Introduction to Aerodynamics Let’s keep it simple, shall we? Why is it important and how does it...

Introduction to AerodynamicsLet’s keep it simple, shall we?

Why is it important and how does it affect my car?

Greenpower schools education eventCastle Bromwich Manufacturing Facility1st May 2013

Edward Sorby

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Contents

• What is aerodynamics?

• So where do the forces come from?

• Why is aerodynamics so important in the design of a Greenpower racer?

• How can I make my car faster?!

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Contents





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Force on air Reaction on wall

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WHAT IS AERODYNAMICS?

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What is aerodynamics?

• Definition:> “The study of the properties of moving air and the interaction between the air and solid

bodies moving through it”

• But what does that really mean?!


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What is aerodynamics?

• Definition:> “The study of the properties of moving air and the interaction between the air and solid

bodies moving through it”

• But what does that really mean?!


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WHY IS AERODYNAMICS SO IMPORTANT IN THE DESIGN OF A GREENPOWER RACER?

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Why is aerodynamics so important in the design of a Greenpower racer?

• In a race you have to work against 3 sources of resistance :

> Aerodynamics> We will discuss where these forces come from> Approximately 40% of the total

> Rolling Resistance> Primary cause is the rubber tyre compound deforming at the contact patch (with

ground). Some of the energy required to deform the tyre is lost as heat; a process called hysteresis. The process is continuous as the tyre rolls.

> A small component of rolling resistance is from bearings and other mechanical parts.> Approximately 30% of the total

> Mass> Uses energy to accelerate the car, or go up hills. > Rolling resistance goes up with mass as deformation of tyre at contact patch increases> Approximately 30% of the total

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• To go faster, we must make each of these 3 things better

• We can do this by being clever when we design the car> This is free, and just requires some creative thinking> This is the fun way to do it!

• We can also go to the shop and buy better parts> This can get expensive> Also is time consuming, requires lots of testing and is very boring!

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• We can also go to the shop and buy better parts> This can get expensive> Also is time consuming, requires lots of testing and is very boring!

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• We can also go to the shop and buy better parts> This can get expensive…> …and is very boring!

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• Improving mass

• To make the car lighter, we can go from something like this> Heavy, using cheap materials and designed to be

stronger than it needs to be!

http://upload.wikimedia.org/wikipedia/commons/b/ba/1965_Turin_Salon_Miura_Chassis_-1.jpg

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• Improving mass

• To make the car lighter, we can go from something like this> Heavy, using cheap materials and designed to be

stronger than it needs to be!

• To something like this> Light, expensive

materials, welldesigned

• To make you car light and safe needs a lot of engineering know-how, exotic materials, or both!

http://upload.wikimedia.org/wikipedia/commons/b/ba/1965_Turin_Salon_Miura_Chassis_-1.jpg

http://www.zolfe.com/Images/Zolfe_chassis.gif

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• Improving Rolling Resistance

• Friction is mostly due to the components you use…> Tyres> Bearings> Chain> Gears

• Improving it requires you to buy better components!

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• Improving aerodynamics

• We want to go from this

http://cdn.speedhunters.com/wp-content/uploads/2012/05/desktop1.jpg

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• Improving aerodynamics

• We want to go from this to this

• Within the rules of Greenpower, improving aerodynamics is down to your design

• You can improve this just by being creative and clever when you design you car!

• It is therefore the cheapest and easiest way to make your car faster!

http://cdn.speedhunters.com/wp-content/uploads/2012/05/desktop1.jpg

http://www.jaguar.com/content/global/Assets_12MY/Nameplate/xf/Exterior/LHD_EU_ROW/A6_LHD_XF_12MY_012_046_Polaris_White-1008x567.jpg

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SO WHERE DO THE FORCES COME FROM?

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So where do the forces come from?

• There are 2 main reasons why the air creates forces:

> Friction drag> Most important for very aerodynamic shapes like aeroplane wings

> Pressure drag> Most important on blunt shapes like cars

High pressureLow pressure

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> Friction drag> Most important for very aerodynamic shapes like aeroplane wings



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> Skin friction drag> Most important for very aerodynamic shapes like aeroplane wings



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So where do the forces come from?A simple explanation…

• So what is pressure?> The force created when air particles bounce off things

• More air particles?> Higher pressure > bigger force

• Fewer air particles?> Lower pressure > less force


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• More air particles?> Higher pressure > bigger force



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• More air particles?> Higher pressure…bigger force


Force on air Reaction on wallHigh pressure

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• More air particles?> Higher pressure…bigger force

• Fewer air particles?> Lower pressure…less force

Force on air Reaction on wallLow pressure

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• But how does a moving car change the air pressure?

Car

Air

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Air

Car

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Car

Air

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Car

Air


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So where do the forces come from?A complicated explanation…

• For the older amongst you, we’ll take a look at the Bernoulli principle and turbulence

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• Bernoulli principle> The theory explaining why aeroplanes fly

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• Bernoulli principle> The theory explaining why aeroplanes fly

• Turbulence> A method through which energy and pressure is lost

http://www.efluids.com/efluids/gallery/gallery_images/wake.jpg

http://www.hiflykites.co.za/kite-online-shop/x-zylo-flying-principles-3.gif

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So where do the forces come from?Bernoulli principle…

• If air moves faster, its pressure will decrease.

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• The distance between air particles is bigger on the top surface than the bottom!

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High pressure

Low pressure


• If air moves slower, its pressure will increase.

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• Look at the image to the right…

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• Imagine you can see the air as you move with the car

• Air in front of the car is slowed down

• It’s pressure therefore increases

> This pushes the car back and tries to slow you down!

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• But what about behind the car?

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• “The air behind the car is moving slowly too, doesn’t that push the car forward?!”

• Ah, now that’s where turbulence comes in!

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So where do the forces come from?Turbulence…

• Blunt bodies like cars will generally have a turbulent area in the air behind them, but what is turbulence?

http://www.efluids.com/efluids/gallery/gallery_images/wake.jpg

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• In certain situations, air flow will become very turbulent

• This means it is chaotic and difficult to predict

• The physics involved means energy and pressure is lost

http://www.cvphysiology.com/Hemodynamics/H007%20turbulent%20vs%20laminar%20flow.gif

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• An example of turbulence…aeroplanes!

• Ideal flight:

• As explained before, the air moves faster over than under the wing, creating an upwards force called lift.

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• An example of turbulence…aeroplanes!

• Flying through turbulence:

• When flying through turbulence, the chaotic movement of the air prevents the difference in pressure from happening> This means the pressure above and below the wing is almost the same, so there’s no lift!

• The bouncing you feel when you fly in turbulence is due to the aeroplane losing lift, and then getting it back again!


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• Turbulence can be created a number of ways, the most important to us are:

> Moving over fast changes in shape> eg if the back of your car has a sharp corner

> Air of different speeds meeting each other and mixing> eg fast air from the top of your car meeting slow air from the bottom

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Fast air

Slow air

Turbulent mixture

• Turbulence can be created a number of ways, the most important to us are:

> Moving over fast changes in shape> eg if the back of your car has a sharp corner

> Air of different speeds meeting each other and mixing> eg fast air from the top of your car meeting slow air from the bottom

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• The Bernoulli principle and turbulence combine to create a drag force on your car slowing it down!

• Now I’ll show you some easy tricks to minimise these effects

High pressure Low pressure

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HOW CAN I MAKE MY CAR FASTER?!

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How can I make my car faster?!

• It’s simple, minimise the pressure change at the front and back of your car!> But how?

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• It’s simple, minimise the pressure change at the front and back of your car!> But how?

• Imagine we start with a very bad car

Track

High pressure Low pressureHigh pressure Low pressure

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• Begin by rounding the front end to prevent pressure building up at the front

Track

High pressure Low pressureHigh pressure Low pressure

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• But air flowing under the car also slows us down because it’s close to the track> So we lower the tip of the nose to make more air go over the top

Track

High pressure

Low pressure High pressureLow pressure

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• Taper the rear of the car to increase the pressure there, and reduce that pesky turbulence!

Track


Low pressureHigh pressure

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• Finally, make your car as small as possible, so there’s less of it for the air to push on!



Track

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• The difference can be very big…

Before…………High pressure Low pressure

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• The difference can be very big…

Before………… …………After



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• Differences made to our car this year…

Before………… …………After

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ANY QUESTIONS?

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YOUR TURN!

Introduction to Aerodynamics Let’s keep it simple, shall we? Why is it important and how does it...

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