasing the vent area leads to higher velocities near the ducts and air shadow regions along the perimeter of the room. Moreover, there is much turbulence observed at higher areas. Points a, b and c, near the inlet, show increase in velocity with fluctuations suggesting the general rise in speed near the inlet coupled with erratic uncontrolled speeds sometime. Whereas point d, slight away from outlet and inlet showing no significant correlation. In essence, a higher vent area requires internal reorientation and planning to achieve a better uniformity. For instance the courtyard acts as a deflector, and other such features can be installed with such features. Wind wall is one example, also office furniture will have a diverse effect on the air flow. 2.3.4 Temperature Based on the optimal configurations from the previous studies, the critical points were evaluated for thermal comfort. Likewise, the inlet velocity was held constant at 1m/s for all the setups, external and internal vent areas were 1.815mÂ˛ and 0.825mÂ˛ respectively. The air flow pattern so achieved had the highest uniformity index of 75.28%, with the average velocity of 0.24m/s. The PPD for each point at various temperatures was consequently recorded and is shown in the table 5.

Figure 21: Work plane velocity contours for different internal vent areas

Temperature (Â°C) 26.000

27.000

28.000

29.000

Velocity Magnitude

PPD

Velocity Magnitude

PPD

Velocity Magnitude

PPD

Velocity Magnitude

PPD

Point-a Point-b Point-c Point-d

0.27 0.19 0.18 0.31

6.80% 5.30% 5.20% 7.80%

0.27 0.19 0.18 0.31

5.40% 6.80% 7.10% 5.10%

0.27 0.19 0.18 0.31

12.20% 15.70% 16.30% 11.30%

0.27 0.19 0.18 0.31

27.60% 32.30% 33.00% 25.70%

Average Velocity (area-wt)

0.24

6.10%

0.24

5.80%

0.24

13.40%

0.24

29.10%

Name

Table 5: PPD at critical points for different temperature conditions:

Percentage Dissatisfied at 1m/s inlet velocity

35.00% 30.00% 25.00% 20.00% 15.00% 10.00% 5.00% 0.00% 25.5

26 Point-a

26.5

27 Point-b

27.5 Point-c

28 Point-d

28.5

29.5

Average Velocity

Figure 22: PPD at critical points for various temperature range

It was not a surprise to observe that the PPD

tion when compared to points a and d. On

increased with higher temperatures. Higher

closer observation, it can be seen that points

temperature range values demand

b and c fall in a local dull region with a slightly

ventilation rate for thermal comfort since

lower velocity as compared to their surround-

more heat has to be dissipated. Since the ex-

ings. Points a and d on the other hand are in

periment limits the inlet velocity, the indoor

a draught region which accounts for a lower

space gets uncomfortable after a certain tem-

PPD at higher temperatures. In general, this

perature for a large section of the people.

design configuration works well for a temper-

What is interesting to observe is that the points b and c have overall higher dissatisfac-

ature up to 28Â°C, beyond which higher ventilation rate is needed.

Outdoor Thermal Sensation

3.2 Outdoor Ventilation Parametric Study

3.1 Modelling Combined Geometry on Site

For the study of outdoor comfort, the parameters listed n section A were varied to ob-

This part of the study deals with the response of the building with its context, mainly the

serve the effect on parameters listed in section B.

speed and direction of the prevalent wind. This is assumed to be blowing from the south, since the windcatchers were designed to take the benefit of pressure difference across the inlet and outlet. Thus, the inlet towers are in the south and outlet towers in the north. The basic geometry along with the ventilation system (with single side multiple opening) was placed in the centre of a site, repre-

Section A Site Inlet Velocity (m/s) 0.5 2.0 3.5 Wind Direction (due South) 0 45 90 Solar Radiation (W/m²) 50 250 450 Surface Temperature (°C) 30 60 90 Ambient Air Temperature (°C) 26 27 29

Section B Air Velocity

At four key points Thermal Sensation Index (TSI) For the four key points

Table 6: Parameters under study

sented by a body of dimension 19.5m by 16.5m, and a height of 15m. The resulting

Givoni et el developed an equation express-

subtracted volume, produced by subtracting

ing thermal sensation (TSI) as a function of

building geometry from site geometry, gives

five variables including surface temperatures

us the air around to simulate wind conditions.

of surrounding materials and solar radiation

The volume of the air enclosed is 4668.4m³.

[14]. The index is based on a 7-point scale with 4 as the neutral value. Index below 4 is for cold sensation and higher values for hot. Each parameter was individually studied, and TSI was calculated at five key point to evaluate user sensation at 1.2m height (mid-chest level for standing person). For this purpose, 2 points were chosen at 2 different windward edges (in the small building niche). One point was chosen in the, northern, wind shadow region, which has two arms of the building around it. The fourth point was taken in the courtyard, where users may sometime step out for fresh air, and the fifth was taken at the

Figure 23: Building on site; the grey region representing the volume of air around it.

south-eastern corner of the block.

The southern and western side surfaces was given an inlet velocity of varying magnitude but a constant direction. This was set at X=1, Y=0, Z=1; simulating the condition of southwesterly wind. Other parameters were kept constant for this part, namely direct solar radiation (250 W/m²), ground surface temperature (60°C), humidity (70% RH) and ambient air temperature (27°C). The velocities at key points were recorded and the thermal sensation index was calculated using WINCOM12.

Figure 24: Location of key points on site

The table below lists the point velocities and

3.2.1 Site Inlet Velocity

corresponding TSI for each different site inlet

To assess the effect of velocity on the ther-

condition.

mal comfort, CFD simulation was carried out on Fluent with the wind blowing from a single direction, 45° due south.

Site Inlet Air Velocity (m/s) 0.5 Point

2.0

3.5

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

0.42 0.50 0.06 0.05 0.70

5 5 5 5 5

1.69 2.01 0.26 0.20 2.80

4 4 5 5 4

2.95 3.52 0.46 0.34 4.91

4 4 5 5 3

Point-a Point-b Point-c Point-d Point-e

Table 7: Velocity at key points for different site inlet velocity

TSI at Key Points Thermal Sensation Index

7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.5

2.0

3.5

SIte Inlet Air Velocity Point-a

Point-b

Point-c

Point-d

Point-e

Figure 25: Graph of TSI vs site inlet velocity

At low velocities, most people would feel

The slope of velocity curve of points a, b and

warm at any point around the building, since

e is much greater than c and d, resulting in

the heat from the solar radiation and ground

this disparity.

creates a warm environment. At higher velocities, points a, b and e have a comfortable index. While points c and d are still considered warm. It is evident that these points lying in the wind shadow region never really get high wind intensities to feel comfortable. Point e being locate in the corner experiences higher wind speeds since the building offers resistance to the air flow, and deflects it around the corners. These corners are areas of high turbulence and velocity. At higher wind speeds it can become quite cold to stand at these points. We do however observe that the points c and d have slightly increased velocities with increasing inlet speed. As such there will be a stage when comfort will be achieved at those points also, but those high velocities will cause further cooling at the windward points.

Velocity at key points 5.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 0.5

2.0 Point-a

Point-b

Point-d

Point-e

Figure 26: Velocity at key points

3.5 Point-c

Figure 27: Velocity contours at 1.2m height

3.2.2 Site Inlet Wind Direction Wind direction is usually very erratic at a location, it can blow from one side for a while and instantly change direction after some time. A prevailing direction is usually considered when designing structures which may obstruct wind. For this part of the study, the wind speed is fixed at 2.0m/s with the direction changing from southerly to westerly wind.

Figure 28: Wind direction simulated for CFD

Wind Direction (measured from due South) 0° Name Point-a Point-b Point-c Point-d Point-e

45°

90°

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

1.18 0.47 0.16 0.04 2.34

5 5 5 5 4

1.69 2.01 0.26 0.20 2.80

4 4 5 5 4

0.33 1.26 0.35 0.07 0.35

5 5 5 5 5

Table 8: Velocity at key points at varying wind direction

TSI at Key Points

Thermal Sensation Index

7.0 6.0 5.0 4.0 3.0 2.0 1.0 0

Wind Direction (from due South) Point-a

Point-b

Point-c

Point-d

Point-e

Figure 29 A graph of TSI vs wind direction for each selected point:

For most cases the comfort index reads

We can conclude that the best wind direction

warm. This is quite prominent for points c and

for this building configuration is when the

d which are in the, calm, wind shadow zone

wind blows from a corner position, leading to

for all scenarios. Point e experiences com-

higher number of comfort regions.

fortable conditions since it is a region of high velocity due to bending of wind around the building. For the case of 90Â°, it however, falls in a shadow zone and experiences higher (warmer) TSI. It is conclusive from the velocity contours that the areas around the corner experience higher velocities and thus should be made use of during site planning of the building. As for the case of the windcatchers, the two inlet towers are in a high velocity (high pressure) region, while the outlet is in a low velocity (low pressure) zone, this creates a suction force and drives the wind through the building. Only when the wind blows parallel to both the towers when it will be essential to use mechanical system for ventilation in the building.

Velocity at key points Wind velocity at key point

3.00 2.50 2.00 1.50 1.00 0.50 0.00 0

Inlet wind direction Point-a

Point-b

Point-d

Point-e

Point-c

Figure 30: Velocity at points vs wind direction

Figure 31: Velocity contours at 1.2m height

3.2.3 Direct Solar Radiation

A positive correlation is observed between

For the optimum condition of wind blowing

TSI and solar radiation, which is no surprise,

from south-west at a speed of 2m/s, the effect

since the sun’s rays have heating capacity. It

of varying solar radiation on TSI was calcu-

is, in fact, worth noting that the relationship is

lated at the key points to determine comfort

a linear one, and every point has the same

level.

gradient. The site exhibits all kind of thermal conditions ranging from as low as 3.7 to as

For this study, the actual value of TSI was

high as 5.3 TSI. Users can choose to move

plotted against solar radiation to give better

to a more comfortable region of their liking.

resolution for comparison.

Direct Solar Radiation (W/m²) 50 Name Point-a Point-b Point-c Point-d Point-e

250

450

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

1.69 2.01 0.26 0.20 2.80

4.08 3.98 4.54 4.56 3.73

1.69 2.01 0.26 0.20 2.80

4.46 4.36 4.92 4.94 4.11

1.69 2.01 0.26 0.20 2.80

4.84 4.74 5.30 5.32 4.49

Figure 32: TSI at varying solar radiation

effect of solar radiation. At higher radiation

TSI at varying Solar Radiation 5.5

the wind speed is not strong enough to produce thermally comfortable condition. During

5.1

the course of a day, the points will have dif-

Thermal Sensation Index

5.3 4.9

ferent solar radiation falling on the ground

4.7 4.5

due to presence of buildings and other site

4.3

conditions. These obstructions will lead to

4.1

simulation of different thermally comfortable

3.9

conditions, of which the user can choose to

3.7 3.5 50

250

450

Direct Solar Radiation (W/m²) Point-a

Point-b

Point-d

Point-e

move from one to another. 3.2.4 Ground Surface Temperature

Point-c

Figure 33: Graph of TSI plotted at varying solar radiation

The urban heat island (UHI) phenomenon rises as urbanization increases. A key characteristic of urban areas is artificial surfaces,

Points c and d being in a lower wind speed

which can absorb and store great amounts of

region have a higher TSI i.e. are quite hot,

heat throughout the day [15]. Thus, it is im-

and will become unbearable after a certain

portant to study the impact of ground surface

solar radiation. Point e is quite cool to begin

temperatures on user comfort.

with since the high wind speed nullifies the 20

For this study the TSI was calculated for the

of temperatures as high as 93°C reaching in

key points with ground temperatures of 30°C,

the Death Valley, California. However, tem-

60°C and 90°C. The other parameters were

peratures even as high as this can be com-

kept constant, namely wind blowing at opti-

pensated by designing a properly ventilated

mal condition with 250W/m² solar radiation

region. The same can be observed when in

falling on site. For each point the TSI was cal-

the graph below. Clearly, at higher tempera-

culated for the three temperature values.

tures a velocity of more than 2m/s can create quite comfortable environment.

Again, we observe a similar trend, mainly, the linear relation of ground temperature with

Since the same velocity can create a chilly

TSI. Suffice to say that either extremes of

environment at cooler ground temperature, it

ground surface temperatures will cause dis-

is advisable to control the ventilation rate.

comfort for users. There have been reports Ground Surface Temperature (°C) 30

Name

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

Point-a Point-b Point-c Point-d Point-e

1.69 2.01 0.26 0.20 2.80

4.30 4.19 4.76 4.78 3.94

1.69 2.01 0.26 0.20 2.80

4.46 4.36 4.92 4.94 4.11

1.69 2.01 0.26 0.20 2.80

4.62 4.52 5.08 5.10 4.27

Table 9: TSI at key points at varying ground surface temperatures

TSI at various Ground Surface Temperature 5.5

Thermal Sensation Index

5.3 5.1 4.9 4.7 4.5 4.3 4.1 3.9 3.7 3.5 30

Ground Surface Temperature (°C) Point-a

Point-b

Point-c

Point-d

Point-e

Figure 34: Graph of TSI plotted against various Ground Surface Temperatures

3.2.5 Ambient Air Temperature

ture. Even a degree rise in temperature of air

Finally, the ambient air temperature was par-

has led to higher TSI, which also means that

ametrized to observe the corresponding

rate of increase of ventilation is much more

change in the thermal index. TSI is essen-

for air temperature rather than ground tem-

tially similar to the PMV model but includes

perature.

more factors. Since both the models were

This is a major reason why during site plan-

generated based on intensive regression of

ning more care is taken in controlling the air

experimental data.

temperatures around, and people prefer airy

The graph is quite steep as compared to the

locations for higher wind speeds. In the cur-

relation of ground surface temperature. In the

rent context, points c and d require proper

first case we observe that the gradient is

measures to prevent over heating as well in-

0.005 while in the current case it is 0.109.

troduction of features to divert wind into the

This means that the air temperature play a

north and east sides of the building and the

more important role in determining outdoor

courtyard. Only then can there be thermal

comfort rather than ground surface tempera

comfort for these regions at all times.

Ambient Outdoor Air Temperature (Â°C) 26.000 Point Point-a Point-b Point-c Point-d Point-e

27.000

29.000

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

Velocity Magnitude

Thermal Sensation Index

1.69 2.01 0.26 0.20 2.80

4.35 4.24 4.81 4.83 3.99

1.69 2.01 0.26 0.20 2.80

4.46 4.36 4.92 4.94 4.11

1.69 2.01 0.26 0.20 2.80

4.68 4.58 5.15 5.17 4.33

Table 10: TSI for key points at various ambient air temperatures

TSI at varying Ambient Air Temperature 5.5

Thermal Sensation Index

5.3 5.1 4.9 4.7 4.5 4.3 4.1 3.9 3.7 3.5 26

Ambient Outside Air Temperature Point-a

Point-b

Point-c

Point-d

Point-e

Figure 35: Graph of TSI at key points plotted against air temperatures

Conclusion and Recommendations 4.1 Conclusions Drawn from the Project The indoor thermal comfort is a challenge for designers and consultants if decided to provide by natural means. In recent years it has become important to conserve and save en-

Thus, for a good uniformity index it is advisable to have more number of outlet vents as compared to inlet vents. They should also be spaced out depending on the building design so that the flow is well spread.

ergy. Factors which determine comfort needs

In case of pockets of draught or no wind

to be understood in detail and their relation-

zones, architectural features like book-

ship with each other.

shelves, desks, interior walls etc should be

For the indoor air flow, it is evident from the contour maps that outlet vent plays more important role in ventilation rather than inlet vents. Speed of inlet vent determines the flow characteristic of the air in the room,

places to divert wind. These features will act like wing walls and ventilated the shallow spaces. The design should be a gentle curve or similar nature to prevent further eddies in the room.

whether it is laminar or turbulent with eddies

The speed of inlet velocity also helps in better

in some room corners. The speed is again a

ventilation of the space, but at higher veloci-

function of the vent area, room dimension,

ties there is much turbulence in the air flow.

design of ducts etc.

Thus, an optimum balance has to be reached

For outdoor comfort, a site cannot have the same condition throughout, since there will be variation in landscape and landform, either natural or man-made. Thus, there are points of different TSI around the building. The building also provides shading from sun

for each design to determine the maximum allowable speed. Similarly, the external duct area also needs to be optimized for each design. A higher area generally leads to increased velocity and turbulence, and a balance has to be sought.

and wind on certain zones around it, which

Inlet vent area, if maintained small and well

may become region of comfort or discomfort.

distributed will give a good uniformity index

Thus, built form orientation is important for

and devoid of turbulences. In the case of us-

site planning and urban growth.

ing grilles and vertical dampers the UI was

4.2 Design Recommendations

much higher compared to the other cases., but due to decreased area the speed of air

The design and position of the duct is a key

was less. To compensate that numerous

in achieving indoor comfort. A higher number

vents can be placed which has the same area

of outlet vents, placed in more parts of the

as an uncovered went. Offices do not gener-

room will lead to a good spread of air. This is

ally also want velocities beyond 0.6m/s, as

the criteria of a good ventilation system. 23

then papers and other desktop object begin

changing pattern. The current model uses

to fly off and create discomfort.

only 4-5 points for measurement. This

The inlet vent when placed higher up on a wall generally has a better uniformity index on the work plane below. The current model with vents near the ceiling was developed after finding poor UI when the same vents were placed near the floor. Hence the experiment takes into account both positions to determine the suitability of inlet vents.

leaves margins for error, but gives scope for future experiment to be done and behaviour to be ascertained. b) The main windcatcher design has been inspired by the styles of the traditional architecture. In vernacular ventilation systems, straight sections were employed to di-vert wind into the room. There is much pressure lost in this system, with generation of

For outdoor thermal comfort the main prob-

turbulence. In practice this is avoided by

lem of having some region with high TSI is

having curved edges and gentle bends in

because they are in wind shadow region. The

the duct section to prevent such losses. In

design intent should be such that these

the present experiment, dampers have

spaces are also well ventilated to create cool-

been installed at the terminal openings of

ing effect. For instance, the central courtyard

a duct. to generate laminar flow.

seems to have very little air flow to cause sufficient cooling, if the building does not provide shading from the sun then the region can get severely hot inside. Shading is required in many parts of the site to prevent high values of solar irradiance falling on the people. Shading is also needed to prevent high wind area on the wind ward side, this can be done by planting trees and plants. They have a two-fold advantage. Apart from providing shade they also help in reducing heat island effect by absorbing the heat, corollary the ground surface temperature is also kept low. 4.3 Limitations a) The number and location of test points vary from design to design and a higher number of them are required to be simu-

Figure 36: Windtower with curved edges to reduce air friction

lated for air velocity and observe the 24

c) The user plane is assumed to be at

plane (900mm) is given in the table below.

750mm, which is also the height of working

It shows a better UI, while the average ve-

desk, but in reality, people may move

locity is more or less the same. We may

around the place or there may be other fur-

observe better UI at even higher heights of

niture with higher level. Thus, diverse

about 1.2m (standing people). Moreover,

range of work plane is required to monitor

office furniture can cause turbulence in

comfort. For instance, if people are sitting

wind flow at lower levels. The table below

on low chairs, their neck will be around the

lists the average speeds on two different

heights of 900mm to 1000mm. The aver-

level for comparison, along with the stand-

age velocity and uniformity index at that

ard 750mm. Inlet Air Velocity

0.5

1.0

Area wt. Avg Velocity

0.75 m

0.10

73.38%

0.9 m

0.09

1.2 m

0.09

IsoSurface @

1.5

2.0

Area wt. Avg Velocity

0.24

75.28%

0.35

74.96%

0.43

74.20%

74.50%

0.23

75.50%

0.33

75.70%

0.41

75.11%

73.65%

0.21

75.29%

0.31

76.86%

0.37

74.44%

Table 11: Average velocities and UI at higher work plane:

Appendices The convergence criteria for the simulation is as shown in figure 37.

Figure 40: Convergence at 300 for case 2.3.1 (1.5m/s)

Figure 37: Convergence criteria

For each simulation controls have been adjusted to achieve convergence. The number of iterations needed to reach convergence for each case is shown in Figures 38 to 45.

Figure 41: Convergence at 358 for case 2.3.1 (2.0m/s)

Figure 38: Convergence at 358 for case 2.3.1 (0.5m/s)

Figure 42: Convergence at 161 for case 2.3.2 (1.395mÂ˛)

Figure 39: Convergence at 202 for case 2.3.1 (1.0m/s) Figure 43: Convergence at 391 for case 2.3.2 (2.515mÂ˛)

Figure 48: Convergence at ~3700 for case 3.2.1 (2m/s) Figure 44: Convergence at 1480 for case 2.3.3 (0.610m²)

Figure 49: Convergence at ~5500 for case 3.2.1 (3m/s)

Figure 45: Convergence at 2491 for case 2.3.3 (1.125m²)

Figure 50: Convergence at ~7716 for case 3.2.2 (0°)

Figure 46: Convergence at 1102 for case 2.3.3 (2.25m²)

Figure 51: Convergence at ~3830 for case 3.2.2 (45°) Figure 47: Convergence at ~1900 for case 3.2.1 (0.5m/s)

The solution for case 3.2.2 (90°) had not reached convergence, the program was allowed to run for more than 9000 iterations multiple times. The least residual value is presented in this report. 27

References [1]. Ben Richard Hughes, John Kaiser Calautit, Saud Abdul Ghani. The development of commercial wind towers for natural ventilation: A review (2011) [2]. Global Footprint Network, Annual report 2011 [3]. Chan H-Y, Riffat SB, Zhu J. Review of passive solar heating and cooling technologies. Renewable and Sustainable Energy Reviews 2010. [4]. Omidreza Saadatian, Lim Chin Haw, K. Sopian, M.Y. Sulaiman.

Review of

windcatcher technologies (2012). [5]. Bahadori MM. Passive cooling systems in Iranian architecture. Scientific American Journal 1978. [6]. Hughes BR, Chaudhry HN, Ghani SA. A review of sustainable cooling technologies in buildings. Renewable and Sustainable Energy Reviews 2011. [7]. Yaghoubi MA, Sabzevari A, Goneshan AA. Wind towers: measurement and performance. Solar Energy 1991 [8]. Battle McCarthy Consulting E. Wind towers: Academy Editions; 1999 [9]. Fathy H. Natural energy and vernacular architecture: principles and examples with reference to hot arid climates. University of Chicago Press; 1986. [10]. Busch, J.F. A tale of two populations: thermal comfort in air-conditioned and naturally ventilated offices in Thailand. EnergyBuild (1992). [11]. de Dear,R., Brager,G.S. Thermal comfort in naturally ventilated buildings: revisions to ASHRAE Standard 55. Energy Build (2002) [12]. E. Olsen, Q. Chen, Energy consumption and comfort analysis for different low energy cooling systems in a mild climate, Energy & Buildings 35 (2003). [13]. Naghman Khan, Yuehong Su, Saffa B. Riffat. A review of wind driven ventilation techniques (2007). [14]. Wendy Walls, Nicki Parker and Jillian Walliss. Designing with thermal comfort indices in outdoor sites [15]. Surface and ambient air temperatures associated with different ground material: a case study at the University of California, Berkeley. Katharine K. Guan Figure 2. Neufert Architectsâ&#x20AC;&#x2122; Data, third edition Figure 3. US Department of Energy Annual report 2008 Figure 4. https://www.designingbuildings.co.uk/ wiki/Windcatcher Figure 5, 6. Review of windcatcher technologies (2012)

List of Figures Figure 1: Office space under consideration, (dimensions in meters) ........................................... 2 Figure 2: Suggested working height ............................................................................................. 2 Figure 3:Commercial Sector Buildings Energy End Use .............................................................. 3 Figure 4: A typical wind catcher in the form of a tower ................................................................. 3 Figure 5: Types of traditional windcatchers .................................................................................. 4 Figure 6: University of Qatar, Doha .............................................................................................. 4 Figure 7: Cooling effect by air movement..................................................................................... 5 Figure 8: Step 1 - Sketching the office footprint ........................................................................... 6 Figure 9: Step 2 - Sketching courtyard, ........................................................................................ 6 Figure 10: The base geometry, isometric view............................................................................. 6 Figure 11: A, B- Bidirectional windcatcher; C, D- Unidirectional windcatcher .............................. 6 Figure 12: Building elevations, (not to scale) ............................................................................... 7 Figure 13: Location of critical points ............................................................................................. 7 Figure 14: Graph of point velocity vs inlet speed ......................................................................... 8 Figure 15: Work plane velocity contour at different inlet speed .................................................... 9 Figure 16: Design of windcatcher ............................................................................................... 10 Figure 17: Graph of velocity at a point vs the design type.......................................................... 10 Figure 18: Work plane velocity contour for different vent areas ................................................. 11 Figure 19: Damper types installed for controlling vent area ....................................................... 12 Figure 20: Graph of point velocities vs internal vent area .......................................................... 12 Figure 21: Work plane velocity contours for different internal vent areas ................................... 13 Figure 22: PPD at critical points for various temperature range ................................................. 14 Figure 23: Building on site; the grey region representing the volume of air around it. ................ 15 Figure 24: Location of key points on site .................................................................................... 16 Figure 25: Graph of TSI vs site inlet velocity .............................................................................. 16 Figure 26: Velocity at key points ................................................................................................ 17 Figure 27: Velocity contours at 1.2m height ............................................................................... 17

Figure 28: Wind direction simulated for CFD ............................................................................. 18 Figure 29 A graph of TSI vs wind direction for each selected point: .......................................... 18 Figure 30: Velocity at points vs wind direction ........................................................................... 19 Figure 31: Velocity contours at 1.2m height ............................................................................... 19 Figure 32: TSI at varying solar radiation .................................................................................... 20 Figure 33: Graph of TSI plotted at varying solar radiation .......................................................... 20 Figure 34: Graph of TSI plotted against various Ground Surface Temperatures ....................... 21 Figure 35: Graph of TSI at key points plotted against air temperatures ..................................... 22 Figure 36: Windtower with curved edges to reduce air friction ................................................... 24 Figure 37: Convergence criteria ................................................................................................. 26 Figure 38: Convergence at 358 for case 2.3.1 (0.5m/s) ............................................................. 26 Figure 39: Convergence at 202 for case 2.3.1 (1.0m/s) ............................................................. 26 Figure 40: Convergence at 300 for case 2.3.1 (1.5m/s) ............................................................. 26 Figure 41: Convergence at 358 for case 2.3.1 (2.0m/s) ............................................................. 26 Figure 42: Convergence at 161 for case 2.3.2 (1.395m²)........................................................... 26 Figure 43: Convergence at 391 for case 2.3.2 (2.515m²)........................................................... 26 Figure 44: Convergence at 1480 for case 2.3.3 (0.610m²)......................................................... 27 Figure 45: Convergence at 2491 for case 2.3.3 (1.125m²)......................................................... 27 Figure 46: Convergence at 1102 for case 2.3.3 (2.25m²) .......................................................... 27 Figure 47: Convergence at ~1900 for case 3.2.1 (0.5m/s) ......................................................... 27 Figure 48: Convergence at ~3700 for case 3.2.1 (2m/s) ............................................................ 27 Figure 49: Convergence at ~5500 for case 3.2.1 (3m/s) ............................................................ 27 Figure 50: Convergence at ~7716 for case 3.2.2 (0°) ................................................................ 27 Figure 51: Convergence at ~3830 for case 3.2.2 (45°) .............................................................. 27