Design of Angles of High Speed Aerodynamic Cars

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Debojyoti Mitra / International Journal of Engineering Science and Technology

Vol. 2(5), 2010, 952-956

Design Estimation of Aerodynamic Angles of

High Speed Cars

DEBOJYOTI MITRA*

Associate Professor & Head

Department of Mechanical Engineering

Sir Padampat Singhania University

Udaipur 313601, Rajasthan.

Abstract

The study of aerodynamic design of high-speed cars is mainly based on the wind-tunnel experiments andcomputational methods till date. In this particular study three car models of 100,200,300 pitch angles and 500,600,700

yaw angles are employed, and by wind-tunnel experiments we obtain pressure distributions over them. Now the

correlations between drag-coefficient, lift-coefficient, pitch-angle and yaw-angle with Reynolds number areobtained by regression analysis of experimental data using MATLAB software. After plotting graphs it can be

concluded that for minimum aerodynamic drag the optimized value of pitch and yaw angle should be 300 and 500.

This type of study is expected to give a fair idea of aerodynamic angle design of high-speed cars.

KeywordsPitch angle, Yaw angle, Drag coefficient, Lift Coefficient, Regression analysis.

Introduction

In early days most of the high speed cars even racing cars showed no concession to aerodynamicconsiderations despite of surprisingly high speeds. In recent decades, in proper designing of high speed cars

aerodynamic aspects have been greatly considered. This study is concerned with the problem of how to design

vehicle shapes that produce desirable flow characteristics. At present, nearly all aerodynamic design for road vehicle

relies on a combination of experimental results, experience, and physical understanding of the way that air flows

behave. Much aerodynamic development involves with experiments using high speed car models in wind tunnel.That does not mean, however that the subject is totally free of mathematics. It is still necessary to have an analytical

basis for the methods used to treat the experimental data, to predict the performance, and to relate wind tunnelresults to full scale behavior.

Drag, in surface vehicle aerodynamics, is the measure of the aerodynamic force, which resists the forward

motion of the vehicle (Bernard, 1996). A low drag coefficient implies that the vehicle body can move easily through

the surrounding viscous air with minimum air resistance, whereas a high negative lift coefficient indicates more

stability and less chances of skidding.

Researchers throughout the world are carrying out extensive research works to lower the drag coefficient

and increase the negative lift coefficient by properly designing the shape of the vehicles. Palowski may beconsidered the pioneer in this field having explored the wind resistance on automobiles way back in 1930. Carr

(1968) has commendable contributions in the field of aerodynamics of road vehicles and its dependency on vehicle

shapes. Reduction in drag implies less fuel consumption: it has been rightly pointed out by Sovran et al. (1983).Bernard (1996), Hucho (1998), Heinz (2002) and Julian (2002) are a few more who have dedicated work on this

field in the recent past. Recently, Mitra (2010) studied the effect of relative wind on Notch Back cars.

The flow field of a car is the result of its shape, its driving speed and the speed and the direction ofthe ambient wind. The present scope of study consists of estimation of proper pitch angle and yaw angle for high

speed cars. The purpose of this study is to ascertain the most suitable pitch, yaw angles for the high speed car tominimize the drag force and to maximize the negative lift. The study is confined to 10

0,200,300 pitch angled car

models and 500,600,700 yaw angles. The definitions are clear from Fig.1.

Experimental ProcedureFor the practical study of movement of high speed vehicles on the ground, i.e. with in the atmospheric

boundary layer, vehicles of different shapes can be easily modeled and experiments can be carried out in windtunnel through it is very challenging task to simulate the actual atmospheric conditions in the wind tunnel.

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The present study is confined to determination of pressure distributions past different pitch angled high

speed cars. For this purpose 100,200,300 pitch angled car models are considered. The experiment was carried out

in the lower test section of Jadavpur University Low Turbulence Subsonic Closed Circuit Wind Tunnel. These

experiments are carried out by keeping yaw angle as 600. The car is placed at a distance 3m from the inlet of the test

section. The experiment is carried out placing 3 models one after another and taking pressure readings at differenttaps for different Reynolds numbers for each model.

Analytical StudyOne of the other important factors for the high speed car design is the value of yaw angle. To check the

comparability of the results, we have used an analytical method for the determination of proper yaw angle. The

method used is regression analysis by Method of least squares. The objective of this is to determine the regressionequation and best fitted curve for the given experimental data. Pressure coefficient Cp depends on shape of the carand properties of fluid. Hence, we can write

Cp=a Re+b (/) +c,

where a, b, c are constants, Re=Reynolds number; = yaw angle, and =pitch angle.

In the above equation Re takes care of the fluid properties and / takes care of the shape of the car.

To determine the constants we have used the results of the above experiment on pitch angle. After gettingthe correlation between Cp, Re and /, we have studied the variation of pressure coefficient for different values of

yaw angles. Finally we estimated the proper yaw angle for high speed cars. Similar analysis has done for Drag, Lift

coefficients also.

Results and Discussions

The drag coefficient CD is plotted against Re as shown in Fig.2. It shows that the values of CD are muchlesser for 300 pitch angled car model compared to the values for 200 pitch angled car model which is again less than

the values for 100 pitch angled car model. This is true for any Reynolds number.

The lift coefficient CL is plotted against Re as shown in Fig.3. It shows little variation for different car

models.

Similar plots are obtained while varying the yaw angles. The CD vs Re graph (Fig.4) shows that the valuesof CD are much less for 50

0 yaw angled car model compared to the values of 600 yaw angled car model which is

again less than the values for 700 yaw angled car model. Fig. 5 shows very little variation in Lift Coefficient for

different yaw angles.

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1 2 3 4

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1->22907, 2->39189, 3->45594, 4->47896

VARIATION OF DRAG COEFFICIENT WITH REYNOLDS NUMBER

FOR DIFFERENT PITCH ANGLED CAR MODELS

DRAGCOEFFICIENTCD

REYNOLDS NUMBER

100PITCH ANGLE CAR MODEL


300

PITCH ANGLE CAR MODEL

Fig.2 Variation of Drag Coefficient with Reynolds Number for different Pitch-angled car models

1 2 3 4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

VARIATION OF LIFT COEFFICIENT WITH REYNOLDS NUMBER

FOR DIFFERENT PITCH ANGLES

1->22907, 2->39189, 3->45594, 4->47896

LIFTCOEFFICIENTCL

RENOLDS NUMBER




Fig.3 Variation of Lift Coefficient with Reynolds Number for different Pitch-angled car models

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1 2 3 4

0.28

0.29

0.30

0.31

0.32

0.33

0.34

0.35

0.36

VARIATION OF DRAG COEFFICIENT WITH REYNOLDS NUMBER

FOR DIFFERENT YAW ANGLED CAR MODELS

1->22907, 2->39189, 3->45594, 4->47896

DRAGCOEFFICIENTCD

REYNOLDS NUMBER

500YAW ANGLE CAR MODEL



Fig.4 Variation of Drag Coefficient with Reynolds Number for different Yaw-angles

1 2 3 4

1.40

1.41

1.42

1.43

1.44

1.45

1.46

1.47

1.48

1.49

VARIATION OF LIFT COEFFICIENT WITH REYNOLDS NUMBER

FOR DIFFERENT YAW ANGLED CAR MODELS

1->22907, 2->39189, 3->45594, 4->47896

LIFTCOEFFICIENTCL

REYNOLDS NUMBER




Fig.5 Variation of Lift Coefficient with Reynolds Number for different Yaw-angles

Conclusions:

The above results pin-point that the drag force acting on high speed car will be less when the pitch angle ismore, although, the chance of reattachment is more in case of low pitch angled car models. But in this particular

study the lengths of low pitch angled models being more, the separated zone on the two sides of the model become

wider at the rear portion of the model, thus causing a longer length of separated region downstream of the model.

This delay in closure of the separated region increases the pressure drop and hence the pressure-drag on longer

models. Thus it may be concluded that high speed cars should have smaller pitch angles to help reattachment ofseparated boundary layer and then a larger pitch angle to reduce the length of the car. That is why racing cars

generally have double slopes at their rear sides. Also, the drag force acting on the high speed car will be less when

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the yaw angle is less. As the yaw angle increases the stream lines will be distorted more. So, the formation of wake

region will be high. That causes separation and then the back flow of the particles will be more. So the drag force

increases drastically.

References

[1] Palowski, F.W. (1930); Wind resistance of automobiles; SAE Journal, Vol. 27.[2]

Barnard, R H 1996, Road Vehicle Aerodynamic Design, Longman, ISBN 0-582-24522-2[3] Heisler Heinz, 2nd Edition 2002, Advanced vehicle Technology, pp.584-634.

[4] Happian-Smith Julian, 2002, An introduction to Modern Vehicle Design, pp.111-124[5] Carr, G.W. (1968). The aerodynamics of basic shapes for road vehicles, Part 2, Saloon car bodies, MIRA, Report no. 1968/9.[6] Hucho,W.H, (1998). Aerodynamics of Road Vehicles: from Fluid Mechanics to Vehicle Engineering, 4 th edition, S.A.E., ISBN 0-7680-

0029-7.

[7] Sovran, G. (1983); Tractive-energy-based formulae for the impact of aerodynamics on fuel economy Over the EPA Driving Schedules,SAE Paper No. 830304..

[8] Mitra, D. (2010). Effect of Relative Wind on Notch-Back Cars with Add-on Parts, International Journal of Engineering Science andTechnology, Vol.2, No.4, pp. 472-476.

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Design of Angles of High Speed Aerodynamic Cars