ijmet 06 10_019

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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 6, Issue 10, Oct 2015, pp. 171-193, Article ID: IJMET_06_10_019

Available online at

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=10

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication

CFD STUDY OF AERODYNAMIC

PERFORMANCE OF A POPULAR

VEHICLE’S OUTER BODY SHAPE AND

ANALYSIS OF THE EFFECT OF

AERODYNAMIC AIDS

Siddhant Ruia

Department of Automobile Engineering,

Manipal Institute of Technology, Manipal, Karnataka, India

Organization: AMW Motors Ltd

Akash Dixit

Department of Automobile Engineering

Manipal Institute of Technology, Manipal, Karnataka, India

Cite this Article: Siddhant Ruia and Akash Dixit. CFD Study of

Aerodynamic Performance of A Popular Vehicle’s Outer Body Shape and

Analysis of The Effect of Aerodynamic Aids, International Journal of

Mechanical Engineering and Technology, 6(10), 2015, pp. 171-193.

http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=6&IType=10

ABSTRACT

Aerodynamic characteristics of a high performance car are of significant

interest in reducing accidents due to wind loading and improving fuel

consumption. With the availability of road cars capable of speeds well in

excess of 300km/h and the growing popularity of ‘track days’, wherein owners

of such street-legal super sports cars engage in competitive driving on race

circuits, it has become imperative to ensure such production cars are

aerodynamically stable at high speeds. For this reason, modifying production

cars for improved aerodynamic performance is becoming increasingly

popular and even several manufacturers are now offering aerodynamic aids

such as air dams with front splitters, canards, side skirts, vortex generators,

spoilers, rear wings and rear diffusers as standard from the factory.

The main purpose of an air dam and front splitter combination is to aid in

the optimization of the flow of air over the rest of the car and reduce drag

while creating downforce. The target is to achieve minimum drag and

maximum downforce aiding the front tires to get better grip and reduce

http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=6&IType=7

Siddhant Ruia and Akash Dixit


understeer tendencies. The front splitter is attached to the bottom of the air

dam. It serves to increase the amount of downforce at the front of the car. Air

flow is brought to stagnation above the splitter by the air dam creating an

area of high pressure. The front splitter redirects air away from this

stagnation point and accelerates that air under the car which, in turn, causes

a low pressure. High pressure over the splitter caused by the air dam and low

pressure of the air flow under the car creates downforce by way of the

Bernoulli Effect which states that fluid is drawn from high pressure areas to

low pressure areas.

The design goal of a rear wing is to increase downforce. This is achieved

by using an inverted aerofoil profile which causes the air to flow faster

underneath the wing creating a low pressure on the bottom surface of the wing

while the slower moving air above the wing exerts high pressure on the top

surface of the wing.

The limitations of conventional wind tunnel experiments and the rapid

development of suitably advanced computer hardware and software have led

to considerable efforts being invested in the last decade to study vehicle

aerodynamics computationally.

This project will present a numerical simulation of air flow around a

popular road car using commercial fluid dynamics software ANSYS

FLUENT®. The project will focus on Computational Fluid Dynamics (CFD)

based lift and drag predictions for the vehicle outer body shape in standard

specifications and with air dam with front splitter and rear wing attached first

individually and then in combination.

A three-dimensional computer model of a first-generation Bentley

Continental GT designed using commercial modelling software CATIA

v4R16® will be used as the base model.

The conclusions drawn from the results obtained are as follows.

The air dam with front splitter on its own is not considerably effective. While it does

reduce the Drag Coefficient (CD) by a very small degree, it has practically no

noticeable effect on the Lift Coefficient (CL).

The rear wing on its own significantly improves downforce and gives a large negative

value of CL but it also causes a noticeable increase in CD.

The air dam with front splitter working in combination with the rear wing is the most

aerodynamically efficient configuration. The downforce CL generated is even greater

than that in the case of rear wing acting alone but the increase in drag CD is

significantly smaller.

1. INTRODUCTION

Automobile designers and engineers have been constantly engaged in an effort to

optimize vehicle body shape to reduce aerodynamic drag since as early as the 1920s.

Similarly, attempts to maximize aerodynamic downforce, which allows a car to travel

faster through a corner by increasing the vertical force on the tires creating more grip,

also began to be made in the late 1920s and resulted in ‘ground-effects’ finding their

way into the mainstream in motorsport in the 1960s and eventually onto production

road cars. With advancements in powertrain, suspension and electronics technologies,

a vehicle’s aerodynamics have become crucial in determining its competency. Some

of the advantages of an aerodynamically designed vehicle are listed below.

CFD Study of Aerodynamic Performance of A Popular Vehicle’s Outer Body Shape and

Analysis of The Effect of Aerodynamic Aids


Speed – The frictional force of aerodynamic drag increases significantly with vehicle

speed. Therefore, a vehicle body designed to minimize drag is capable of achieving

greater speeds.

Safety – An aerodynamically sound vehicle body design ensures greater aerodynamic

stability and achieving greater straight-line and cornering speeds safely and

confidently is made easier.

Fuel Efficiency – Streamlined vehicles require less energy to overcome air resistance

and are, therefore, more fuel efficient.

Fuel Economy – Better fuel efficiency ensures more economical operation.

1.1 OBJECTIVES

To create a three-dimensional Computer Aided Design (CAD) model of the outer

body shape of the first-generation Bentley Continental GT using Dassault Systèmes

Computer Aided Three-Dimensional Interactive Application (CATIA v4R16®)

software and blueprints obtained from www.the-blueprints.com.

To measure the aerodynamic performance parameters of lift and drag of the standard

outer body shape of the Bentley Continental GT by simulating suitable aerodynamic

conditions around its CAD model in commercial Computational Fluid Dynamics

(CFD) software ANSYS FLUENT®.

This shall be referred to as follows.

Case 1 Base Model

To repeat the aerodynamic performance parameter measurements under identical

aerodynamic conditions in ANSYS FLUENT® after attaching suitable aerodynamic

aids individually and finally in combination to the CAD model of the Bentley

Continental GT in CATIA v4R16®.

These shall be referred to as follows.

Case 2 – With Air Dam with Front Splitter

Case 3 – With Rear Wing

Case 4 – With Air Dam with Front Splitter and Rear Wing

To study the velocity streamlines and pressure contours in the flow fields generated in

each of the four cases in order to determine the causes for the variation in the values

of the aerodynamic performance parameters of lift and drag measured and to,

therefore, compare the results of the simulation tests run on all four cases and

determine the most aerodynamically efficient configuration of the four.

1.2 LIMITATIONS

This was our first time modelling an entire vehicle body using CATIA V5® so the

result does not boast of professional grade perfection. Also, the blueprints that were

used to sketch out and generate the model are of rudimentary detail and do not

provide all the required dimensions and proportions required to generate a perfectly

accurate replication with all the design nuances of the original but more detailed

drawings are neither easily available nor affordable so we had to do the best we could

with what we had at hand. That said, the base model used is still a reasonably

accurate representation of the first-generation Bentley Continental GT outer body

shape and the results obtained were accurate to a satisfactory degree. More about the

actual inaccuracies that may have been contained in the base model used will be



discussed later while elaborating the results obtained from the simulation test

performed on the base model.

Certain limitations were arbitrarily imposed while designing the aerodynamic aids in

order to keep the core concept relevant. Considering the objective was to test the

effectiveness of aerodynamic aids that could be realistically and relatively easily and

affordably implemented on an actual Bentley Continental GT to make it more

competitive and aerodynamically efficient on a race track while preserving its road-

worthiness, we did introduce convenient limits such as restricting ground clearance

even at the lowest point at the front splitter from being reduced to less than 6cm,

restricting the depth of the front splitter to a maximum of 10cm, restricting the span

and chord length of the rear wing from exceeding the vehicle’s footprint and

generally refraining from modifying the fundamental shape and dimensions of the

base model. Such arbitrary limits, wherever imposed, will be explicitly mentioned

later while discussing the design of the aerodynamic aid they concern.

Time constraint was another factor that helped limit the scope of this project. Even

with a not inadequate 16GB RAM on tap, the computer systems used required up to

almost a week to execute one complete simulation test provided there were no

glitches or setbacks and not counting considerable time required for meshing the

models implying that all the required simulation tests presented in this project report

took well over a month and a half to complete. As a result, we could not design and

execute more simulation tests for more models. However, given more time or more

computing power in the future, this project could be extended to include comparison

of models with different designs for the same aerodynamic aids, comparison of

models with a greater number of aerodynamic aids in a greater number of

combinations, comparison of identical models under a variety of aerodynamic

conditions and so on. With more time and computing power at hand, it would also be

possible to design and implement more detailed and complex aerodynamic aids than

the simplified designs analyzed in this project.

2. TEST SUBJECT SELECTION

The reason the Bentley Continental GT was chosen as the test subject is because, as a

powerful two-door grand tourer, there is no doubt that it was designed with sporting

intent and that it possesses a definite potential for motorsport. It featured few of the

aerodynamic aids for downforce that its contemporaries employed. Despite its

relatively large bulk, the standard car has an impressively low manufacturer

calculated CD of just 0.33 so we consider it an ideal candidate for receiving some

downforce improving aero upgrades because, even though the air dam with front

splitter and rear wing we planned to put on it would increase its CD, its inherently

slippery design and huge engine power would ensure that its performance remained

competitive. Also, while the standard car’s CD is favorably small, its CL is a positive

value implying the car experiences lift at high speed resulting in reduction in traction

or grip and stability making it an even more eligible recipient of some downforce

improving aerodynamic aids.

3. RESEARCH DESIGN AND METHODOLOGY

This section discusses the design of the Computer Aided Design (CAD) models upon

which the CFD analysis was performed as well as the methodology employed in order

to correctly complete the other steps of the pre-processing stage of the numerical

method.




3.1 MODEL GENERATION

As discussed earlier, a total of four models were created using CATIA v4R16®

software and subjected to CFD analysis in ANSYS FLUENT® for the purpose of this

project. These four models have been designated as follows.

Case 1 Base Model

Case 2 with Air Dam with Front Splitter

Case 3 with Rear Wing

Case 4 with Air Dam with Front Splitter and Rear Wing

In order to create these four models, the base model was generated first which

would serve as Case 1. This was followed by modelling of the air dam with front

splitter on the base model to create Case 2. The rear wing was then modelled and

added to the base model to create Case 3 and to the base model with air dam with

front splitter to create Case 4.

3.1.1 Generation of Base Model

In order to generate the base model, the set of blueprints seen below in Figure 3.1

were sourced from www.the-blueprints.com.

Figure 3.1 Bentley Continental GT Blueprints

The following steps were followed for modelling the base model in CATIA v4R16®.

Blueprint files each comprising one of the four views were separately imported using

the Sketch Tracer function.

Box Type Selection was used to adjust each view in accordance with the known full-

scale dimensions of the car.

The views were then correctly arranged in space and aligned with each other as

shown below in Figure 3.2.

Figure 3.2 Scaling, Arrangement and Alignment of Blueprint Views



1. Freestyle Sketching function was used to trace three-dimensional curves as they

appear in different blueprint views. Each view to which a curve must be aligned was

successively selected by changing Compass Settings to ‘Make Privileged Most

Visible’.

Figure 3.3 Curve Tracing 2. Generative Shape Design Workbench was used for surface modelling.

3. The Fill Surface Definition tool was used to select each three-dimensional curve

containing the closed contour of each surface required.

Figure 3.4 Surface Modelling 4. All surfaces were modelled in this way to create an enclosed volume.

5. Wheels were modelled separately within the same part file and added to complete the

base model.

Figure 3.5 Creation of an Enclosed Volume

Figures 3. 6 shows the completed base model that formed the basis for models of

all four cases that have been subjected to CFD analysis in this project.




Figure 3.6 Completed Base Model

3.1.2 Generation of Air Dam with Front Splitter

An air dam is essentially a narrow vertical wall that extends below the front bumper

while the front splitter is a horizontal element that extends forward perpendicular to

the air dam from the bottom edge of the air dam. In theory, the effectiveness of this

combination can be increased by increasing the height of the air dam and the length of

the front splitter. However, in order to ensure that the car remained fairly usable on

the road even after the addition of the air dam with front splitter, certain restrictions

were imposed on the dimensions of this aerodynamic aid.

1. The Bentley Continental GT has a ground clearance of 14cm at the front bumper in

standard specification. In order to allow the car to travel over small road bumps and

to prevent small rocks and gravel from damaging the air dam and front splitter, the

ground clearance was arbitrarily limited to 6cm or more. We decided to opt for the

maximum possible air dam height keeping this restriction in mind. Therefore, a

height of 8cm (from the bottom of the bumper to the bottom of the air dam) was

selected for the air dam reducing the ground clearance in this region to its lowest

permissible value of 6cm.

2. The length of the front splitter was arbitrarily chosen as 10cm. This ensures that it is

long enough to serve its purpose of reducing drag and generating downforce but not

so long that it scrapes and digs into the ground when going over bumps or due to

pitching action under heavy braking.

3. The front splitter thickness was arbitrarily selected as 1.5cm as this is the average

thickness of most commercially available carbon fibre and plastic front splitters.



Figure 3.7 Air Dam with Front Splitter Design

3.1.3 Generation of Rear Wing

3.1.3.1 Aerofoil Profile Selection

After spending a lot of time looking at some of the most common motorsport rear

wing profiles, the top two most efficient ones, namely, the Eppler 664 and Benzing

BE 122-125, were short-listed. The graph in Figure 3.8 that was sourced from Andrew

Shedden’s BMW Z3 Track Car ‘Build Blog’ presents a comparison between the

efficiencies of these two inverted aerofoil profiles at increasing angles of attack.

Despite there not being much difference between the peak efficiency of the two

profiles, the Benzing BE 122-125 profile will clearly perform better under dynamic

conditions, that is, it is more efficient across all the different attack angles. This is an

advantage as the car pitches under braking and acceleration causing the attack angle

of the wing to change accordingly.

A small Gurney Flap was added to the trailing edge of the aerofoil. Its height is

about 5% of the chord length. These are a great addition to any aero device as they

increase downforce significantly with only a very small addition in drag. Basically,

the flap creates an area of low pressure at the tip of the wing which attracts the airflow

on the lower wing surface helping it remain attached for longer.

This allows the wing to run at steeper attack angles which would otherwise cause

flow separation and stall the wing.

Figure 3.8 Effect of Increasing Attack




Figure 3.9 Benzing BE 122-125 Aerofoil Profile with 5% Gurney Flap

[Source-Andrew Shedden’s BMW Z3 Track Car ‘Build Blog’]

3.1.3.2 Rear Wing Positioning and Design

With the rear wing aerofoil profile selected, the next step was to determine the ideal

wing mounting position. This was done by studying the velocity streamlines obtained

in the symmetry plane as a result of the CFD analysis of Case 1 – Base Model.

This allowed us to locate the region above the rear end where the air flow was fast

and uniform characterized by yellow velocity streamlines. This was the ideal position

for the rear wing and not the slow air flow region characterized by green and blue

velocity streamlines.

More importantly, this plot also helped us visualize a fair approximation of the

direction of fast and uniform air flow which is represented by the red reference line in

Figure 3.10. The black reference line was then constructed at an angle of 2° with the

red line because, as indicated in Figure 3.8, the Benzing BE 122-125 aerofoil profile

is most efficient at an angle of attack of approximately 2°. The green line segment

parallel to the black reference line was then constructed in the exact location where

the rear wing would be mounted.

The Sketch Tracing function that was used earlier to import the blueprint views

was used once again this time to import Figure 3.9.

The image was then scaled, adjusted and, most importantly, rotated until the chord

line in Figure 3.9 was aligned perfectly with the green line segment, the latter having

been determined as the required chord line for the rear wing.

Figure 3.10 Rear Wing Positioning



Finally, the Benzing BE 122-125 aerofoil profile was traced using Freestyle

Sketching function. This was followed by surface filling and solid extrusion of the

surface by half the span length in both directions resulting in the creation of the rear

wing seen in Figure 3.11. The position of the rear wing was then fixed in space so it

could be imported into ANSYS FLUENT® separately with the base model to create

the model for Case 3 – With Rear Wing and with the model for Case 2 – With Air

Dam with Front Splitter to create the model for Case 4 – With Air Dam with Front

Splitter and Rear Wing.

Figure 3.11 Rear Wing Design

The chord length of the rear wing was arbitrarily selected as 40cm and the span

length as 196.6cm (equal to the width of the car) in order to ensure that the rear wing

did not protrude beyond the standard footprint of the Bentley Continental GT.

3.2 VIRTUAL WIND TUNNEL VEHICLE ORIENTATION

The models of all four cases were successively oriented in a virtual wind-tunnel in

order to carry out CFD analysis. A virtual air box was created around each three-

dimensional CAD model which represents the wind tunnel in real life. Since we were

most interested in the behavior of air flow at the rear end of the vehicle where the

wake phenomenon occurs, more space was provided inside the air box behind the rear

end of each model to capture the air flow characteristics in greater detail behind the

vehicle.

Figure 3.12 Air Box Dimensions

Due to the complexity of the simulation with limited computer resources and time,

the complete domain was split using a ‘symmetry’ plane which was the Y-Z plane in

this case. Therefore, the simulation would be calculated for just the one side of the

vehicle since the other side is symmetrical. The Y-Z plane was defined as the

symmetric boundary in the solver to make the boundary condition at this plane

identical to that of a ‘slip wall with zero shear forces’. The simulation results,

however, are valid for full model. All six surfaces of the virtual wind tunnel or air box

were named such that the numerical solver of ANSYS FLUENT® would recognize

them and apply the appropriate boundary conditions automatically.

Since the right and top surfaces of the air box are very far away from the model

and have no influence on the model at all, they were named ‘symmetry top’ and

‘symmetry side’. This does not imply that they are symmetric. This was only done to

give them the same boundary conditions as the symmetric surface, a slip wall with

zero shear force.



Figure 3.13 Naming of Surfaces

3.3 MESH GENERATION

3.3.1 Mesh Sizing and Inflation

The triangular shape surface mesh was used due to its proximity to changing curves

and bends. These elements easily adjust to the complex shapes used in automobile and

aerospace bodies. With the default settings for mesh generation, ANSYS Meshing®

generated the coarse meshes as seen in Figure 3.14.

Figure 3.14 Mesh Generation with Standard Settings

With the global mesh sizing settings, ANSYS Meshing® recognized that there

were some curvatures around the vehicle body. But the meshing was very coarse and

it was only an initial guess by the software. In order to capture more accurate data

through the solver, we needed to improve the mesh. The first thing to do was change

the mesh sizing parameters. All mesh sizing parameters that were altered for the

models of the four cases under consideration are given in the following Tables 3.1 and

3.2.

Table 3.1 Altered Mesh Sizing Parameters




Table 3.2 Altered Mesh Sizing Parameters

The new mesh looked more decent but it was still lacking a layer of inflation

around the vehicle body. The inflation layer was, therefore, enabled and ‘Automatic

Inflation – Program Controlled’ was selected to capture the boundary effects of the

flow around the body more accurately. The vehicle body itself and the road were

included within ‘Program Controlled Inflation’ while the other named selections were

excluded. Based on what is advised for vehicle external aerodynamics with ANSYS

FLUENT® by Marco Lanfrit’s guideline, the inflation option has been set as ‘First

Aspect Ratio’ instead of the default ‘Smooth Transition.’

Figure 3.15 Mesh Generation with Altered Sizing Settings



Figure 3.16 Mesh Generation with Inflation Layers

Another recommendation from the guidelines by FLUENT® Germany that was

incorporated for the mesh generation with altered sizing settings and with inflation

layers and which can be observed in Figures 3.15 and 3.16 was the creation of a new

volume control box around the body where the elements can be limited to a certain

size. The advantage of using a control volume box and limiting the mesh sizing within

that control volume is that it lets us improve the mesh quality only within the region

where we need high resolution mesh instead of having to use fine mesh in the entire

domain. This reduces the time required to run the simulations.

There are several ways of doing this. One method is to create a ‘sphere of

influence’ but this method is mainly used for helicopter or aeroplane simulations

mostly when there is no wall or road involved. Another way of doing this is to create

a virtual box around the vehicle that is about 2.5 times longer in the Z-direction and

about 1.5 times longer in the X and Y-directions. The orientation of this virtual box

which we called the ‘car box’ can be seen in Figure 3.17.

Figure 3.17 Virtual Car Box Orientation

Once the virtual car box was generated and its mesh sizing was limited to 40mm

with the Body Sizing function in the software, the new mesh became very detailed

and was finally ready to run in the solver.

3.4 SOLVER SETTINGS

CFD analysis of the air flow around a car requires the solver settings to be completed

before starting the simulations. The solver setting includes selection of the type of

solver, namely, two-dimensional (2D) or three-dimensional (3D), definition of fluid

properties, setting of boundary conditions and solution initialization.




Table 3.3 Solver Settings

3.4.1 Definition of Fluid Properties

Table 3.4 Viscous Model and Turbulence Model Settings

3.4.2 Boundary Conditions

Table 3.5 Boundary Condition Settings



3.4.3 Solution Initialization

Table 3.6 Solution Initialization Settings

4. SIMULATION RESULTS

This section presents the results of the CFD analysis that was performed on the

following four models in ANSYS FLUENT®.

Case 1 Base Model

Case 2 with Air Dam with Front Splitter

Case 3 with Rear Wing

Case 4 with Air Dam with Front Splitter and Rear Wing

CFD analysis performed using the ANSYS FLUENT® numerical solver involves

a three-dimensional steady-state incompressible solution of the Navier-Stokes

Equations. For an incompressible flow of uniform viscosity, the Navier-Stokes

Equations can be read as follows.

The same meshing resolution, k – ε turbulence model and boundary conditions

were used for simulations of all four cases. The free stream velocity was set at 40m/s

or approximately 144km/h for all simulations.

CASE 1

Figure 4.1.1 Base Model




Figure 4.1.2 Velocity Streamlines in Symmetry Plane

Figure 4.1.3 Pressure Contours on Outer Body Surfaces

CASE 2

Figure 4.2.1 with Air Dam with Front Splitter





CASE 3

Figure 4.3.1 with Rear Wing






CASE 4

Figure 4.4.1 with Air Dam with Front Splitter + Rear Wing





Table 4.1 presents a comparison of the convergent values of CD and CL obtained

from the results of the CFD simulations performed on the models of all four cases.

Table 4.1 Convergent CD and CL Values

5. OBSERVATIONS AND CONCLUSIONS

In this section, we shall endeavor to explain and interpret these findings by studying

the individual contributions of the two aerodynamic aids under consideration, namely,

the air dam with front splitter and the rear wing, with the help of velocity streamline,




velocity vector and pressure contour plots obtained in ANSYS FLUENT® as a result

of the CFD simulations.

5.1 BASE MODEL

As already mentioned in Section 2, the first-generation Bentley Continental GT in

standard specification has a manufacturer calculated CD of 0. 33. As seen in Table

4.1, our base model approaches the same value.

Figure 5.1 Minimal Wake Formation at the Rear End of the Base Model

The reason behind the standard car’s impressively low CD is its fastback shape and

boat tailed rear bumper which delay flow separation and consequently minimize

wake, which is the major contributor to aerodynamic drag, as shown in Figure 5.1.

Ideally, the absence of wing mirrors and the overall lack of three-dimensional

details on our CAD model should have resulted in it having a CD slightly lower than

the value claimed by the manufacturer for the actual car. However, as discussed in

Section 1.2, one of the limitations we faced was that, this being our first time

modelling an entire vehicle body using CATIA V5®, the resultant CAD model does

not boast of professional grade perfection. Also, the blueprints that were used to

sketch out and generate the model are of rudimentary detail and do not provide all the

required dimensions and proportions required to generate a perfectly accurate

replication with all the design nuances of the original but more detailed drawings are

neither easily available nor affordable so we had to do the best we could with what we

had at hand.

The slight inaccuracies that may exist in our CAD model manifest as edges and

contour changes than are perhaps a little sharper and more abrupt than they are on the

actual car and surfaces that are very possibly flatter or more concave than they are on

the actual car. These drag inducing inaccuracies are very likely the cause for the CD of

our base model not being as low as was expected given the lack of wing mirrors and

other protruding details.

Nevertheless, our base model has still proven to be a fairly accurate representation

of the first-generation Bentley Continental GT outer body shape since the results

obtained are clearly reasonable and justifiable.

It is also known that the Bentley Continental GT in standard specification

experiences positive aerodynamic lift at high speed as is the case with most factory

standard road-legal production passenger cars. This too is corroborated by Table 4.1

as the value of CL obtained as result of CFD simulations is positive.

5.2 AIR DAM WITH FRONT SPLITTER

As seen in Table 4.1 for Case 2, while there is a noticeable but not substantial

decrease in CD, the change in CL is practically negligible. In Table 4.1 for Case 4, the



air dam with front splitter is more effective at considerably reducing the aerodynamic

drag induced by the rear wing and the CD of this combination is considerably lower

than that in Case 3 in which the rear wing acts alone. Moreover, it is also noted that

while the air dam with front splitter generates practically no downforce when acting

along as in Case 2, its contribution to downforce is not inconsiderable when it acts in

combination with the rear wing as in Case 4 which results in the air dam with front

splitter and rear wing combination of Case 4 having a significantly larger negative

value of CL than the solitary rear wing configuration of Case 3.

The air flow is brought to stagnation above the splitter by the air dam creating an

area of high pressure. The front splitter redirects air away from this stagnation point

and accelerates that air under the car which, in turn, causes a low pressure. High

pressure over the splitter caused by the air dam and low pressure of the air flow under

the car creates down force by way of the Bernoulli Effect which states that fluid is

drawn from high pressure areas to low pressure areas.

Figure 5.2 Air Dam with Front Splitter Air Flow

5.3 REAR WING

It is very clear from Table 4.1 for Case 3that the rear wing is the ultimate downforce

generating aerodynamic aid. The positive value of CL for the base model is

transformed dramatically into a large negative value indicating a noticeable and

desirable change from lift to downforce generation when the rear wing is attached.

The flipside is the considerable and undesirable increase in aerodynamic drag that

accompanies the downforce generated by the rear wing as is also witnessed in Table

4.1. However, this increase in CD can be counteracted to some extent by using the

rear wing in combination with the air dam with front splitter as in Case 4. It is

observed that the drag reduction quality of the air dam with front splitter is also

noticeably enhanced when it is used in combination with the rear wing!

In an inverted aerofoil profile such as that used for sports car or racing car rear

wings, the lower surface is shaped such that air rushing below the wing speeds up

because it must cover a greater distance than the air flowing above the wing between

the leading and trailing edges decreasing the pressure below the wing while the slow

moving air above the wing exerts high pressure pushing down the wing and

generating downforce




Figure 5.3 Air Flow Characteristics Around the Rear Wing

REFERENCES

[1] Bertin, John J, ‘Aerodynamics for Engineers’, Prentice Hall

[2] Hucho, Wolf-Heinrich, ‘Aerodynamics of Road Vehicles – From Fluid

Mechanics to Vehicle Engineering’, Society of Automotive Engineers

[3] Jiyuan Tu, Guan Heng Yeoh and Chaoqun Liu, ‘Computational Fluid Dynamics

– A Practical Approach’, Butterworth-Heinemann

[4] Lanfrit, Marco, ‘Best Practice Guidelines for Handling Automotive External

Aerodynamics with FLUENT’, Fluent Deutschland GmbH

[5] Williamson, C H K, ‘Three-Dimensional Vortex Dynamics in Bluff Body

Wakes’, Experimental Thermal and Fluid Science

[6] Zikanov, Oleg, ‘Essential Computational Fluid Dynamics’, John Wiley & Sons

[7] Mohd Azman Abdullah, Azhar Ibrahim, Yohei Michitsuji and Masao Nagai.

Active Control of High-Speed Railway Vehicle Pantograph Considering Vertical

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Technology, 4(6), 2013, pp. 263 - 274.

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[10] www.asracingblog.com/6/post/2012/12/rear-wing-design-1.html

[11] www.the-blueprints.com

[12] www.bentleymotors.com