CALCULATION AND OPTIMIZATION OF THE AERODYNAMIC DRAG OF AN OPEN-WHEEL RACE CAR

Transcription

1 Journal of Engineering Science and Technology EURECA 2013 Special Issue August (2014) 1-15 School of Engineering, Taylor s University CALCULATION AND OPTIMIZATION OF THE AERODYNAMIC DRAG OF AN OPEN-WHEEL RACE CAR ABDULKAREEM SH. MAHDI AL-OBAIDI*, LEE CHUNG SUN School of Engineering, Taylor s University College, No. 1 Jalan SS 15/ Subang Jaya, Selangor DE, Malaysia *Corresponding Author: Abdulkareem.mahdi@taylors.edu.my Abstract Aerodynamic drag reduction is one of the important factors to make a race car achieve a faster lap time. Additional drag is produced due to the air channel for radiator cooling of the student designed open-wheel race car. This paper presents the aerodynamic drag optimization of the race car through studying the effect of the angle of the radiator air channel numerically using ANSYS Fluent and experimentally using wind tunnel. A reduction of 12.7% in drag coefficient compared to the current setup is achieved by tilting the angle of cooling channel to 72.5 degree. Numerical results and experimental results show good agreement, a maximum deviation of 7.7% between numerical and experimental drag coefficient for case of the race car with driver included. Keywords: Aerodynamic drag, Drag reduction, Drag optimization, ANSYS Fluent, Automobile drag, CFD. 1. Introduction A number of students from Taylor s University had conceived, designed and built an open-wheel race car to participate in the national competitions. Due to the competition rules and constrains, the radiator is placed directly at the back of the driver which blocks most of the cooling air from entering the radiator. Thus, an air channel is fabricated to direct the air above the driver s head into the radiator which results in additional aerodynamic drag. The cooling of the engine is extremely important during the endurance race. Engine overheating will affect the efficiency of the engine or even engine failure might occur during the race. Figure 1 shows the actual race car with the attachment of radiator cooling channel. 1

2 2 A. S. M. Al-Obaidi and C. S. Lee Nomenclatures A Frontal area, m 2 C D Total drag coefficient C Df Skin-friction drag coefficient C Dp Pressure drag coefficient D p Pressure drag force, N P Pressure, Pa Reynolds number based on reference length R l Greek Symbols θ Angle between relative velocity to the normal pressure force, rad Radiator Cooling Channel Fig. 1. The Open-wheel Race Car Imperica with the Cooling Channel Attached. Vortex generators are normally used on aircraft to delay the flow separation and are beginning to see applicable to sedan car providing the same function. Although vortex generators create drag themselves, they also reduce drag by delaying the flow separation at the downstream. It had been proven by Mitsubishi Motors Research & Development department that the vortex generators designed and properly positioned on the roof top of their production car, the Mitsubishi Lancer Evolution can reduce a drag coefficient of [1]. Furthermore, it is also proven numerically that rear-spoiler on a Camry model is able to reduce the drag by 1.7% [2]. A wind friction drag reduction system has been suggested by Krishna Mohan Raju [3]. The drag reduction device is placed at the rear of the vehicle with collapsible rear attachment to reduce the aerodynamic during high speeds without increasing the engine capacity. The research showed that the hydraulic controlled wind friction reduction device increase the velocity by 1.3 times compare to the conventional configuration [3]. The objective of this paper is to study and compare the effect of the angle of the radiator cooling channel on the overall drag coefficient of the race car. Furthermore, conduct computational studies using Computational Fluids Dynamic (CFD) and experimental wind tunnel studies. Then compare and analyse the trend of the results from the numerical method and experimental method. Lastly, an optimum solution for drag reduction of the race car from the

3 Calculation and Optimization of the Aerodynamic Drag of an Open-Wheel Race Car 3 study of the effect on the angle of the radiator cooling channel on aerodynamic drag is suggested. Figure 2 shows the parametric study conducted in this paper. Flow direction Flow direction Flow direction Flow direction Flow direction Flow direction Fig. 2. The Parametric Study on the Angle of the Radiator Cooling Channel. 2. Numerical Approach using Computational Fluid Dynamic (CFD) ANSYS Fluent 14 software was used to simulate the aerodynamics of the automobile to obtain the drag coefficient numerically. A simplified vehicle shape called Ahmed Body was used for verification and validation of the software [4]. It is also used to determine the proper meshing, turbulence model and CFD solver input settings for the external flow simulation for the race car model. Table 1 shows three different simulations on Ahmed Body using different turbulence models; Realizable k-ε solves two transport equations to obtain turbulent kinetic energy, k and dissipation rate, ε. The Reynolds Stress Model (RSM) solves 6 components of Reynolds stresses and dissipation rate, ε and Large Eddy Simulation (LES) solves the large eddies and models the smaller eddies [5]. The LES Smagorinsky-Lilly model was also used for the Ahmed Body verification simulation. The results don t differ much from experimental value [4, 6, 7]. Hence, using less computational time to obtain similar results is highly recommended. The results from Table 1 shows that the numerical results is within acceptable range hence the similar meshing and solver input methods were used for the external flow race car simulations.

4 4 A. S. M. Al-Obaidi and C. S. Lee Table 1. Comparison of CFD results of Ahmed Body. Realizable k-ε RSM LES Drag coefficient (C D ) Experimental Drag Coefficient C D accuracy (%) Computational Time (hours) Geometry and meshing A full-scale simplified drawing of Taylor s University Race Car named Imperica in produced using Solidworks A computational domain similar to a wind tunnel test section is created around the car. A domain of 3 car lengths upstream and 5 car lengths downstream is created to accommodate for the flow development at the front and turbulence formation at the rear end. A hybrid meshing approach tested using Ahmed Body is also used on Imperica. Prismatic layers are created near the surface of the body and also the road. This type of meshing is suggested by Marco Lanfrit using inflation of first aspect ratio of 5, growth rate of 1.2 and a total of 5 layers [8]. A rectangular box mesh with smaller element size is created near the body to capture the flow condition near the surface body and also the wake region at the rear end [8]. Figure 3 shows the full scale CAD drawing and Fig. 4 shows the mesh condition near the surface body of the car. Fig. 3. Full Scale CAD Drawing. Fig. 4. Mesh Condition near the Surface Body.

5 Calculation and Optimization of the Aerodynamic Drag of an Open-Wheel Race Car Turbulence model Realizable k-ε with non-equilibrium wall function is selected based on the results obtained from the simulation of Ahmed Body. This is to reduce the time consumption for simulations and also to help the convergence of the simulation. The selected turbulence model is enough to give an understanding of the overall external flow conditions and drag coefficient relationship [8]. Pressure based coupled algorithm is selected for the solver. Pressure based coupled algorithm solve both continuity and momentum equation simultaneously and this could help the overall convergence of the simulations but more computational memory is needed [9]. 3. Wind Tunnel: Validation and Verification Three different size of spheres of diameter 3 cm, 6 cm and 9 cm were manufactured using the Rapid Prototype Machine (RPM). The purpose of these three spheres are to validate the results obtained using the force transducer in the wind tunnel to ensure validity and accuracy of the data obtained. Drag coefficient for sphere at different Reynolds number are well defined in the literature. The wind tunnel testing of the sphere model shows a drag coefficient of 0.4 to 0.5 for all three size of sphere. The Reynolds number calculated for these three sized sphere are in the range of 10 4 to as shown in Fig. 5. From the reference data [10], it shows that the drag coefficient obtained using the force transducer in the wind tunnel is acceptable. Fig. 5. Drag Coefficient of Sphere at Different Reynolds Number [10]. 4. Results and Discussion The investigation of the radiator cooling channel angle of the race car with driver included is conducted according to Fig. 2. ANSYS Fluent v14 was used for the numerical analysis. Scaled race car model was produced using the Rapid Prototyping Machine (RPM) and placed inside the wind tunnel for experimental analysis.

6 6 A. S. M. Al-Obaidi and C. S. Lee 4.1. Computational results Current basic setup total drag coefficient The first CFD simulation was conducted to obtain the total drag coefficient of the current basic setup on the race car with the radiator cooling channel relative to velocity. Figure 6 shows the relationship of the drag coefficient relative to velocity obtained through CFD. The figure shows that the drag coefficient does not vary much when the velocity increases. The total drag coefficient obtained is approximately with the current race car setup. The results also match with theory where the drag coefficient does not vary much when velocity increased at subsonic speed [11]. Fig. 6. Drag Coefficient of the Race Car Simulated at Different Speeds. Drag coefficient for different angled radiator cooling channel From the graph shown in Fig. 6, a velocity of 18.3 m/s is chosen as an overall average speed for next simulation by considering the race car is maintained at an average speed of 18.3 m/s throughout the Melaka International Motorsport Circuit (MIMC). The effect of the angle of radiator cooling channel is investigated through CFD at the mentioned speed. The original setup of the cooling channel is angled at 36 degree where it completely directs the flow into the radiator to cool the engine. Multiple simulations were conducted to analyse the effect of increasing the angle of the radiator cooling channel. The effect of drag coefficient by varying the angle of the cooling channel is plotted in Fig. 7. Fig. 7. Drag Coefficient as a Function of Angled Radiator Cooling Channel.

7 Calculation and Optimization of the Aerodynamic Drag of an Open-Wheel Race Car 7 The current setup of the race car which attached the radiator cooling channel at 36 degree produces a drag coefficient of which increased the drag coefficient by 14.84% compared to the one without. Attaching the radiator cooling channel produced negative pressure region at the rear of the car which increases the drag coefficient. The cooling channel is then angled to reduce the drag coefficient but on the other hand it reduces the air flowing into the radiator. Increasing the angle of the cooling channel from 46.5 degree actually reduces the drag coefficient towards the drag coefficient of the race car angled at 90 degree. This phenomena agrees well with theoretical explanation whereby increasing the angle of the cooling channel reduces the restriction of air flow whereby reduces the total drag coefficient. Theoretically, 90 degree angled cooling channel should produce the less overall drag coefficient. When the radiator cooling channel is angled at 72.5 degree, the drag coefficient is which is lower than the current setup and 90 degree angled cooling channel. A reduction of 9.9% in drag coefficient compared to the current setup is achieved. This phenomenon might be caused by the turbulence created from the cooling channel. This particular angle introduced turbulence into the negative pressure region at the rear end of the car which increases the pressure coefficient, Cp at the rear reducing the pressure difference Experimental results Drag coefficient for different angled radiator cooling channel Figure 8 below shows the drag coefficient for different angled radiator cooling channel obtained through wind tunnel force measuring system. From the original basic setup, the overall drag coefficient for the race car reduces as the radiator cooling channel angle increases. In the case of 54.5 degree and 72.5 degree, the drag coefficient obtained experimentally are lower than the drag coefficient obtained for 90 degree which theoretically with less air flow restriction. This shows the same phenomena in the numerical analysis described above. At 72.5 degree setup, a reduction of 12.67% compare to the current setup is achieved shown in the experimental results. Fig. 8. Drag Coefficient as a Function of Angled Radiator Cooling Channel.

8 8 A. S. M. Al-Obaidi and C. S. Lee 4.3. Comparison Figure 9 compares the trend for the results obtained from numerical analysis and experimental analysis. As shown in the figure, the comparison of numerical results and experimental results show good agreement in trend especially after 46.5 degree. A maximum deviation of 7.7% between the experimental and numerical results was detected at the 90 degree radiator cooling channel setup. This shows that at both analysis, 72.5 degree setup provide the less total drag coefficient compare to all other setup. Fig. 9. Comparison of Experimental and Numerical Drag Coefficient. 5. Conclusion An investigation has been conducted for the effect of the angle of radiator cooling channel on the total drag coefficient of the race car. Angle of the radiator cooling channel at 36 degree, 46.5 degree, 54.5 degree, 72.5 degree, and 90 degree are analysed. Numerical method and experimental method are used in the analysis. Comparison is made between the numerical and experimental results. Some conclusion from the analysis and suggestion are given below. The results obtained from both numerical and experimental methods show good agreement. Increasing the radiator cooling channel could reduce the total drag coefficient of the race car compare to the current setup where the cooling channel is angled at 36 degree degree setup provide the less drag coefficient, but this setup provide less air flow into the radiator which will overheat the engine during the race. Hence, optimization and compromise need to be done to obtain the optimum solution. An automated control feedback system could be implemented to compromise and optimize the drag produced. The angle of the radiator cooling channel increases when the engine is cooled to reduce the air flow restriction thus reducing the total drag coefficient and vice versa. Future work can be done in the area of modelling the heat transfer and obtain the relation between the heat transfer of the radiator and the drag coefficient of the car.

9 Calculation and Optimization of the Aerodynamic Drag of an Open-Wheel Race Car 9 References 1. Koike, M.; Nagayoshi, T.; and Hamamoto, N. (2004). Research on aerodynamic drag reduction by vortex generators. Mitsubishi Technical Review, 16, Hu, X.-X.; and Wong, E.T.T. (2011). A numerical study on rear-spoiler of passenger vehicle. World Academy of Science, Engineering and Technology, 5(9), Raju, K.M.; and Reddy, G.J. (2012). A conceptual design of wind friction reduction attachments to the rear portion of a car for better fuel economy at high speeds. International Journal of Engineering Science and Technology, 4(5), Ram, G.; and Faltin, G. (1984). Some salient features of the time-average ground vehicle wake in Detroit. Society of Automotive Engineers. 5. Versteeg, H.K.; and Malalasekera, W. (1995). An introdution to Computational Fluid Dynamics. Harlow: Pearson Education Limited. 6. Gabriel. A.; Drage, P.; Lindbichler. G.; Hormann. T.; Brenn, G.; and Meile, W. (2008). Efficient use of computational fluid dynamics for the aerodynamic developmet process in the automotive industry. 26 th AIAA Applied Aerodynamics Conference. AIAA Lienhart, H.; and Becker, S. (2003). Flow and turbulence structure in the wake of a simplified car model in Michigan, USA. SAE 2003 World Congress, SAE Paper Marco, L. (2005). Best practice guidelines for handling Automotive External Aerodynamics with Fluent. Birkenweg: Fluent Deutschland GmbH. 9. Keating, M. (2011). Accelerating CFD Solutions. ANSYS, Inc. 10. Schlichting, H. (1979). Boundary Layer. (7 th Ed.), McGraw-Hill Book Company, New York. 11. Hoerner, S.F. (1965). Fluid-Dynamic Drag. Hoerner Fluid Dynamics, Brick Town N.J.

10 10 A. S. M. Al-Obaidi and C. S. Lee Appendix A Residual plots and Convergence graphs in ANSYS Fluent For the numerical analysis, each drag coefficient is obtained with residual plots and convergence graph in the ANSYS Fluent software. The drag coefficient is obtained when the convergence graph stop converging and stabilizes. Fig. A1-1a. Residual for Imperica with Driver without Cooling Channel. Fig. A1-1b. C D Convergence Graph for Imperica with Driver without Cooling Channel.

11 Calculation and Optimization of the Aerodynamic Drag of an Open-Wheel Race Car 11 Fig. A1-2a. Residual for Imperica with Driver with 36 Degree Cooling Channel. Fig. A1-2b. C D Convergence Graph for Imperica with Driver with 36 Degree Cooling Channel.

12 12 A. S. M. Al-Obaidi and C. S. Lee Fig. A1-3a. Residual for Imperica with Driver with 46.5 Degree Cooling Channel. Fig. A1-3b. C D Convergence Graph for Imperica with Driver with 46.5 Degree Cooling Channel.

13 Calculation and Optimization of the Aerodynamic Drag of an Open-Wheel Race Car 13 Fig. A1-4a. Residual for Imperica with Driver with 56.5 Degree Cooling Channel. Fig. A1-4b. C D Convergence Graph for Imperica with Driver with 56.5 Degree Cooling Channel.

14 14 A. S. M. Al-Obaidi and C. S. Lee Fig. A1-5a. Residual for Imperica with Driver with 72.5 Degree Cooling Channel. Fig. A1-5b. C D Convergence Graph for Imperica with Driver with 72.5 Degree Cooling Channel.

15 Calculation and Optimization of the Aerodynamic Drag of an Open-Wheel Race Car 15 Fig. A1-6a. Residual for Imperica with Driver with 90 Degree Cooling Channel. Fig. A1-6b. C D Convergence Graph for Imperica with Driver with 72.5 Degree Cooling Channel.