Mini Final Main Body
Transcript of Mini Final Main Body
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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
3.5.2.1 Pressure contour plot
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
Fig 3.9 Audi R8 and A4
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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Fig 3.10 Audi R8 and A4
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
3.5.1.1 Pressure contour plot
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
3.5.2.1 Pressure contour plot
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.