Aerodynamics of Car

7/27/2019 Aerodynamics of Car

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JSPMS Rajarshi Shahu College OfEngineering, Pune-33Seminar Report

Introduction to Aerodynamics

of Cars.

Submitted to the University of Pune,

with the partial fulfillment of the requirements,

For the degree ofBachelor of Engineering in Mechanical Engineering,

By Akshay .S. Mishra under the guidance ofProf. Harshal .B. Tambat.

For the academic year 2013-2014.

2013

Akshay .S. MishraB3426

5/10/2013


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ACKNOWLEDGEMENT

First of all i thank the almighty for providing me with

strength and courage to present the seminar.

I avail this opportunity to express my sincere gratitude

towards Dr. Abhay Pawar Head of Mechanical

Engineering Department, for providing me to conduct theseminar.

I also at the outset thank and express my profound

gratitude to my seminar guide Prof. Harshal Tambat.

I am also indebted to all teaching and non-teaching staff

of the department of Mechanical Engineering for their

co-operation and suggestions which is the spirit behind

this report.

Last but not the least, I wish to express my sincere thanks

to all my friends and family for their goodwill and

constructive ideas.

-AKSHAY MISHRA


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ABSTRACT

When objects move through air, forces are generated by the relative

motion between the air and surfaces of the object. Aerodynamics is the

study of these forces, generated by the motion of air. Usually

aerodynamics is categorized according to the type of flow as subsonic,

hypersonic, supersonic etc.

It is essential that aerodynamics be taken into account during the design

of cars, as an improved aerodynamics in car would attain higher speeds

and more fuel efficiency. For attaining this aerodynamic design the cars

are designed lower to the ground and are usually sleek in design and

almost all corners are rounded off, to ensure smooth passage of air

through the body. In addition to it a number of enhancements like

spoilers, wings are also attached to the cars for improving aerodynamics.

Wind tunnels are used for analyzing the aerodynamics of cars, besides

this a number of softwares are also available nowadays to ensure the

optimal aerodynamic design.


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CONTENTS

Acknowledgement

Abstract

Contents

List of figures

1. Introduction

2. Aerodynamic forces on a

Body

a) Lift

b) Weight

c) Drag

d) Thrust

3. History and evolution of aerodynamics

4. Study of Aerodynamic forces

on cars a) Drag

b) Lift or Down force

5. Aerodynamic devices

6. Drag Co-efficient

7. Methods for evaluating Aerodynamics

in cars

a) Wind tunnels

b) Software

8. Conclusions

9. References


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INTRODUCTION

When objects move through air, forces are generated by the relative motion between

air and surfaces of the body, study of these forces generated by air is called

aerodynamics. Based on the flow environment it can be classified into external

aerodynamics and internal aerodynamics; external aerodynamics is the flow around

solid objects of various shapes, where as internal aerodynamics is the flow through

passages in solid objects, for e.g. the flow through jet engine air conditioning pipe etc.

The behavior of air flow changes depends on the ratio of the flow to the speed of

sound. This ratio is called Mach number, based on this mach number the aerodynamic

problems can be classified as subsonic if the speed of flow is less than that of sound,

transonic if speeds both below and

above the speed of sound are present, supersonic if characteristics of flow are greater

than that of sound and hypersonic if flow is very much greater than that of sound.

Aerodynamics have wide range of applications mainly in aerospace engineering

,then in the design of automobiles, prediction of forces and moments in ships and

sails, in the field of civil engineering as in the design of bridges and other buildings,

where they help to calculate wind loads in design of large buildings.


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AERODYNAMIC FORCES ON A BODY:

LIFT

It is the sum of all fluid dynamic forces on a body normal to the

direction of external flow around the body. Lift is caused by Bernoullis effect which

states that air must flow over a long path in order to cover the same displacement in

the same amount of time. This creates a low pressure area over the long edge of object

as a result a low pressure region is formed over the aerofoil and a high pressure region

is formed below the aerofoil,It is this difference in pressure that creates the object to rise

F= (1/2)CLdV2A

Where:

CL= Coefficient of Lift, dependent on the specific geometry of the

object, determined experimentally

d= Density of air

V=Velocity of object relative to air, A=Cross-sectional area of object, parallel to wind

DRAG

It is the sum of all external forces in the direction of fluid flow, so it

acts opposite to the direction of the object. In other words drag can be explained as

the force caused by turbulent airflow around an object that opposes the forward

motion of the object through a gas or fluid.

F= (1/2)CDdV2A

Where: CD= Coefficient of Drag, dependent on the specific geometry of the

object, determined experimentally.

d= Density of air.

V=Velocity of object relative

to air.


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A= cross section of frontal

area.

Since drag is dependent on square of velocity it is most predominant

when object is traveling at very high speeds. It is the most important aerodynamic

force to study because it limits both fuel economy of a vehicle and the maximum

speed at which a vehicle can travel.

WEIGHT

It is actually just the weight of the object that is in motion .i.e. the mass of

the object multiplied by the magnitude of gravitational field. This weight has a

significant effect on the acceleration of the object.

THRUST

When a body is in motion a drag force is created which opposes the motion

of the object so thrust can be the force produce in opposite direction to drag that is

higher than that of drag so that the body can move through the fluid. Thrust is a

reaction force explained byNewtons second and third laws, The total force

experienced by a system accelerating in mass m is equal and opposite to mass

m times the acceleration experienced by that mass.


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HISTORY & EVOLUTION OF AERODYNAMICS

Ever since the first car was manufactured in early 20th century the attempt has been

to travel at faster speeds, in the earlier times aerodynamics was not a factor as the

cars were traveling at very slow speeds there were not any aerodynamic problems

but with increase of speeds the necessity for cars to become more streamlined

resulted in structural invention such as the introduction of the windscreen,

incorporation of wheels into the body and the insetting of the headlamps into the

front of the car. This was probably the fastest developing time in automobiles history

as the majority of the work was to try and reduce the aerodynamic drag. This

happened up to the early 1950s, where by this time the aerodynamic dray had been

cut by about 45% from the early cars such as the Silver Ghost. However, after this the

levels of drag found on cars began to slowly increase. This was due to the way that the

designing was thought about. Before 1950, designers were trying to make cars as

streamlined as possible to make it easier for the engine, yet they were restricting the

layout of the interior for the car. After 1950, the levels of aerodynamic drag went up

because cars were becoming more family friendly and so as a consequence the

shapes available to choose were more limited and so it was not possible to keep the

low level of aerodynamic drag. The rectangular shape made cars more purposeful for

the family and so it is fair to say that after 1950 the designing of cars was to aid the

lifestyle of larger families.

Although this was a good thing for families, it didnt take long before the issue of

aerodynamics came back into the picture in the form of fuel economy. During the

1970s there was a fuel crisis and so the demand for more economical cars became

greater, which led to changes in car aerodynamics. During the 1970s there was a fuel

crisis and so the demand for more economical cars became greater, which led

to changes in car aerodynamics. If a car has poor aerodynamics then the engine has

to do more work to go the same distance as a car with better aerodynamics, so if theengine is working harder it is going to need more fuel to allow the engine to do the

work, and therefore the car with the better aerodynamics uses less fuel than the

other car. This quickly led to a public demand for cars with a lower aerodynamic drag

in order to be more economical for the family.

Only about 15% of the energy from the fuel you put in your tank gets used to move

your car down the road or run useful accessories, such as air conditioning. The rest of


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the energy is lost to engine and driveline inefficiencies and idling. Therefore,

the potential to improve fuel efficiency with advanced technologies is enormous.

Now a days almost all cars are manufactured aerodynamically , one misconceptionthat everyone has is aerodynamics is all about going faster, in a way it is true but it is

not all about speed, by designing the car aerodynamically we can reduce the

friction that it encounters and there by power needed to overcome would be less thus

fuel can be saved; In the modern era where our fuel resources are fast depleting all

the efforts are to find alternate sources of energy or to save our current resources or

minimize the use of current resources like fuels, so nowadays aerodynamics are

given very much importance as everyone like to have a good looking, stylish and fuel

efficient car.


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STUDY OF AERODYNAMICS OF CARS

In order to improve the aerodynamics we must first know how the flow of

air past a car, if we visualize a car moving through the air. As we all know, it takes

some energy to move the car through the air, and this energy is used to overcome a

force called Drag.

DRAG

A simple definition of aerodynamics is the study of the flow of air around

and through a vehicle, primarily if it is in motion. To understand this flow, you can

visualize a car moving through the air. As we all know, it takes some energy to move

the car through the air, and this energy is used to overcome a force called Drag.

Drag, in vehicle aerodynamics, is comprised primarily of two forces. Frontal

pressure and rear vaccum.

DRAG FORCE AT LOW SPEEDS

The total drag force decreases, meaning that a car with a low drag force

will be able to accelerate and travel faster than one with a high drag force. This

means a smaller engine is required to drive such a car, which means less

consumption of fuel.

CARWEIGHT

As with the parts inside the engine, when the entire car is made lighter, through the use

of lighter materials or better designs, less force is required to move the car. This is

based on F=MA or more accurately, A=F/M, so as mass of the car decreases, the

acceleration increases, or less force is required to accelerate the lighter car.


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FRONT END

Frontal pressure is caused by the air attempting to flow around the front of the car. As

millions of air molecules approach the front grill of the car, they begin to compress, and

in doing so raise the air pressure in front of the car. At the same time, the air molecules

traveling along the sides of the car are at atmospheric pressure, a lower pressure

compared to the molecules at the front of the car. The compressed molecules of air

naturally seek a way out of the high pressure zone in front of the car, and they find it

around the sides, top and bottom of the car. Improvements at the front can be made by

ensuring the front end is made as a smooth, continuous curve originating from the line

of the frontbumper. Making the screen more raked (i.e. not as upright) tends to

reduce the pressure at the base of the screen, and to lower the drag. However, much of

this improvement arrives because a more sloped screen means a softer angle at the topwhere it meets the roof, keeping flow attached. Similar results can be achieved through

a suitably curved roof.

REAREND

Rear vacuum (a non-technical term, but very descriptive) is caused by the

"hole" left in the air as the car passes through it. To visualize this, imagine a bus

driving down a road. The blocky shape of the bus punches a big hole in the air, with

the air rushing around the body, as mentioned above. At speeds above a crawl, the

space directly behind the bus is "empty" or like a vacuum. This empty area is a result

of the air molecules not being able to fill the hole as quickly as the bus can make it.

The air molecules attempt to fill into this area, but the bus is always one step ahead,

and as a result, a continuous vacuum sucks in the opposite direction of the bus. This

inability to fill the hole left by the bus is technically called Flow detachment .At the

rear of vehicles, the ideal format is a long and gradual slope. As this is not practical, it

has been found that raising and/or lengthening the boot generally reduce the drag. In

plan view, rounding corners and all forward facing elements will reduce drag.

Increases in curvature of the entire vehicle in plan will usually decrease drag provided

that frontal area is not increased. Tapering the rear in plan view, usually from the rear

wheel arch backwards, can produce a significant reduction in drag. Under the vehicle,

a smooth surface is desirable as it can reduce both vehicle drag and surface friction


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drag. Fora body in moderate proximity to the ground, the ideal shape would have

some curvature on the underside.

Flow detachment applies only to the "rear vacuum" portion of the drag equation, and it

is really about giving the air molecules time to follow the contours of a car's bodywork,and to fill the hole left by the vehicle, The reason keeping flow attachment is so

important is that the force created by the vacuum far exceeds that created by frontal

pressure, and this can be attributed to the Turbulence created by the detachment.

LIFT ORDOWNFORCE

One term very often heard in race car circles is down force. Down force is

the same as the lift experienced by airplane wings, only it acts to press down, instead

of lifting up. Every object traveling through air creates either a lifting or down force

situation. Race cars, of course use things like inverted wings to force the car

down onto the track, increasing traction. The average street car however tends to

create lift. This is because the car body shape itself generates a low pressure area

above itself.

For a given volume of air, the higher the speed the air molecules are traveling, the

lower the pressure becomes. Likewise, for a given volume of air, the lower the speed

of the air molecules, the higher the pressure becomes. This of course only applies to

air in motion across a still body, or to a vehicle in motion, moving through still air.

When we discussed Frontal Pressure, above that the air pressure was high as the air

rammed into the front grill of the car. What is really happening is that the air slows

down as it approaches the front of the car, and as a result more molecules are packed

into a smaller space. Once the air stagnates at the point in front of the car, it seeks a

lower pressure area, such as the sides, top and bottom of the car.

Now, as the air flows over the hood of the car, it's loses pressure, but when it reachesthe windscreen, it again comes up against a barrier, and briefly reaches a higher

pressure. The lower pressure area above the hood of the car creates a small lifting

force that acts upon the area of the hood (Sort of like trying to suck the hood off the

car). The higher pressure area in front of the windscreen creates a small (or not so

small) down force. This is akin to pressing down on the windshield.

Where most road cars get into trouble is the fact that there is a large surface area on

top of the car's roof. As the higher pressure air in front of the wind screen travels over


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the windscreen, it accelerates, causing the pressure to drop. This lower pressure

literally lifts on the car's roof as the air passes over it. Worse still, once the air makes

its way to the rear window, the notch created by the window dropping down to the

trunk leaves a vacuum or low pressure space that the air is not able to fill properly.

The flow is said to detach and the resulting lower pressure creates lift that then acts

upon the surface area of the trunk.

Not to be forgotten, the underside of the car is also responsible for creating lift or

down force. If a car's front end is lower than the rear end, then the widening gap

between the underside and the road creates a vacuum or low pressure area, and

therefore "suction" that equates to down force. The lower front of the car effectively

restricts the air flow under the car. So, as you can see, the airflow over a car is filled

with high and low pressure areas, the sum of which indicates that the car body either

naturally creates lift or down force.


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WINGS & SPOILERS

The Wings or spoilers prevent the separation of flow and thereby preventing the

formation of vortices or helps to fill the vacuum in the rear end more effectively

thus reducing drag. The wing works by differentiating pressure on the top and

bottom surface of the wing. As mentioned previously, the higher the speed of a

given volume of air, the lower the pressure of that air, and vice-versa. What a wing

does is make the air passing under it travel a larger distance than the air passing

over it (in race car applications). Because air molecules approaching the leading

edge of the wing are forced to separate, some going over the top of the wing, and

some going under the bottom, they are forced to travel differing distances in order

to "Meet up" again at the trailing edge of the wing. This is part of Bernoulli's theory.

What happens is that the lower pressure area under the wing allows the higher

pressure area above the wing to "push" down on the wing, and hence the car it's

mounted to.

The way a real, shaped wing works is essentially the same as an airplane wing, but

it's inverted. An airplane wing produces lift, a car wing produces negative lift or in

other words what we call us, down force. That lift is generated by a difference in

pressure on both sides of the wing. .

But how is the difference in pressure generated? Well, if you look closely at the

drawings, you'll see that the upper side of the wing is relatively straight, but the

bottom side is curved. This means that the air that goes above the wing travels a

relatively straight path, which is short. The air under the wing has to follow the curve,

and hence travel a greater distance. Now there's Bernoulli's law, which basically states

that the total amount of energy in a volume of fluid has to remain constant. (Unless

you heat it or expose an enclosed volume of it to some form of mechanical work) If

you assume the air doesn't move up and down too much, it boils down to this: if air (or

any fluid, for that matter) speeds up, its pressure drops. From an energetic point of

view, this makes sense: if more energy is needed to maintain the speed of the

particles, there's less energy left do work by applying pressure to the surfaces.


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Nowadays the aerodynamic studies are not constrained to the flow of air past

cars but also a number of other factors like new methods are developed to provide a

greater level of detailed information. Special pressure sensitive paint is now used in

the wind tunnel to graphically show levels of air pressure on a vehicle how it is done is

that ,Two different images are obtained, one at normal room air pressure (wind-off)

and a second in which the wind tunnel is running (wind-on) at a desired test speed.

These differences in color, from wind-off to wind-on, are used to calculate surface

pressure.

A bank of blue lights illuminates the car to be tested that has pressure-

sensitive paint applied on the driver's side window. The car and lights are in a wind

tunnel at Ford Motor Company's Dearborn Proving Ground. Ford researchers have

developed a computerized, pressure-sensitive paint technique that measures

airflow over cars, shaving weeks off current testing methods. A digital camera near

the blue lights captures this information and feeds it into a computer, which displays

the varying pressure as dramatically different colors on a monitor.

The images obtained from tests in the wind tunnel are captured on computer.

They can then be used to study air flow patterns across a vehicle, highlighting areas of

possible refinement or improvement. Additionally, actual data from a production ready

model can be compared with pre-production computer predictions which can in turn

help improve the accuracy of the early design stages.

SOFTWARES

Nowadays a large number of softwares are developed for the analysis and optimization of

aerodynamics in automobiles. Earlier times the cars were worked directly on wind tunnels

where they prepared different shapes or cross sections and tested upon the cars, during

those times it was not possible to test the for small areas that is for a small part of front area

etc there testing were made for the entire cross sections, But with the introduction of

computational fluid dynamics i.e. the use of computers to analyze fluid flows where the

entire area is divided into grids and each grid is analyzed and suitable algorithms are

developed to solve the equations of motion. Based on CFD large number of softwares are

developed for the design and analyzing aerodynamics the most commonly used softwares

are ANSYS, CATIA.


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ALIAS SURFACE AND AUTO STUDIO

Alias Surface Studio is a technical surfacing product designed for the developmentsurfaces. It offers advanced modeling and reverse engineering tools, real-time

diagnostics and scan data processing technology. Surface Studio is comprised of a

complete suite of tools for creating surface models to meet the high levels of quality,

accuracy and precision required in automotive styling.

This software performs all the basics of design right from the sketching to

evaluation.

Features:

1)User InteractionA user interface that enables creativity and efficiency

2) SketchingA complete set of tools for 2D design work tightly integrated into a 3D modeling

Environment.

3)2D / 3D IntegrationTake advantage of your sketching skills throughout the design process. Add details

and explore ideas quickly by sketching over 3D forms before taking the time to model

them.

4) Modeling

Industry-leading, NURBS-based surface modeler.

5) Advanced Automotive Surfacing Tools

Surface creation tools that maintain positional, tangent or curvature continuity

between surfaces - for high quality, manufacturability results.

6) Reverse Engineering

Tools for importing and configuring cloud data sets from scanners for visualizing, as

well as extracting feature lines and building surfaces based on cloud data.


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AERODYNAMIC DESIGN TIPS

Keep the vehicle low to the ground, with a low nose, and pay attention tothe angle of wind shield.

Cover the wheel wells, Open wheels create a great deal of drag and air flowturbulence

Enclose the under carriage (avoid open areas-convertibles, etc.) Make corners round instead of sharp The underbody should be as smooth and continuous as possible, and should

sweep out slightly at rear.

There should be no sharp angles (except where it is necessary to avoidcrosswind instability ).

The front end should start at a low stagnation line, and curve up in acontinuous line.

The front screen should be raked as much as is practical. All body panels should have a minimal gap. Glazing should be flush with the surface as much as possible. All details such as door handles should be smoothly integrated within the

contours.

Minor items such as wheel trims and wing mirrors should be optimized usingwind tunnel testing.

Using spoilers or wings.


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CONCLUSION

Earlier cars were poorly designed with heavy engines, protruding parts and rectangular

Shapes due to which they consumed large quantities of fuel became unaffordable all

these factors lead to the development and need of aerodynamics in the design of cars

now it would be fair to say that all most all cars are tested for getting the optimum

aerodynamic configuration.

REFERENCES

BOOKS1) Road Vehicle Aerodynamic Design, Barnard R.H.

2) Introduction to Aerodynamics by Anderson.

WEBSITES1) www.wikipedia.com

2) www.cardesignonline.com

Documents

Aerodynamics of Car