calculation and optimization of the aerodynamic drag...

Journal of Engineering Science and Technology EURECA 2013 Special Issue August (2014) 1 - 15 © School of Engineering, Taylor’s University

1

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 15/8

47500 Subang Jaya, Selangor DE, Malaysia

*Corresponding Author: [email protected]

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.

2 A. S. M. Al-Obaidi and C. S. Lee

Journal of Engineering Science and Technology Special Issue 8/2014

Nomenclatures

A Frontal area, m2

CD Total drag coefficient

CDf Skin-friction drag coefficient

CDp Pressure drag coefficient

Dp Pressure drag force, N

P Pressure, Pa

Rl Reynolds number based on reference length

Greek Symbols

θ Angle between relative velocity to the normal pressure force, rad

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 0.006 [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

Radiator Cooling Channel

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.

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.

Flow

direction

Flow

direction

Flow

direction

Flow

direction

Flow

direction

Flow

direction



Table 1. Comparison of CFD results of Ahmed Body.

Realizable k-ε RSM LES

Drag coefficient (CD) 0.316 0.316 0.284

Experimental Drag Coefficient 0.299 0.299 0.299

CD accuracy (%) 5.3 5.3 5.2

Computational Time (hours) 0.5 3 8

2.1. Geometry and meshing

A full-scale simplified drawing of Taylor’s University Race Car named Imperica in

produced using Solidworks 2011. 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.



2.2 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 104 to 2×10

5 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.



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

0.618 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.



The current setup of the race car which attached the radiator cooling channel

at 36 degree produces a drag coefficient of 0.619 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 0.563 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.

4.2. 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.



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.

• 72.5 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.



References

1. Koike, M.; Nagayoshi, T.; and Hamamoto, N. (2004). Research on

aerodynamic drag reduction by vortex generators. Mitsubishi Technical

Review, 16, 11-16.

2. 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), 575-582.

3. 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), 2366-2372.

4. 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.

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W. (2008). Efficient use of computational fluid dynamics for the

aerodynamic developmet process in the automotive industry. 26th AIAA

Applied Aerodynamics Conference. AIAA 2008- 6735.

7. 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 2003-01-0656.

8. 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. (7th

Ed.), McGraw-Hill Book

Company, New York.

11. Hoerner, S.F. (1965). Fluid-Dynamic Drag. Hoerner Fluid Dynamics, Brick

Town N.J.



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. CD Convergence Graph for Imperica

with Driver without Cooling Channel.




with Driver with 36 Degree Cooling Channel.






with Driver with 46.5 Degree Cooling Channel.



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