ijmear-y12-tj-g053

Vol 03, Issue 03; July 2012 International Journal of Mechanical Engineering applications Research IJMEAR

http://technicaljournals.org ISSN: 2249- 6564

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CFD ANALYSIS OF SEDAN CAR WITH VORTEX

GENERATORS K.SAI SUJITH

1, G.RAVINDRA REDDY

2

1PG Student, CAD/CAM, Siddharth Institute of Engineering & Technology, Puttur - 517583 2Asso. Prof., Mech. Engg. Dept., Siddharth Institute of Engineering & Technology, Puttur - 517583

[email protected]

ABSTRACT

The efficiency of a sedan car is increased by using different shapes of vortex generators at the roof of the car,Aims to evaluate the effects of cross wind gusts and also to visualize the flow pattern behind the car using

Computational Analysis. The separation of flow near the vehicles rear end due to Aerodynamic drag. To delay flow separation, Delta shaped vortex generators (VG) are tested for application at the front wind shield of a

sedan car, commonly used on aircrafts to prevent flow separation .The effects depends up on the shape, size and

also location of the vortex generators, those on the vehicle roof will be optimized.

Keywords: Sedan car, Delta shaped Vortex Generators (VG), Computational analysis, Aerodynamic drag

1. INTRODUCTION Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts

with a moving object. Drag and lift are two aerodynamic forces acting on anybody travelling through the air

whether it's an airplane, car or truck. Drag is far more important for vehicles, unless it's a race car or high

performance sports car travelling at very high speeds. In areas where the body transitions at a rate of more than 120, vortex generators, Diffusers and other devices can be used to "Trip the Airflow". The idea is that areas like

the transition between the roof and rear window on the average car creates a large vortex, which effectively

grabs the car and try to hold it back as it tries to slip through the air. If the air that makes up the vortex can be

"tripped" before it leaves the back of the car, it will make smaller vortices, which will have a smaller effect on

the overall aerodynamics of the vehicle.

By using CFD it is easily to validate pressure coefficient on the center line of the car, it is possible to find where

the low pressure exits and what is the drag created in the model.

2. EXPERIMENTAL WORK

Design of vortex generators

A well-known example for intensifying the flow separation delaying effect is utilizing a dimple (like the ones on golf balls). Adding dimple-shaped pieces can lower the Drag coefficient (CD) to a fraction of its original value.

This is because dimples cause a change in the critical Reynolds number. There are reported examples of aircraft

wings controlling the boundary layer, in which vortex generators (hereinafter referred to as VG(s)) successfully

delayed flow separation even when the critical Reynolds number is exceeded Although the purpose of using

VGs is to control flow Separation at the roof end of a sedan, it is so similar to the purpose of using VGs on

aircraft. To determine the shape of sedan VGs, the data on aircraft VGs are referred.

Fig.1 Vortex Generators

Mechanism of Flow Separation The flow velocity profile on the vehicles Center line plane near the roof end shown in Fig. 2. Since the vehicle height in this section becomes progressively lower as the flow moves downstream, an expanded airflow is

formed there. This causes the downstream pressure to rise, which in turn creates reverse force acting against the

main flow and generates reverse flow at downstream Point C. No reverse flow occurs at Point A located further

upstream of Point C because the momentum of the boundary layer is prevailing over the pressure gradient

(dp/dx). Between Points A and C, there is separation Point B, where the pressure gradient and the momentum of

the boundary layer are balanced.






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As shown in Fig. 2, the lower zone close to the vehicles surface within the boundary layer, the airflow quickly loses momentum as it moves downstream due to the viscosity of air. The purpose of adding VGs is to supply the

momentum from higher region where has large momentum to lower region where has small momentum by

stream wise vortices generated from VGs located just before the separation point, as shown in Fig. 3. This

allows the separation point to shift further downstream. Shifting the separation point downstream enables the

expanded air flow to persist proportionately longer, the flow velocity at the separation point to become slower,

and consequently the static pressure to become higher. The static pressure at the separation point governs over

all pressures in the entire flow separation region. It works to reduce drag by increasing the back pressure.

Shifting the separation point downstream, therefore, provides dual advantages in drag reduction: one is to

narrow the separation region in which low pressure constitutes the cause of drag; another is to raise the pressure

of the flow separation region. A combination of these two effects reduces the drag acting on the vehicle.

However, the VGs that are installed for generating stream wise vortices bring drag by itself. The actual effectiveness of installing VGs is therefore deduced by subtracting the amount of drag by itself from the amount

of drag reduction that is yielded by shifting the separation point downstream. Larger-sized VGs increase both

the effect of delaying the flow separation and the drag by itself. The effect of delaying the flow separation point,

however, saturates at a certain level, which suggests that there must be an optimum size for VGs.

Calculation of Optimum Vortex Generator

To select appropriate shape and size of the VG which generates stream wise vortex the most efficiently (with the

least drag by itself) is important to achieve objectives. Consequently, the optimum height for the VG is

estimated to be up to approximately 30 mm. As to the shape, a bump-shaped piece with a rear slope angle of 25

to 30 is selected. This is based on the fact that a strong stream wise vortex is generated on a hatchback-type car

with such rear window angle. A half-span delta wing shape is also recommended for the VG.

Fig.4 Shape of Vortex Generator

As to the location of VGs, a point immediately upstream of the flow separation point was assumed to be

optimum, and a point 100 mm in front of the roof end was selected as shown in Fig. 5. The effects of bump shaped VGs mounted at this point are presented in Fig. 6. The front half contour of the bump-shaped VG was

smoothly curved to minimize drag and its rear half was cut in a straight line to an angle of approximately 27 for

maximum generation of a stream wise vortex. However, a taller VG might cause a decrease in the lift. The

rather small change in drag coefficient resulting from change in height can be accounted for as follows.

An increase in height of the VG simultaneously causes two effects: one is reduced drag resulting from delayed

flow separation and the other is increased drag by the VG itself. These two effects are balanced when the VGs height is between 20 and 25 mm. From these results, a reduction of CD is 0.003 with this bump-shaped VG

when the shape and size are optimized. The effectiveness of the delta-wing-shaped VG is also examined.

Table.1 Design Conditions

Recommended shape for Delta-wing shape Created shape for Delta- wing shape

Length/height = 2

Yaw angle = 15

Interval/height = 6

Length/height = 2

Height=15mm,20 mm &25mm (three types)

Thickness = 5mm

The delta-wing-shaped VGs should be installed at a yaw angle of 15 to the airflow direction. In order to meet this condition, the direction of airflow at the roof end was investigated by oil flow measurement. Airflow






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direction was found to be different between sideways positions on the roof. The airflow is aligned directly with

the backward direction at center of a car, but it increasingly deviates toward the center as the measurement point

shifts away from the central position. For this reason, the delta-wing-shaped VGs must be installed at an angle

of 15 against the vehicle centerline for the central position, whereas they must be installed at an angle near 0

for outermost positions.

The results of these tests are shown in Fig. 7. Delta wing- shaped VGs were found to be less sensitive to change in height than bump-shaped VGs; the drag reduction effects for the VGs of three different heights (15 mm, 20

mm and 25 mm) were all equivalent to 0.006.

The drag reduction also differed only slightly with changes in the number of VGs and their positions. The

number and positions of the tested VGs seems to be in their optimum ranges. From these results, delta-wing-

shaped VGs were capable of reducing drag by 0.006. The reason for why delta-wing-shaped VGs are more effective than bump-shaped VGs can be explained as follows: Delta-wing-shaped VGs have a smaller frontal

projection area, which means that they themselves create smaller drag. Moreover, the vortex generated at the

edge of a delta-wing-shaped VG keeps its strength in the flow downstream of the edge since it barely interferes

with the VG itself because of the VGs platy form. With bump-shaped VGs, on the other hand, the vortex is generated at a point close to the downstream edge of the bump, which causes the vortex to interfere with the

bump and lose its strength.

3. ANALYSIS OF EXPERIMENTAL RESULTS

Computational Analysis

For creating the car model in Gambit the following procedures are adopted. Import the vertices of a model in a

dat format.

Specify the way in which the geometry will be colored.

Create domain.

Subtract model from domain.

Merge faces to facilitate meshing.

Mesh a face with a tetrahedral mesh.

Prepare the mesh to be read into FLUENT 5/6

Apply boundary conditions.

Fig.8 Geometry of Model with Vortex Generators






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Fig.9 Meshed Geometry Model Fig.10 Meshed Geometry Model with vortex generator

4. RESULTS AND DISCUSSION

The Meshed geometry is analyzed using the given Boundary conditions specified for FLUENT and the variation

of flow parameters are plotted and studied.

Flow Parameters Taken For Study

Four important results were obtained from the analysis

1. Pressure coefficient variation along the car model 2. Static pressure variation along the car model 3. Total pressure variation along the car model 4. Velocity vectors The results are plotted in the form of contour plot for

Pressure coefficient

Static Pressure

Total pressure And Vector Plot for Velocity Vectors

And Scatter plot for variation of following parameters along the contour

Static Pressure

Total pressure

Pressure coefficient Contour Plots (Filled) For Model with Vortex Generators

Fig.17 Pressure coefficient variation Fig.18 Static pressure variation

Fig.19 Total pressure variation






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Vector Plot for Model with Vortex Generators

Fig.20 Velocity vectors

Scatter Plot for Model with Vortex Generators

Fig.21 Pressure coefficient variation along car Fig.22 Static pressure variation along car

5. VERIFICATION OF VORTEX GENERATOR MECHANISM The effect of VGs is estimated that the separation point is shifted to downstream, which in turn narrows the flow

separation region. The flow field was thus investigated in order to verify the correctness of this estimation.

Fig.23 Static pressure distribution in the wake flow with & without Vortex Generators

Fig.23 shows static pressure distribution in the wake flow immediately upstream of the rear for both cases with

and without VGs. High total pressure regions correspond to high velocity regions. As the figure shows, the high

velocity region is expanded downward by addition of VGs, signifying that the flow separation region is

narrowed.

Fig.24 Vortices distribution behind the Vortex Generators (VGs)






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Fig.24 shows vortices distribution behind the VGs. Stream wise vortices are generated behind the VGs. Our

estimation that the stream wise vortex causes the separation point to shift downstream is confirmed by CFD

results. The case with VGs shows flow separation occurring further downstream than in the case without VGs.

CONCLUSION

Vortex generators (VGs) were studied to install immediately upstream of the flow separation point in order to control separation of airflow above the sedans rear window and improve the aerodynamic characteristics. It was found that the optimum height of the VGs is almost equivalent to the thickness of the boundary layer (15 to

25 mm) and the optimum method of placement is to arrange them in a row in the lateral direction 100 mm

upstream of the roof end.

Application of the VGs shows a drag reduction in sedan car.

As a result of the verifications, it is confirmed that VGs create stream wise vortices, the vortices mix higher and lower layers of boundary layer and the mixture causes the flow separation point to shift downstream,

consequently separation region is narrowed. From this, we could predict that VGs cause the pressure of the

vehicles entire rear surface to increase therefore decreasing drag,

REFERENCES

Hoerner, S.F., Fluid-dynamic Drag, Published by the author, 1958

Hoerner, S.F., Fluid-dynamic Lift, Published by the author,1985

Shibata, H., MMCs Vehicle Wind Tunnel, Automobile Research Review (JARI) Vol. 5, No. 9, 1983

Hucho, W. H., Aerodynamics of Road Vehicles, Fourth Edition, SAE International 1998

www.racecaraerodynamics. com

www.sedancar/aerodynamics/flow.187.com

Hevesi D Claus Luthe, Car Design Innovator, Is Dead at 75New York Times, 10 April 2008

Buckley M Used Car Buying Guide: Range Rover Channel 4 (UK) 24 Jan 2005

Norman, Henry (April 1902). "The Coming of the Automobile". The World's Work: A History of Our Time V: 3304-3308.

http://books.google.com/books?id=DoDNAAAAMAAJ&pg=PA3304. Retrieved 2009-07-10.

Navier, C. L. M. H. (1823). Memoire sur les lois du mouvement des fluides. Memoires de l'Academie des Sciences (6), 389-416

Lanchester, F. W. (1907). Aerodynamics.

Anderson, John D. (2007). Fundamentals of Aerodynamics (4th ed.). McGraw-Hill. ISBN 0071254080.

Bertin,J.J.;Smith,M.L.(2001). Aerodynamics for Engineers (4th ed.).Prentice Hall. ISBN 0130646334. .

Craig, Gale (2003). Introduction to Aerodynamics. Regenerative.

Authors Biography

K.Sai Sujith was born in India, Andhra Pradesh, in 1984. He received B.E. (Mechanical) degree from Anna University, Chennai & doing M.E. (Mechanical CAD/CAM), degree from JNTUA University, Ananthapur,

Andhra Pradesh, India. Since 2006 he has been with Mechanical Engineering Department at Siddharth

Institute of Engineering & Technology, Puttur, Andhra Pradesh. He has published more than 5 papers at

National & International Seminar / Conferences / Journals etc. His area of research includes I.C.Engine,

Automobile engineering.

G.Ravindra Reddy is born in India, Andhra Pradesh, Kadapa (Dist). He received B.E. (Mechanical), & M.E.

(Mechanical Heat Power Engineering), from Anna University, Chennai, India. Since 2010 he has been with

Mechanical Engineering Department at Siddharth Institute of Engineering & Technology, Puttur, Andhra

Pradesh. as Associate Professor, P.G. Co-ordinator. He has published 8 papers at National & International

Seminar/ Conferences / Journals etc. His area of research includes Heat Transfer, I.C.Engine, Automobile engineering.

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