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To Improve the Aerodynamic Performance of a model

hatchback car with the addition of a rear roof spoiler.

Dileep P. Menon*, Samir Kamat G.*, Yagnavalkya S. Mukkamala*, Prakash S. Kulkarni #

*- Department of Mechanical Engineering, Vellore Institute of Technology, Vellore, India

cooresponding author : email id - dileepmenon92@yahoo.com

postal address - 4, Luckmudoss Street. Chennai-600003#- Department of Aerospace Engineering, Indian Institute of Science, Bangalore, India

Abstract

In the present study, a model hatchback car is considered to be an Ahmed Body. The Ahmed Body represents a

simple ground vehicle geometry in the form of a bluff body. It has a shape that is simple enough to allow for

accurate flow simulation but retains some important practical features relevant to automobile bodies. As the demand

for fuel increases, it is important to design vehicles with increased aerodynamic performance to obtain a higher fuel

economy and produce lesser emissions. The drag force on a vehicle is greatly dependent on the wake formed behind

the vehicle, hence it is essential to reduce the flow separation at the rear to recover the downstream pressure. The

aim of this project is to decrease the drag force and increase the cross-wind stability of the model by the addition of

a rear roof spoiler. Further the effect of varying the curvature and length of the spoiler on the drag force and

normalized yaw moment will be studied.

Keywords: Bluff body, Aerodynamic performance, Cross-wind stability, Spoiler

16th Annual CFD Symposium, August 11-12,2014, Bangalore

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Table. 1: Output parameters with different cell densities for Ahmed Body with rear roof spoiler

Cells cd cm Drag Force(N) Lift Force(N)

4.6 million 0.2655 0.22053 14.97 0.623

6 million 0.27135 0.21799 15.2948 1.0008

8.7million 0.26965 0.21657 15.19913 0.6953

Results and Discussions

Fig. 3.Contours of velocity in symmetric plane for flow over: (a) Ahmed Body; (b) Ahmed Body with roof spoiler

Fig. 5.Streamlines in symmetric plane for flow over: (e) Ahmed Body; (f) Ahmed Body with roof spoiler

Fig. 4.Contours of static pressure in symmetric plane for flow over: (c) Ahmed Body; (d) Ahmed Body with roof spoiler

b

d

f

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Fig. 6. Surface plot of pressure coefficient over Ahmed Body on symmetry plane

Fig. 7. Surface plot of pressure coefficient over Ahmed Body with rear spoiler on symmetry plane

Table. 2: Comparison of drag force for the two cases

Ahmed Body Drag Force (N) cd

Without spoiler 15.86 0.281

With rear spoiler 15.1* 0.2688*

*- average

Conclusion

It was found that there was a 4.3% reduction in drag coefficient on adding a rear roof spoiler to the model. This

reduction in drag force is attributed to the increased pressure recovery at the rear of the model near the roof spoiler.

Further work will be carried out to understand how the curvature and dimensions of the spoiler affect the wake

formed at the rear of the model. The cross-wind stability of the model by the addition of the spoiler will also be

investigated.

Upper Surface

Lower Surface

Upper Surface

Lower Surface