Design of Rear wing for high performance cars and Simulation using Computational Fluid Dynamics

INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303

Design of Rear wing for high performance cars and Simulation using Computational Fluid Dynamics S.Sathishkumar2

T.Maniraj1 1

2

Anna University, Automobile Engineering, tamilnambimaniraj4@gmail.com

Anna University, Automobile Engineering, sssatzsathi07@gmail.com

Abstract— The performance of a sports car is not only limited to its engine power but also to aerodynamic properties of the car.

By decreasing the drag force it is possible to reduce the engine power required to achieve same top speed thus decreasing the fuel requirement. The stability of a sports car is considerably important at high speed. The provision of a rear wing increases the downforce thus reducing the rear axle lift and provides increased traction. In this study an optimum rear wing is designed for the high performance car so as to decrease drag and increase downforce. The CAD designed baseline model with or without rear wing is being analyzed in computational fluid dynamics software. The lift and drag coefficient are calculated for all the design thus an optimum rear wing is designed for the considered baseline model.

Index Terms— CAD, CFD, downforce, drag, lift, rear wing, traction ——————————  ——————————

1 INTRODUCTION Aerodynamics makes it major impact on modern vehicles through its contribution to road load. Aerodynamic forces interact with the vehicle causing drag, lift, lateral forces, and moments in roll, pitch, yaw and noise. These impact fuel economy, handling and NVH.The flow around the vehicle not only leads to drag but also causes aerodynamic forces and moments which are components of resulting wind force this affects driving stability[1].The airflow pattern resulting from the forward motion of the vehicle produces lift and pitching moment this affects traction. Due to the lateral side force the yawing moment and rolling moment results. To control the stability of the vehicle these aerodynamic forces can be used positively by producing the needed negative lift that is the downforce to give more traction to the driving wheels [1].

1.1 Aerodynamic Drag force Aerodynamics drag force is the force which opposes the forward motion of the vehicle when the vehicle is traveling. The aerodynamics drag force acts externally on the body of a vehicle.

The aerodynamics drag affects the performance of a car in both speed and fuel economy as it is the power required to overcome the opposing force. Because air flow over a vehicle is so complex, it is necessary to develop semi empirical models to represent the effect [1]. Therefore, aerodynamic drag force is characterized by, D = ½ ρ v2 CD A (1) Coefficient of drag is defined as how the aerodynamic the shape is to the incoming air. CD is determined empirically for the car [2]. It is possible to have an aerodynamic drag coefficient greater than 1 if the air is pushed outward such that the effective area of the air movement is greater than the area of object facing the air.

pressure differential from the top to the bottom of vehicle causes a lift force. These forces are significant concerns in aerodynamic optimization of a vehicle because of their influence on driving stability [2]. The force, L is quantified by the equation L = ½ ρ v2 CL A (2) The lift force dependent on the overall shape of the vehicle. At zero wind angle, lift coefficient normally fall in the range of 0.3 to 0.5 for modern passengers car [1], but under crosswinds conditions the coefficient may increase dramatically reaching value of 1 or more.

1.3 Pressure distribution over the vehicle Most of the lift comes from the surface pressure distribution. A typical pressure distribution on a moving car is shown in figure 1.

The distribution for the most part with simple Bernoulli equation analysis. Location with high speed flow (i.e. over the roof and hood) has low pressure while location with low speed flow (i.e. on the grill and windshield) has high pressure. It is easy to believe that the integrated effect of this pressure distribution would provide a net upward force [3]. This force is negative force, meaning that the force that no need to enhance the stability of a vehicle. The opposite force of upward force is downforce.

1.4 Rear wing

1.2 Aerodynamic Lift force The aerodynamic drag force is acted horizontally to the vehicle and there is another component, directed vertically, called aerodynamic lift. It reduces the frictional forces between the tires and the road, thus changing dramatically the handling characteristics of the vehicle. This will affect the handling and stability of the vehicle. The

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INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303 A Rear wing is an aerodynamic device attached to an automobile rear boot whose intended design function is to modify unfavorable air movement across a body of a vehicle[1].Rear wing aids to produce down force by creating a "dam" at the rear lip of the trunk. This can result in improved vehicle stability by decreasing lift or decreasing drag that may cause unpredictable handling in a car at high speed. Rear wing are often fitted to race and high performance sports car, although they have become common on passenger vehicles as well. Rear wing located on the rear deck may serve several purposes. By deflecting the air upward, the pressure on the rear deck is increase, hence creating a down force at the most advantageous point on the vehicle to reduce lift. If this modified pressure distribution is integrated in the x and y direction, the result is lower drag and lift. A rear wing can have three effects. It can reduce drag, reduce rear-axle lift and reduce dirt on the rear surface. With rear wing also, attention first focused on drag, but increasing emphasis is now placed on negative lift [1][2].

The influence on the pressure distribution is shown in figure below. The possibility of reducing drag is comparatively low. In fact on sporty cars, and even more so on racing cars, even an increase in drag is accepted in order to ensure that the rear-axle lift gets low[2].

The selected wind tunnel is GM wind tunnel whose test section dimensions are as follows: Length: 20.3 m Width: 10.4 m Height: 5.4 m

2 METHODOLOGY 2.1 Modelling in CAD software CAE tools will be used for modeling and analyzing the models.

First, the models will build up in CAD (Computer-Aided Design) software. Mostly people use CAD software to design and build up the model. For this project, SolidWorks and CATIA will be

used to build up the model and the model will be designed according the actual dimension. The design of the rear wing is not just accurate in dimension, but also fixes perfectly with the baseline model that will be use. This precaution step can avoid any errors during analysis and also to make the model of rear wing is easily mate with the baseline model. The baseline model that will be use in this project also must build up according the actual design. The baseline model used is PAGANI HUAYRA and rear wings are NACA designated.

2.2 Dimensions of Rear wing and CAD model The rear wing are modelled in the CATIA software by importing the coordinates from the NACA airfoil generator to produce smooth surface whose dimension are mentioned in the figure below.

2.3 Inputs and selection of Wind tunnel 1.Testing of baseline model

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INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303 2. Testing of aero foils The selected wind tunnel is AUDI wind tunnel whose test section dimensions and area are as follows: Length: 7.5 m Area: 3.01 m2 3. CFD process inputs The various parameters to be set are mentioned as follows: Material: Air with density 1.2754 kg/m3 at 200 C Boundary condition: Inlet: Velocity 80km/h,100km/h,150km/h Outlet: Pressure 0 Pascal Flow: Incompressible Laminar Flow.

2.3 Analyzing in CFD software During this project, Simulation CFD will be use to analyze the car model with its attachment, which is the rear wing. Simulation CFD is the only fluid flow analysis tool for designers fully embedded with Autodesk products. With this software, it can analyze the solid model directly. The model that has been built up in SolidWorks then will be export into Simulation CFD to analyze the model. Through this software, it can analyze parts, assemblies, subassemblies, and multibodies. Detail steps for use this software is include in its tutorial. The design will analyze, the data will interpret, the result will produce and analysis will summarize and present in form of Flow simulation, table, graph, chart or etc. During the analysis, some errors may come. Besides that, some limitations must be considered during analyzing the model to make sure that the results are acceptable. Ground line must be 0.120 m. Ground line is a distance between road surface and bottom part of the car. This distance is important to keep in constant so that the CD and CL are acceptable for high performance car. Another part that is important is the location of installment of rear wing. The location of base of rear wing must be same for all type of rear wing, so that the pressure acting at the back is in same region [3].

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INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303 drag and lift force are calculated by the wall calculator function of the CFD software.

The coefficient of drag and lift are calculated manually with result data from the CFD software for the NACA 2412 and modified NACA 2412. It is seen that NACA 2412 has less drag and more lift. The modified NACA 2412 has more drag than NACA2412 but produces the required downforce which is needed for the designed baseline model.

On comparison it is seen that the baseline model equipped with NACA 2412 has reduced the lift by varying the flow at the baseline models rear end. The baseline model with modified NACA 2412 has reduced the baseline models drag and increased the downforce thereby achieving the objective of the study. The rear wing modified NACA 2412 has altered the flow at the rear end of baseline model considerably. The calculated lift and drag coefficients show that the baseline model equipped with modified NACA 2412 has low drag and lift coefficient comparing to the other two.

ACKNOWLEDGMENT We wish to thank Autodesk for providing licensed software for carrying out our research work and Mr. J.Dhanabal ,Assistant Professor,SVCE, for being our mentor, teaching us basics of Automotive Aerodynamics and providing us SAE edition books for reference.

REFERENCES [1] Wolf Heinrich Hucho, Aerodynamics of road vehicles Fourth SAE edition.

[2] Thomas D.Gillespsie, Fundamentals of Vehicle Dynamics Third SAE edition.

[3] Dainel bell, Aerodynamic analysis and optimization of rear wing of a WRC car, MSc project, Oxford Brookes University.

Author Profile:  T.Maniraj is currently pursuing bachelor’s degree program in automobile engineering in Anna University, India PH-9884268182.

E-mail: tamilnambimaniraj4@gmail.com  S.Sathishkumar is currently pursuing bachelor degree program in automobile engineering in Anna University, India .

3 CONCLUSION The baseline model created using CAD software (solid works & CATIA) has been imported into Autodesk CFD software. On the baseline model flow analysis has been performed. The velocity and pressure distribution has been simulated. The NACA 2412 and modified NACA 2412 has been analyzed in CFD for various velocity. The baseline model with NACA 2412 and modified NACA 2412 has been analyzed in CFD software. The velocity and pressure distribution has been simulated. The corresponding

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