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AERODYNAMIC ANALYSIS OF SEDANS USING CFD

Mini Project Interim Report

Submitted in partial fulfillment of the requirements for the award of the degree of

Bachelor of Technologyin

Mechanical Engineering

by

ASHWIN PRABHAKARAN

DEEPAK K

JAISON LOUIS

RAHUL G R

SANDEEP VIJAYAKUMAR


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Group Members

ASHWIN PRABHAKARAN B080299PE

DEEPAK K B080160PEJAISON LOUIS B080134PE

RAHUL G R B080174PE

SANDEEP VIJAYAKUMAR B080293PE

Faculty Guide

Dr. Sarvoththma Jothi

(Guide)Assistant Professor

Dept. of Mechanical Engineering

Faculty in charge


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Department of Mechanical Engineering

NATIONAL INSTITUTE OF TECHNOLOGY CALICUT

APRIL 2010

CERTIFICATE

This is to certify that the report entitled AERODYNAMIC ANALYSIS OF

SEDAN is a bonafide record of the Mini Project done by ASHWIN PRABHAKARAN

(Roll No.: B070178ME), DEEPAK K (Roll No.: B070070ME), JAISON LOUIS (Roll

No.: B070154ME), RAHUL G R (Roll No.: B070116ME), and SANDEEP

VIJAYAKUMAR (Roll No.: B070193ME) under my supervision, in partial fulfillment

of the requirements for the award of the degree of Bachelor of Technology in

Mechanical Engineering from National Institute of Technology Calicut, and this work

has not been submitted elsewhere for the award of a degree.

Dr. Sarvoththma Jothi(Guide)

Assistant Professor

Dept. of Mechanical Engineering

Professor & HeadDept. of Mechanical Engineering

Place : NIT CalicutDate : 16 February 2011


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ACKNOWLEDGEMENT

We wish to express our sincere gratitude to our project guide Dr T J Sarvoththama Jothi

for his invaluable guidance and encouragement throughout the course of project. The

suggestions given by him during our discussions played an important role in constructing

the methodology of the project and in developing ideas for successful completion of the

project.

Ashwin Prabhakaran

Deepak K

JaisonLouis

Rahul G R

Sandeep Vijayakumar


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ABSTRACT

In simple terms, aerodynamics has something to do with the shape of an object

affecting the flow of air to generate force. A car is designed taking into consideration

various aspects like manufacturing complexity, cost, aerodynamic efficiency and

aesthetic beauty. Among the previously mentioned factors, one of the most vital factors is

the aerodynamic efficiency of the car. This is because a car with bad aerodynamic design

will lack fuel efficiency, stability and economic fuel feasibility. So this is why racecars as

well as road cars have good aerodynamic designs.

The flow of air surrounding a car can affect its performance. Shaping a cars body

so that the car can pass through the air with minimum amount of resistance, at the sametime that air flow pushes the car unto the ground for stability, is the goal of car

aerodynamics.

In our project we propose to design and study three different sedan models using

Ansys CFX and compare the results and draw suitable conclusions.


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CONTENTS

1 Introduction

1.1 Introduction

1.1.1 Aerodynamic design of Audi R8

1.1.2 Computational Fluid Dynamics

1.1.3 Software Used

1.2 Problem Definition

1.3 Working methodology

1.4 Outline of the report

2 Theory

2.1 Fluid Mechanics

2.1.1 Laminar and turbulent flows

2.1.2 Forces Acting on Immersed bodies

2.1.3 Flow separation

2.2 Standard K- model

2.2.1 Transportation equations for standard K- model

3 Modeling and Analysis using software

3.1 Analysis of Simple Car model

3.1.1 Pressure Contour plot

3.1.2 Velocity Contour plot

3.1.3 Velocity Streamline plot3.1.4 Velocity Vector plot


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3.2 Analysis of Aerofoil

3.2.1 Pressure Contour plot

3.2.2 Velocity Contour plot

3.2.3 Velocity Vector plot

3.3 Audi R8 and Audi A4 modeling in Solidworks

3.4 Ansys analysis of Audi R8 and Audi A4 in Ansys CFX

3.5 Plot Results

3.5.1 Plot results for Audi A4

3.5.1.1 Pressure contour plot

3.5.1.2Pressure time plot

3.5.1.3 Velocity time plot

3.5.1.4 Velocity contour plot

3.5.1.5 Velocity streamline plot

3.5.1.6 Velocity vector

3.5.2 Plot Results for Audi R8


3.5.2.2 Pressure Time Plot

3.5.2.3 Velocity Time Plot

3.5.2.4 Velocity contour Plot

3.5.2.5 Velocity Streamline Plot

3.5.2.6 Velocity Vector Plot

3.5.2.7 Velocity Vector of Rear

3.6 Results

3.7 Scope for future work

3.8 Reference


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CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION

1.1.1 Aerodynamic design of AUDI R8 and AUDI A4

Low drag is an important consideration when it comes to design of automobiles as

it increases the top speed and reduces fuel consumption. Down forces are very important

for stability of the vehicle. This is because the air flowing around a high speed car can

induce considerable lift forces.This reduces the weight applied to the wheels and thus

impairs directional stability.

Although seemingly only a marginal issue on sports cars, aero acoustics plays a

very important part in determining long-distance comfort and everyday suitability. Audiwas able to call on its wealth of experience as a manufacturer of premium saloon cars in

making the R8 the sports car with the lowest level of wind noise.The aim is ultimately to

keep the driver and passenger in top shape over long distances.

1.1.2 Computational Fluid Dynamics

Applying the fundamental laws of mechanics to a fluid gives the governing

equations for a fluid. The conservation of mass equation and the conservation of

momentum equation and the conservation of energy equation form a set of coupled,

nonlinear partial differential equations. It is not possible to solve these equations

analytically for most engineering problems. However, it is possible to obtain approximate

computer-based solutions to the governing equations for a variety of engineering

problems. This is the subject matter of Computational Fluid Dynamics (CFD).

CFD is attractive to industry since it is more cost-effective than physical testing.

However, one must note that complex flow simulations are challenging and error-prone

and it takes a lot of engineering expertise to obtain validated solutions.

1.1.3 Software Used

Ansys CFX is the software used for the aerodynamic analysis of the model. It is

widely used software in the industry as it provides a comprehensive range of models. The

solid model of the car was modeled using Solidworks and it can be exported to Ansys.


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1.2 PROBLEM DEFINITION

The primary objective of our work is to understand the aerodynamics of typical

sedan. The airflow around the sedan will be analyzed and aerodynamic forces will be

calculated under a range of speeds using the software Ansys CFX. Thus with the help

of this data, the laws of mechanics will be used for the motion analysis of the sedan.

1.3 WORKING METHODOLOGY

The work started by familiarizing with the software Ansys CFX and Solidworks.

The first work we started was modeling the Audi R8 model in Solidworks. Parallel to that

we also started the analysis of simple 2D figures and then moved onto 3D figures like

sphere and cylinder.

The first complex shape we considered was that of an aerofoil. The knowledge

gained was used to model and analyze a simple car model. Once the original car model

was completed we analyzed it in Ansys CFX. From the data obtained, basic laws of

motion were used for further analysis. Then we started the modeling of our second

variant of sedan Audi A4 on Solidworks. Its analysis was done similar to our first model

using Ansys CFX. We then found out various characteristics like coefficient of drag and

compared the results.

1.4 OUTLINE OF THE REPORT

The report consists of an introduction to the aerodynamics of a sedan and the field

of CFD. This is followed by a theoretical discussion on the various flow types and flow

past immersed bodies. After this, we move into the analysis of an aerofoil and simple car

model using ANSYS software and the interpretation of the result. We then moves onto

details of modeling and analysis of the two car models. Finally we conclude with further

investigations results, which can be carried so as to make this project even more

meaningful and advantageous to the car design field as well as for academic purposes.


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CHAPTER 2

THEORY

2.1 FLUID MECHANICS

2.1.1 Laminar and turbulent flow

There are two radically different states of flows that are easily identified and

distinguished: laminar flow and turbulent flow. Laminar flows are characterized by

smoothly varying velocity fields in space and time in which individual laminae (sheets)

move past one another without generating cross currents. These flows arise when the

fluid viscosity is sufficiently large to damp out any perturbations to the flow that may

occur due to boundary imperfections or other irregularities. These flows occur at low-to-

moderate values of the Reynolds number.

Fig 2.1 Laminar and turbulent flow

In contrast, turbulent flows are characterized by large, nearly random fluctuations

in velocity and pressure in both space and time. These fluctuations arise from instabilities

that grow until nonlinear interactions cause them to break down into finer and finer

whirls that eventually are dissipated (into heat) by the action of viscosity. Turbulent flows

occur in the opposite limit of high Reynolds numbers.

2.1.2 Forces Acting on Immersed bodies

Any arbitrary body placed in a flow field experiences forces and moments from

the field. These act in three dimensions and hence a coordinate system is selected with

one axis along the free stream and positive downstream.


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Fig 2.2 Forces on immersed bodies

Drag:Drag is the force acting on the body along the axis parallel to free stream. It is

essentially a flow loss and must be overcome if the body is to move against the stream.

The moment acting about this axis is called rolling moment. The drag equation calculates

the force experienced by an object moving through a fluid at relatively large velocity (i.e.

high Reynolds number), also called quadratic drag.

Lift:

Lift acts perpendicular to the direction of drag and usually performs a useful job

like bearing weights of bodies. The moment about this axis is called yaw.

Side force:

Side force acts along an axis perpendicular to lift and drag. This force is neither a

loss nor gain. The moment about this axis is pitching moment.

2.1.3 Flow separation

Flow separation occurs when the boundary layer travels far enough against an

adverse pressure gradient that the speed of the boundary layer falls almost to zero. The

fluid flow becomes detached from the surface of the object, and instead takes the forms

ofeddies and vortices.
http://en.wikipedia.org/wiki/Drag_equationhttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Reynolds_numberhttp://en.wikipedia.org/wiki/Boundary_layerhttp://en.wikipedia.org/wiki/Adverse_pressure_gradienthttp://en.wikipedia.org/wiki/Eddy_%28fluid_dynamics%29http://en.wikipedia.org/wiki/Vortexhttp://en.wikipedia.org/wiki/Vortexhttp://en.wikipedia.org/wiki/Eddy_%28fluid_dynamics%29http://en.wikipedia.org/wiki/Adverse_pressure_gradienthttp://en.wikipedia.org/wiki/Boundary_layerhttp://en.wikipedia.org/wiki/Reynolds_numberhttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Drag_equation


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Fig 2.3 Flow separation

In aerodynamics, flow separation can often result in increased drag, particularly

pressure drag which is caused by the pressure differential between the front and rear

surfaces of the object as it travels through the fluid. For this reason much effort and

research has gone into the design of aerodynamic and hydrodynamic surfaces which

delay flow separation and keep the local flow attached for as long as possible.

Stagnation point

Stagnation point is a point in a flow field where the local velocity of the

fluid is zero. Stagnation points exist at the surface of objects in the flow field, where the

fluid is brought to rest by the object. The Bernoulli equation shows that the static pressure

is highest when the velocity is zero and hence static pressure is at its maximum value at

stagnation points. This static pressure is called the stagnation pressure.

Fig 2.4 Stagnation Point
http://en.wikipedia.org/wiki/Aerodynamicshttp://en.wikipedia.org/wiki/Drag_%28physics%29http://en.wikipedia.org/wiki/Pressure_draghttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Aerodynamichttp://en.wikipedia.org/wiki/Hydrodynamichttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Bernoulli%27s_principlehttp://en.wikipedia.org/wiki/Static_pressurehttp://en.wikipedia.org/wiki/Stagnation_pressurehttp://en.wikipedia.org/wiki/Stagnation_pressurehttp://en.wikipedia.org/wiki/Static_pressurehttp://en.wikipedia.org/wiki/Bernoulli%27s_principlehttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Hydrodynamichttp://en.wikipedia.org/wiki/Aerodynamichttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Pressure_draghttp://en.wikipedia.org/wiki/Drag_%28physics%29http://en.wikipedia.org/wiki/Aerodynamics


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The Bernoulli equation applicable to incompressible flow shows that the

stagnation pressure is equal to the dynamic pressure plus static pressure. Total pressure is

also equal to dynamic pressure plus static pressure so, in incompressible flows, stagnation

pressure is equal to total pressure

2.2 STANDARD K - MODEL

2.2.1 Transport equations for standard K - model

For turbulent kinetic energy

+

=

[ +

+ + + For dissipation

+ = + + 1 + 3 2 2

+ Turbulent viscosity is modeled as:

= 2

Model constants 1 = 1.44,2 = 1.92, = 0.09, = 1.0, = 1.3
http://en.wikipedia.org/wiki/Bernoulli%27s_principlehttp://en.wikipedia.org/wiki/Incompressible_flowhttp://en.wikipedia.org/wiki/Static_pressurehttp://en.wikipedia.org/wiki/Dynamic_pressurehttp://en.wikipedia.org/wiki/Stagnation_pressurehttp://en.wikipedia.org/wiki/Incompressible_flowhttp://en.wikipedia.org/wiki/Bernoulli%27s_principle


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CHAPTER 3

MODELING AND ANALYSIS USING SOFTWARE

3.1 Analysis of Simple Car Model

A rough car shape was modeled and analyzed using Ansys CFX.M Its

aerodynamic analysis was performed and plot results were obtained.

3.1.1 Pressure Contour Plot

Fig 3.1

3.1.2VelocityContour Plot

Fig 3.2


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3.1.3 Velocity Streamline Plot

Fig 3.3

3.1.4 Velocity Vector Plot

Fig 3.4


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3.2 Analysis of Aerofoil

An aerofoil was roughly modeled and analyzed using Ansys CFX.

Its aerodynamic analysis was performed and plot results were obtained.

3.2.1 Pressure Contour Plot

Fig 3.5

3.2.2 Velocity Contour Plot

Fig 3.6


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3.2.3 Velocity Vector Plot

Fig 3.7

3.3 Audi R8 and Audi A4 Modeling in Solidworks

The modeling was done using Solidworks software. The various steps are listed below

1) The front, side, back and top view of Audi R8 and A4 were obtained from theofficial website of Audi company with dimensions.

2) These views were imported into the Solidworks along their respective planes i.e.side view mounted on right plane, front view mounted on front plane parallel to

front plane and at a distance equal to the length of the car, and top view mounted

on the top plane.

These were used as the base reference for all the drawings


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Fig 3.8 Audi R8 and A4

3) For locating the corner points the following steps were followed:a) A corner point which was present in two different views was found - ex Plane

1 and Plane 2.

b) In plane 1 the point was markedc) A plane parallel to plane 2 passing through the point in plane 1 was createdd) The original corner point was located in this new plane by tracing

4) Using the spline tool and tracing through different views which were previouslyimported in steps 1 and 2, many sketches were made.

These sketches became the outline of the entire car surface.


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5) Using the loft tool and sweep tool, the surfaces required were made. A loft toolrequires two profile curves, and one or more guide curves. These curves for each

surface was separately given in order to get the required geometry


6) Any surface that extended unnecessarily was cut-off using trim tool. A trim toolrequires a sketch and a given direction.

7) When two adjacent surfaces are made, using the knit tool they were joinedtogether. A knit tool requires two surfaces which are adjacent to each other.


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8) Using the thicken tool, all the surfaces were made into solid parts.9) The part body was given car paint appearance and other parts like headlamps, tail

lamps, rear and front windshields and side glasses were given glass appearance.

10)The wheel was made separately using the following stepsa) A cross section of the wheel was sketched using side view of the car

imported in steps 1 and 2

b) Using circular pattern tool this sketch was made into a circular crosssection of the wheel

c) It was then extruded to surface to obtain a solid geometryd) The deft tool was also used to get the shape and make intricate designs


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Fig 3.11

e) Unwanted material was cutoff using trim to surface toolf) For the tyre a small cross section was made above the wheelg) Using the revolve tool this sketch was made into a circular volume around

the wheel.

h) Using the extruded cut option, the threads on the tyre were madei) The wheel part was given metal chromium appearance and tyre pattern

appearance was given to the tyre.

Fig 3.12


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j) The car and the wheel were combined in an assembly.

Fig 3.13 Audi A4 Audi R8(rendered view)

3.4 Ansys Analysis of Audi R8 and Audi A4

Software used

Ansys CFX was used for the analysis part of our project work. Ansys CFX

provides a wide variety of models to suit the demands of individual classes of problems.

It is much simpler than the other CFDs available.

Steps for analysis

1) The model made in Solidworks was saved in IGES format for exporting.2) It was imported into Ansys using Ansys Design Modeler.


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3) Using Boolean operation the volume of the car was subtracted from thecontrol volume.

Fig 3.14 Audi A4

Fig 3.15 Audi R8


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4) Ansys meshing module option was used to mesh the model along with thecontrol volume.

5) For better accuracy finer meshing needed to be done which in turn wasproduced using virtual topology.

6)

Now the model is further imported into Ansys CFX-Pre where boundaryconditions were specified.

Wind velocity = 10m/s

Fluid = air at 250C

Buoyancy = -g in y direction

Wind tunnel is kept open to the atmosphere

7) Expressions used for finding Co-efficient of dragU = 10m/s

Fx = force_@car surfaceFy = force_y@car surfaceLift = Fy

Drag = Fx

Denom = 0.5 @U^2*

=

=

8) Two monitor points CD and CL were defined with expressions CD and CLrespectively.

9) Solution is done using K - model10)Cd was found out11) Plot results were also obtained


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3.5 Plot results

The Ansys results and plot were obtained using CFD post.

3.5.1 Plot results for A4


Fig 3.16

It is shown on figures that there is a higher pressure concentration on the car front.

Particularly, air slows down when it is approaches the front of the car and results that

more air molecules are accumulated into a smaller space. This pressure stagnant causes

rise in drag. Once the air stagnates in front of the car, it seeks a lower pressure area, such

as the sides, top and bottom of car. As the air flows over the car hood, pressure is

decreasing, but when reaches the front windshield it briefly increasing.

When the higher pressure air in front of the windshield travels over the

windshield, it accelerates, causing the decreasing of pressure. This small lower pressure

region literally produces a lift - force on the car roof as the air passes over it, but there

exists a high pressure region above this low pressure region producing a negative lift.


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3.5.1.2Pressure time plot

Fig3.17

Variation of pressure of a particle moving along the wind tunnel on the surface of the car

is given in the plot. Pressure dip corresponds to the low pressure region on the top of the

car. Thereafter the pressure rises on the surface producing negative lift.

3.5.1.3 Velocity time plot

Fig3.18


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Variation of velocity of a particle moving along the wind tunnel on the surface of the car

is given in the plot. Velocity spike corresponds to the low pressure region on the top of

the car.

3.5.1.4 Velocity contour plot

Fig 3.19Figure shows the variation of velocity. The air velocity is decreasing as it is

approaching the front of the car. Air passes over the surface of the car the velocity

increases.


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3.5.1.5 Velocity streamline plot

Fig 3.20

Figure shows the velocity streamline around the car.

3.5.1.6 Velocity vector

Fig 3.21

Velocity vector plot is shown in the diagram. The direction of velocity is at each point of

the control volume is shown.


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3.5.2 Plot Results for Audi R8


Fig 3.22

Fig 3.16

It is shown on figures that there is a higher pressure concentration on the car front.

Particularly, air slows down when it is approaches the front of the car and results that

more air molecules are accumulated into a smaller space. This pressure stagnant causes

rise in drag. Once the air stagnates in front of the car, it seeks a lower pressure area, such

as the sides, top and bottom of car. As the air flows over the car hood, pressure is

decreasing, but when reaches the front windshield it briefly increasing.

When the higher pressure air in front of the windshield travels over the

windshield, it accelerates, causing the decreasing of pressure. This small lower pressure

region literally produces a lift - force on the car roof as the air passes over it, but there

exists a high pressure region above this low pressure region producing a negative lift.


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3.5.2.2 Pressure Time Plot

Fig 3.23

Variation of pressure of a particle moving along the wind tunnel on the surface of the car

is given in the plot. Pressure dip corresponds to the low pressure region on the top of the

car. Thereafter the pressure rises on the surface producing negative lift.

3.5.2.3 Velocity Time Plot


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Fig3.24

Variation of velocity of a particle moving along the wind tunnel on the surface of the car

is given in the plot. Velocity spike corresponds to the low pressure region on the top of

the car.

3.5.2.4 Velocity contour Plot

Fig 3.25

3.5.2.5 Velocity Streamline Plot

Fig 3.26

Figure shows the velocity streamline around the car.


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3.5.2.6 Velocity Vector Plot

Fig 3.26

Velocity vector plot is shown in the diagram. The direction of velocity is at each point of

the control volume is shown.

3.5.2.7 Velocity Vector of Rear

Fig 3.27

Velocity vector diagram of the rear part of the car is shown. Turbulence of air can be

seen.


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3.6 Results

Coefficient of drag (Cd)

For Audi R8 = 0.298

For Audi A4 = 0.358

We see that the coefficient of drag is lower for the Audi R8 model which implies

that the aerodynamic designing of Audi R8 is better. Therefore the drag forces will be

comparatively lower for this model.


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3.8 Scope for future work

1. We could improve the model by making minor variations to the design such as adding

a spoiler and study the variation in result

2. Along with the Ansys simulation one could make a prototype of the model and using a

wind tunnel obtain the results experimentally.


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3.7 Reference

(1) http://www.autozine.org/technical_school/aero/tech_aero.htm

(2) Milad Mafi, "Investigation of Turbulence Created by Formula One Cars with the

Aid of Numerical Fluid Dynamics and Optimization of Overtaking Potential",

Competence Centre, Transtec AG, Tbingen, Germany.

(3) Virag, Zdravko, Lectures from course "Numerical methods"

(4) Luke Jongebloed, "Numerical Study using FLUENT of the Separation and

Reattachment Points for Backwards - Facing Step Flow", Mechanical Engineering

Rensselaer Polytechnic Institute, Hartford, Connecticut, December, 2008,

(5) ANSYS Fluent, Release 12.1: Help Topics

(6) http://www.up22.com/Aerodynamics.html.

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