EFFECT OF SPOILER DESIGN ON HATCHBACK CAR Ashpak Kazi 1 *, Pradyumna Acharya 2, Akhil Patil 3 and Aniket Noraje 4 1,2,3,4 Department of Automotive Engineering, School of Mechanical Engineering, VIT University, Vellore, Tamil Nadu, India, 632014 Abstract- The aerodynamic aspects of automobiles have received more interest recently. Detailed knowledge of vehicle aerodynamics is more important to improve fuel efficiency and enhance stability of vehicle at high speed cruising. Additionally it provides improved external aesthetics. There are different ways to improve the aerodynamic performance of a car like using spoilers, air dam, diffuser etc. This paper deals with effect of addition of spoiler on Volkswagen Polo car in terms of coefficient of drag and lift. Main purpose of spoiler is to generate down force or negative lift. More down force is good but at the same time drag produced must be minimum. It is more important to maximize the down force to drag ratio. The VW Polo car was analyzed for two cases namely with and without spoiler. Creo 2.0 was used for modeling and Ansys Fluent 15 was used for CFD analysis. Validation was done with ahmed body. k-ω shear stress transport turbulence model was used for simulation. The results showed the effect of introduction of rear spoiler on drag and lift. Keywords- Spoiler, k-ω turbulence model, polyhedral mesh, Ansys fluent 15, Creo 2.0 * Corresponding Author : Ashpak Kazi I. INTRODUCTION When vehicle is running at speeds greater than 100 kmph it leads to uncontrollable lift and pitching moments hence high drag is produced. Lift is generated due to high velocity low pressure air flowing at the top of the vehicle and low velocity high pressure air flowing at bottom of the vehicle. This causes reduction in tire grip with ground which in turns causes difficulty in vehicle handling and makes it unstable. The term spoiler used in automobiles is also referred to as an inverted aircraft wing. An aircraft wing produces positive lift which in turns helps aircraft to take off. Spoiler generates the negative lift as air passes around it which is also called as down force. This is because spoilers are inverted hence they push the vehicle down against the ground and rather than decreasing the drag, spoilers increase the drag. When vehicle moves through air, their body experiences aerodynamic forces and moments from the air. The force on the vehicle in the direction opposite to moving direction is called drag. The force perpendicular to the drag and normal to the ground is called lift. The higher the drag force is, the more the horsepower is required. Obviously, a vehicle achieves higher mileage when the drag on the vehicle is reduced. The drag and lift forces can be expressed in a non-dimensional form. The drag and lift coefficients are defined, respectively, as [3] C L = Where is the air density, U is the vehicle velocity, A is the frontal projected area of the vehicle. When driver turns the vehicle without spoiler at a high speed Newton s first law comes into picture as vehicle tends to move in its own direction even during cornering. When vehicle is moving at high speed it has strong momentum in the direction of motion at the same time vehicle has less grip with road due to the positive lift. This causes difficulty in turning. When vehicle with spoiler takes a turn it generates down force which pushes the vehicle down. This generates extra load on the DOI:10.21884/IJMTER.2016.3065.THKRO 192
tire which makes them have a good grip with road and hence better handling. Spoiler generates the down force that causes good grip with road. There is no need to increase in vehicle weight for better handling. Spoilers help in maintaining the traction at very high speed and provide the vehicle stability. Installing the rear spoiler on vehicle not only increase the traction but also increases the braking stability because of the down force generated by it. This braking stability makes easier brake timing even at high speed and make safe ride. Many car owners install spoilers as an aesthetic accessory and they do a very good job by providing the stylish look and make them look cool. This idea first introduced in the 1970s, when Porsche launched the 911 Turbo models, which featured whale tail spoiler on the back. III. OBJECTIVE The main objective of this project is to study and analyze the effect of rear spoiler on drag and lift of hatchback. IV. METHODOLOGY A. Modeling of VW Polo with and without spoiler and ahmed body is done using Creo 2.0 as modeling software. B. Validation of solver settings for Ansys fluent 15 by performing simulation for drag analysis on Ahmed body. C. CFD analysis for drag, lift, velocity and pressure contours of VW Polo without spoiler. D. CFD analysis for determining drag, lift, velocity and pressure contours of VW Polo with spoiler. E. Comparison of results of both cases. V. MODELING OF VW POLO IN CREO 2.0 Modeling of Volkswagen Polo with and without spoiler is done in Creo 2.0 modeling software. The dimensions of Volkswagen Polo car are obtained from internet source and accordingly modeled (Source:http://www.volkswagen.co.in/en/models/polo/polo-variant.html) VI. MODELING OF AHMED BODY IN CREO 2.0 Modeling of Ahmed body with rear slant angle of 30 0 is done in Creo 2.0 modeling software. The dimensions of Ahmed body are obtained from internet source and accordingly modeled. (Source: www.cfdonline.com) Figure 1. Dimensions of Volkswagen Polo @IJMTER-2016, All rights Reserved 193
Figure 2. Volkswagen Polo Without spoiler Figure 3. Volkswagen Polo With spoiler Figure 4. Dimensions of Ahmed body Figure 5. Ahmed body with rear slant angle of 30 0 VII. VALIDATION OF SOLVER SETTING WITH AHMED BODY Before starting the actual analysis of the car model for various parameters like drag and lift coefficients; the solver settings required for analysis need to be verified for their correctness. So the validation has been done using ahmed body by comparing the C D value with the paper by Ashish sing et al. [2]. Ahmed model [1] is a simple generic ground vehicle model that represents all flow physics developed by any ground vehicle when slant surface changes its angle. It has become a benchmark for aerodynamic simulation tools. It is seen in previous literature that flow changes its behavior from fully attached to fully separated for a critical angle near 30 degree. Figure 6. shows the meshed model of the ahmed body in Ansys fluent 15. Tetrahedron mesh is converted into polyhedral so as to reduce computational time without considerable loss of accuracy. Figure 6. Meshed model of ahmed body The inflation layer selected is around 20% with five layers and the number of cell elements present were around 0.2 million. The results obtained regarding pressure and velocity contour are shown in the figure 7. and 8. respectively. The gauge pressure found at the stagnation point is around @IJMTER-2016, All rights Reserved 194
1 kpa. Velocity contour shows zero velocity at the stagnation point and wake region behind the vehicle. Figure 7. Contours of Static Pressure (Pascal) Figure 8. Contours of Velocity Magnitude (m/s) The graph of coefficient of drag for the above meshed model of ahmed body is shown in figure 9. C D obtained for the current solver settings is around 0.32 which is found to be nearer to the results obtained in previous studies [2]. Figure 9. C D for ahmed body @IJMTER-2016, All rights Reserved 195
The table below shows the validated results for C D values at various slant angles. At an angle of 30º drag seems to be under-predicted by both Open FOAM and CFD++ software and is closer to the experimental results. Hence it is concluded that the current solver settings are correct for further analysis of actual car model. Table 1. Validation of result for ahmed body [2] Slant Angle Experimental result Open FOAM CFD++ 25º 0.285 0.292 0.314 30º 0.379 0.315 0.291 35º 0.263 0.317 0.301 VIII. CFD ANALYSIS OF VW POLO CAR ON ANSYS FLUENT 15 The cad model was imported to Ansys fluent 15 and an enclosure which represents the wind tunnel is created as shown in figure 10. Figure 10. Enclosure The enclosure volume was meshed using tetrahedron elements and to capture the boundary layer effects an inflation of 20% was created near the car surface as shown in figure 11. and 12. for both with and without spoiler. k-ω sst turbulence model was used for the cfd analysis. The analysis was done at steady state. Table 2. Boundary Conditions Boundary conditions Sr. No. Parameter Value 1 Inlet Velocity 40 m/s 2 Outlet Pressure 1 bar 3 Inflation layers 5 4 Growth rate 20% @IJMTER-2016, All rights Reserved 196
Figure 11. Meshed model Figure 12. Inflation layer IX. RESULTS AND DISCUSSION The figure 13. shows the pressure contour of VW Polo without spoiler. It can be seen that the maximum pressure was observed at front bumper, windscreen and rear view mirrors. Figure 14. shows the velocity contours around the car and figure 15. shows the velocity vectors around the car. It can be seen that the velocity is almost zero near the front bumper lower portion and at the wake region. Because of the absence of rear spoiler the flow is attached to rear end of car for relatively longer length, due to which the wake region formed is smaller and near to the rear end. Figure 13. Pressure contours without spoiler @IJMTER-2016, All rights Reserved 197
Figure 14. Velocity contours without spoiler Figure 15. Velocity vectors without spoiler Figure 16, 17 and 18 shows the pressure contour, velocity contour and velocity vector for VW Polo with spoiler. In this case it is seen that the flow separates at the spoiler and because of which the wake region formed is larger. Figure 16. Pressure contours without spoiler @IJMTER-2016, All rights Reserved 198
Figure 17. Velocity contours without spoiler Figure 18. Velocity vectors without spoiler The graphs for coefficient of drag C D and coefficient of lift C L for VW Polo without and with spoiler are shown in figures 19,20,21 and 22 respectively. Figure 19. C D without spoiler Figure 20. C L without spoiler @IJMTER-2016, All rights Reserved 199
Figure 21. C D with spoiler Figure 22. C L with spoiler The results for drag and lift coefficients for above two cases are shown in Table 3. Table 3. Values of C D and C L Sr. No. Parameter Without spoiler With spoiler % Change 1 C D 0.36 0.39 + 8.33 2 C L 0.44 0.18-59.09 X. CONCLUSION Based on the above analysis conducted on VW Polo with and without spoiler it is evident that the spoiler has a major impact on performance of a hatchback. It is found that the coefficient of drag, coefficient of lift, wake region is affected with the addition of spoiler. Following conclusions can be drawn from the above analysis, A. Coefficient drag was increased by 8.33% with the addition of spoiler. B. Coefficient of lift was reduced by 59.09% with the addition of spoiler. C. It is found that the flow was attached for a longer length of rear end of car without spoiler due to which wake region was smaller. With the introduction of spoiler the flow was separated at the spoiler which resulted in increased wake region. XI. ACKNOWLEDGMENT We would like to thank VIT University for providing us with the laboratory facilities equipped with the required simulation resources, without which we would not have been able to undertake this research study. REFERENCES [1] Ahmed, S. R., G. Ramm, and G. Faitin. Some salient features of the time-averaged ground vehicle wake. No. SAE- TP-840300. Society of Automotive Engineers, Inc., Warrendale, PA, 1984. [2] Singh, Ashish, Santhosh Kumar, and Kishor Nikam. High Performance CFD Computations for Ground Vehicle Aerodynamics. No. 2011-26-0107. SAE Technical Paper, 2011. [3] Kim, I., Chen, H., and Shulze, R., "A Rear Spoiler of a New Type that Reduces the Aerodynamic Forces on a Mini-Van," SAE Technical Paper 2006-01-1631. [4] Boujo, E., Nakasato, K., Shiozawa, H., Miyamoto, W. et al., "Development of a Prediction Method for Passenger Vehicle Aerodynamic Lift using CFD," SAE Technical Paper 2008-01-0801. [5] Pachpund, S., Madhavan, J., Pandit, G., and Chimner, T., "Development of CFD Methodology for Drag Force Prediction on Passenger Car with Rear Mounted Spoiler," SAE Technical Paper 2012-28-0029. [6] Hucho, W.H. (ed.) (1998) Aerodynamics of Road Vehicles, SAE international, Warrendale, PA. [7] Kim, M., Kuk, J., and Chyun, I., "A Numerical Simulation on the Drag Reduction of Large-Sized Bus using Rear- Spoiler," SAE Technical Paper 2002-01-3070. @IJMTER-2016, All rights Reserved 200