CFD Analysis on Ahmed Body and the suggested ... Diffuser, FLUENT, Wind Tunnel Testing Introduction...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 9 (2016) pp 6465-6471

© Research India Publications. http://www.ripublication.com

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CFD Analysis on Ahmed Body and the suggested aerodynamic changes to

the Ahmed Body

Sparsh Sharma

School of Mechanical Engineering, VIT University Vellore, Tamil Nadu, India.

Yagnavalkya Mukkamala

School of Mechanical Engineering, VIT University Vellore, Tamil Nadu, India.

Abstract

“Ahmed Body” is a very standard simple bluff body which is

basically a very simplified version of an automobile. The

body is used many times to validate new CFD simulation

software and to compare the accuracy of one software with

another CFD software by performing simulations in both and

comparing the values for Coefficient of drag (Cd) and lift (Cl)

with the experimental obtained values of Cd and Cl for the

Ahmed body for a particular slant angle. The Ahmed body’s

has a variable parameter which is the slant angle at the back,

the change in the slant angle (the angle which the slant makes

with the horizontal) has an effect on the drag coefficient

obtained in simulations, this is due to the difference in the size

of the wake area and intensity of the vortices formed. This

wake area is a low pressure region of recirculating air which is

caused due to the separation in flow as the flow encounters

change in curvature and shape of the “Ahmed Body”. Since

the Ahmed’s body is a very simplified version of the

automobile body, there has been various experiments and

simulations aiming to introduce slight aerodynamic and other

changes to the base model of the Ahmed’s body to compare

the result of the new obtained body with the original Ahmed

body and try to get a picture if this change can be at some

level be applied to a commercial car which can improve the

performance of the car with very minor operating costs. This

paper explores the addition of certain structural and

aerodynamic changes to the original: “Ahmed body” by

looking into diffusers, rear spoilers, front nose and other

suggested changes made by previous authors in related

projects regarding the Ahmed Body. The paper then compares

the final results obtained in simulation software to the values

of the original Ahmed Body and comments on the impact of

each of these aerodynamic and structural changes.

Keywords: Coefficient of Lift and Drag, Ahmed Body,

Spoiler, Diffuser, FLUENT, Wind Tunnel Testing

Introduction The use of Computational software in today’s world has

increased sharply, it has provided us with a very cost effective

way of carrying out experiments with lots of ease and high

accuracy in terms of results, It has reduced the overall cost of

many projects and has in fact decreased the time of testing as

well, In fact simulation has become so reliant that these days

most testing is done virtually through these software’s itself.

This project also uses a CFD simulation software called

“FLUENT” which is a software concerned mainly about the

flow of fluids like air in a user defined domain with user

defined boundary conditions, it is mainly used to observe the

aerodynamics behind a body such as lift, drag, vortices,

pressure and velocity distributions over the body and also to

visualise the flow. In this paper we will focus on an

automotive application by looking into the simulation of a

base Ahmed body and then looking into certain aerodynamic

and structural modifications to the base Ahmed body in a bid

to produce a significant negative lift without producing

excessive drag. The paper also deals with verification of the

results by performing of the wind tunnel testing.

Purpose and Scope of the Paper The purpose of this project is to visualise and understand very

basic aerodynamics associated with cars, by analysing the

Ahmed Body we can understand the behaviour of the velocity

streamline when it comes in contact with the Ahmed Body.

From the project the change in velocity and pressure regions

at different lengths along the Ahmed Body can be seen.

Moreover the concept of wakes, flow separation, vortices,

slant angle, and the definition and relation between all the

terms become very clear. Most importantly the effect that

certain aerodynamic and structural additions made to the

Ahmed Body has on the Coefficient of Lift and Drag,

turbulence, wake area for the Ahmed Body can be analysed

and compared to the original Ahmed Body and certain

inferences can be main regarding how each

aerodynamic/structural change has a positive or negative

impact. The project also deals with understanding of

manufacturing process like 3D printing and learning about

which type of material and printing method gives us the best

surface finish suitable to our application. The project also

deals with Wind Tunnel Testing and there is great scope in

learning about wind tunnel testing itself and the various

parameters involved.

The scope of this paper can be found in the Automobile

Industry, this is so because, at a very conceptual level, the

Ahmed Body is a very simple version of an automobile,

Hence when certain new aerodynamic features and structural



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changes want to be incorporated into the automobile and the

engineer would like to know the impact that this change

would have on Coefficient of Drag and lift and other factors

like Turbulence, the engineer can first try these changes on the

Ahmed body, this is because it will be very easy to

incorporate these changes to the simple body and moreover it

will take much less time than actually making the change in

the actual design of the automobile. Instead, the engineer can

see if the proposed changes he wishes to bring in the

automobile has a positive impact on the performance of the

Ahmed Body, if yes, then he can go ahead and be confident of

making changes in the actual automobile design, however if

the performance characteristics are not satisfactory, then they

can be discarded and hence a lot of simulation time can be

saved.

Literature Survey In M. Grandemange and his team’s paper [1] have performed

experiments at industrial scales over the Ahmed geometry, i.e.

at a Reynolds number of Re=2.5x106 based on the height of

the body. In their paper the shape of the square back geometry

is first optimized to make an initial substantial drag reduction.

The separated flow at the trailing edge is orientated by

introducing chamfers at the top and bottom edges. A

parametric study based on both chamfered angles leads to an

optimized Ahmed geometry having a drag of 5.8% lower than

the reference square back model. It is evidenced that this

optimized geometry produces 4 intense longitudinal vortices

that still contribute significantly to the drag. Another

suggested method to limit the formation of the local

longitudinal vortices of the optimized geometry is to orient

simultaneously the flow from the sides of the geometry in

order to retrieve a relative axisymmetric in the after body

flow.

In Sneh Hetawal and team’s paper [2] they talk about 3

different FSAE car models and focus on the certain

aerodynamic features such as the addition of front wings to

the car. In the paper they show that the addition of these

aerodynamic devices have a positive impact on the drag

reduction as well as the increase in down force. In the paper

“Characterization of synthetic jet actuation with application to

Ahmed body wake” [3] an active system is used rather than

making structural changes to the original Ahmed Body. An

extensive experimental parametric analysis, based on

aerodynamic forces, velocity and pressure measurements, has

been conducted on a simplified Ahmed body type geometry in

order to investigate and to develop a wake flow control

technique by means of a synthetic jet. Using this jet system,

implemented in an open-loop strategy, the aerodynamic drag

has been successfully reduced. In the paper “Coupling active

and passive techniques to control the flow past the square”[4]

the authors have applied both active control techniques like

steady jets to control and manipulate the formation of wake

areas, this along with certain passive techniques like

introducing porous material layers so as to reduce the velocity

and form small vortices in the nearby area this leads to

decrease the overall drag produced by the Ahmed body and

make it suitable to the automotive industry standards.

In the paper “Effect of aspect ratio on the near-wake flow

structure” [5], the author has talked about a certain parameter

called “Aspect Ratio” which is a parameter which was not

used by others, in the paper he defines what this aspect ratio is

and then shows through his simulations that by maintaining a

certain aspect ratio, he has found that the overall drag has

been reduced in comparison to the original size of the Ahmed

Body itself. He justifies this approach by saying that the vast

array of vehicle aspect ratios existent in the automotive

industry, and the severe implications that slant angle choice

coupled with aspect ratio can have on overall vehicle drag

characteristics.

In the paper “Effects of suppressing the 3D separation on the

rear slant on the flow” [6], the author mainly focuses on the

flow separation aspect which happens when the airflow

around the car comes in contact with the sudden rear slant, the

author suggests that by rounding of that edge from where the

roof of the Ahmed body goes into a rear slant, we can reduce

the drag by nearly 10%. The author says that by rounding off

these edges, the flow becomes more smooth and follows the

surface of the Ahmed Body for a further distance before

eventually flow separation occurs. This way we can

manipulate the wake area slightly. In the paper “Study of F1

car aerodynamic rear wing using computational fluid dynamic

(CFD)” [7], the author performs CFD analysis on 3 NACA

profiles which are commonly used as wing structures for

spoilers in F1 cars. In the study he performs a 2-D as well as a

3-D analysis on these wings and compares the value of the CD

and CL he gets for the different wing profiles. In the paper

“Best practice guidelines for handling Automotive External

Aerodynamics with FLUENT” [8], the author takes us

through various commonly encountered simulations when it

comes to automobiles, the author based on experience,

suggests various good practices which should be followed at

the time of simulation so as to get most accurate results from

ANSYS simulation.

Ahmed Body Ahmed Body is a standard bluff body widely used in the

automotive industry for validating simulation tools. The

Ahmed body shape is simple enough to model, while

maintaining car-like geometry features. The Ahmed body at a

Reynolds number of 780,000 has a coefficient of drag of 0.28

and a coefficient of lift of 0.

Even though the values of coefficient of lift and drag are very

well established for the Ahmed Body, the Ahmed body is still

simulated using FLUENT so that we may validate the ANSYS

mesh we are using on the body. The Ahmed body

specifications used for the ANSYS simulation is shown below

in Fig1. The Ahmed body was simulated in ANSYS FLUENT

at a Reynolds number of 780,000 and the results obtained was

a CD of 0.26 and a CL of 0. This means there was a zero

percent error for the lift calculation and the drag calculation

had an error of nearly 7% which is acceptable and shows that

the mesh used for carrying out the CFD analysis in FLUENT

is indeed a good and valid mesh considering the size of the

Ahmed body and the fluid domain around it.

This means that a similar type of mesh can be used for the

modified Ahmed body which will roughly have the same size



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and will be analysed at the same operating conditions as that

of the standard Ahmed body. Some of the results obtained

from the simulation are shown in the Figures [1-9] below.

Figure 1: Dimensions of the Ahmed Body

Figure 2: Ahmed Body and Meshing

Figure 3: Coefficient of Lift CL obtained= 0

Figure 4: Coefficent of Drag CD = 0.26

Figure 5: Residual values obtained in transient simulation

Figure 6: Velocity vectors on the Ahmed Body

Figure 7: Static Pressure Contours on Ahmed body on the

symmetry plane

Figure 8: Velocity contours on the symmetry plane

Figure 9: Velocity vectors on the Ahmed Body



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Aerodynamic and structural changes considered Spoilers

A spoiler is basically a wing which has been inverted for the

purpose of inducing a down-force or a negative lift force on

the aerofoil. The purpose of producing this negative lift is to

increase the stability of the car by making sure that the

negative lift makes the wheels be in contact with the road at

all times and hence helps in handling and acceleration of the

car. Spoiler is a very essential feature in race car and sports

cars in general. The objective while selecting an ideal spoiler

is to first consider the Reynolds number at which you will be

operating act (which in turn depends on the speed at which

your car is going) and after that also consider the angle of

attack which gives you the maximum lift force but at the same

time less drag. The negative aspect of too much negative lift is

that you end up producing excessive drag, so while selecting

the aerofoil, we must consider an optimization between the 2

parameters. Taking into consideration a few common

aerofoils, Finally NACA 6409 was selected, the following

shows the geometry of the aerofoil and the graphs of CD and

CL versus the angle of attack. This is shown in the figures [10-

12].

Figure 10: NACA 6409 [9]

Figure 11: CD vs α for 2 different Reynolds numbers, purple

for Re = 500,000 and green for Re = 200,000.

Figure 12: CL vs α for different angles of attack for 2

different Reynolds numbers, purple for Re = 500,000 and

green for Re = 200,000

Hence from the figures[10-12], it was concluded that the

NACA 6409 aerofoil will be chosen and the will be

implemented at an angle of 100 to the horizontal as good

amount of lift is produced and less drag is produced as well.

Diffuser A diffuser, is a shaped section of the car underbody which

improves the car's aerodynamic properties by enhancing the

transition between the high-velocity airflow underneath the

car and the much slower free stream airflow of the ambient

atmosphere. It works by providing a space for the underbody

airflow to decelerate and expand (in area, density remains

constant at the speeds that cars travel) so that it does not cause

excessive flow separation and drag, by providing a degree of

"wake infill" or more accurately, pressure recovery. The

diffuser itself accelerates the flow in front of it, which helps

generate down force. The diffuser shape chosen for car is

elliptical in shape because the discharge rate is best for an

elliptical curve compared to a circular curve.

Modified Ahmed Body After implementing the airfoil and the diffuser onto a standard

Ahmed Body, the modified Ahmed body is shown in the

figures below [13-15].

The solid-works body was then imported into the ANSYS, it

was meshed and then solved in the same size domain as the

standard Ahmed Body and under the same boundary

conditions and solution methods used for the standard Ahmed

body. The results obtained from the simulation are shown in

the figures bellow [16-20].



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Figure 13: Modified Ahmed Body side view

Figure 14: Modified Ahmed Body isometric view

Figure 15: Modified Ahmed Body front view

Figure 16: Coefficient of transient drag = 0.423

Figure 17: Coefficient of transient lift =-0.437

Figure 18: Scaled Residuals

Figure 19: Contours of velocity magnitude on symmetry wall

Figure 20: Path lines by particles around Ahmed body



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Results The values obtained from the CFD simulations using ANSYS

FLUENT for both the standard Ahmed body and the modified

Ahmed body are shown in tables 1&2. The values of the

coefficient of drag and lift were tabulated at different wind

velocities ranging from 10m/s to 40m/s which correspond to

different Reynolds numbers and to speeds ranging from

36km/h to 144km/h which are speeds at which commercial

passenger cars usually travel at. The graphs were then plotted

to compare the lift and drag coefficients obtained for both the

bodies. The results can be seen in Fig 21 and Fig 22.

Simulation Results

Table 1: CD and CL obtained for Ahmed body in FLUENT

simulation.

S.No. Velocity of Air (m/s) Re CD CL

1. 10 6.18 x 105 0.27271 0.014814

2. 15 9.27 × 105 0.2642 0.013386

3. 20 1.24 × 106 0.2606 0.0058581

4. 25 1.54 × 106 0.25815 0.0030011

5. 30 1.85 × 106 0.36674 0.002502

6. 35 2.16 × 106 0.25514 -0.000576

7. 40 2.47 × 106 0.25622 0.0031012

Table 2: CD and CL for modified Ahmed body in FLUENT

simulation.

S.No. Velocity of Air (m/s) Re CD CL

1. 10 6.18 x 105 0.42073 -0.57272

2. 15 9.27 × 105 0.41262 -0.5875

3. 20 1.24 × 106 0.41030 -0.52796

4. 25 1.54 × 106 0.40574 -0.49721

5. 30 1.85 × 106 0.40120 -0.47959

6. 35 2.16 × 106 0.39992 -0.45819

7. 40 2.47 × 106 0.400241 -0.491246

Figure 21: CL for the standard Ahmed body and the modified

Ahmed body at different wind velocities.

Figure 22: CD for the standard Ahmed body and the modified

Ahmed body at different wind velocities.

Conclusion From the results obtained above we can see that the

coefficient of drag for the modified Ahmed body in

comparison to the standard Ahmed body has increased from

roughly 0.27 to 0.40. However this increase in drag has come

with a significant increase in negative lift from roughly 0 to-

0.5 which is a bigger increase than the increase in drag, which

is justified with the addition of the spoiler as well as the

diffuser which contribute to increase of drag and negative lift.

This result can be interpreted as both a positive and a negative

result, negative because of the increase in drag but positive

because of the drastic stability due to the negative lift.

This trade-off exists in the automotive industry as well and

especially racing application where car designers and

manufacturers are always trying to increase the stability of the

car on the road by increasing negative lift but without

increasing the drag too much.

Acknowledgment I would like to thank my guide Dr.Yagnavalkya S Mukkamala

and my reviewer Professor Kushal Kumar Chode of VIT

University, for all their important suggestions and guidance at

various different stages of the project, without which I would

not been able to write this paper. I would also like to

acknowledge the contributions of Mr.Manoj C.N, a fellow

student for his help.

References

[1] M. Grandemange, O. Cadota, A. Courbois, V.

Herbert, D. Ricot, T. Ruiz, R. Vigneron (2015), “A

study of wake effects on the drag of Ahmed׳s

squareback model at the industrial scale”, Journal of

Wind Engineering and Industrial Aerodynamics,145

(2015), pp. 282-291

[2] Sneh Hetawal,*, Mandar Gophane, Ajay B.K.,

Yagnavalkya Mukkamala, “Aerodynamic study of

FSAE car”, 12th Global Congress On Manufacturing

And Management, GCMM 2014.



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[3] Azeddine Kourta, Cedric Leclerc (2013),“

Characterization of synthetic jet actuation with

application to Ahmed body wake”, Sensors and

Actuators A: Physical, 192(2013) 13-26.

[4] Charles-Henri Bruneau, Emmanuel Creuse, Delphine

Depeyras, Patrick Gillieron, Iraj Mortazavi,“Coupling

active and passive techniques to control the flow past

the square”, Computers & Fluids , 2010 pg 1875-

1892.

[5] M.Corallo, J. Sheridan, M.C. Thompson,” Effect of

aspect ratio on the near-wake flow structure”, Journal

of Wind Engineering and Industrial Aerodynamics,

147(2015) pg 95103.

[6] A. Thacker, S. Aubrun, A. Leroy, P. Devinant

,“Effects of suppressing the 3D separation on the rear

slant on the flow”, Journal of Wind Engineering and

Industrial Aerodynamics, (2012) pg 237-243

[7] Mohd Shahmal Bin Mohd Shahid,“Study of f1 car

aerodynamic rear wing using computational fluid

dynamic (cfd)”, Faculty of Mechanical Engineering

Universiti Malaysia Pahang.

[8] Marco Lanfrit,“Best practice guidelines for handling

Automotive External Aerodynamics with FLUENT”,

Fluent Deutschland, Version 1.2 (Feb 9th 2005)

http://airfoiltools.com/airfoil/details?airfoil=n6409-il

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