Car aerodynamics

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When air flows over an airfoil, the air flows at the top flow faster than the bottom and hence creates a net pressure difference, and thus an upward force is applied on the wing. Thus the airplane lift from the ground and keep flying in the air.

Now, imagine the side view of a car. Car shape, in my opinion, looks very much like an inefficient airfoil. It is flat at the bottom and has overall "round" shape at the upper. So, why not when the car is moving on the ground and hence the air flows through it, the same effect on airfoil happens on the car, and thus give rise to a lifting force (the force might not sufficient to lift it off the ground)?
Is it because the car is too close to a solid ground but airplane's wing is in the middle of sky so the "ground effect" makes the car doesn't experience what the airfoil experiences?

Looking forward to any inspiring thought and explanation.
 

boneh3ad

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First, a car doesn't have a sharp trailing edge, so it doesn't force there to be a strong circulation, a feature necessary for the functionality of an airfoil.

Some cars do experience lift though. For example, Indy cars and F1 cars require those huge wings to create downforce to make sure they stay on the road as well as provide better traction. Without those, the cars tend to lift off the ground.
 
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First, a car doesn't have a sharp trailing edge, so it doesn't force there to be a strong circulation, a feature necessary for the functionality of an airfoil.

Some cars do experience lift though. For example, Indy cars and F1 cars require those huge wings to create downforce to make sure they stay on the road as well as provide better traction. Without those, the cars tend to lift off the ground.
So are you mean in order to make an ordinary street car experience lift force by its own contour, its tail must be a slender sharp shape kind of contour?

What about the following car designs? They look streamline to me. Will they experience lift?

http://www.1motormart.com/glosimgs/sedanet.jpg

[URL]http://www.allsportauto.com/photoautre5/voisin/c28/1936_avion_voisin_C28_aerosport_07_m.jpg[/URL]

[PLAIN]http://www.carpages.co.uk/volkswagen/volkswagen_images/volkswagen-passat-cc-14-01-08.jpg [Broken]

46179885_3532c4ec20.jpg


[URL]http://www.exelerocar.com/images/exelero/exelero-15.jpg[/URL]

What about those solar cell competition car? They look very much like an airfoil. Do you think they will experience lift force?

[URL]http://www.technology.am/wp-content/uploads/2009/04/cambridge-university-solar-car.jpg[/URL]

[PLAIN]http://www.solarpowerathome.com/image-files/solar-power-car.jpg [Broken]

http://dvice.com/pics/solar_powered_racing_honda_car.jpg

According to you, may I say if the F1 car was to have their front and rear wing removed, it will lift off from the ground once it reaches certain speed? (of course it will fall down again)
 
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boneh3ad

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There is a difference between streamlined and a lifting body. A car likely does experience some degree of lifting force, but it is not designed as an airfoil, so it isn't going to be very good at it, and they weigh so much it won't matter. Compare it with a wing where the wings are made of aluminum and are as light as possible.
 
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There is a difference between streamlined and a lifting body. A car likely does experience some degree of lifting force, but it is not designed as an airfoil, so it isn't going to be very good at it, and they weigh so much it won't matter. Compare it with a wing where the wings are made of aluminum and are as light as possible.
I agree that car has some degree of insignificant lifting force when moving in the air.
Thank you for your explanation.
 
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It's really glad to see your interests in aerodynamics of cars. Actually, the car designers mainly focused their body to create necessary downward force to make cars stable and
help improve turning performance. Note: Cars body are not designed for the lift but for down force. And the other major concerns for car designer is to decide how their windshield shape should be. This help the driver inside not to be so much exposed to the windy conditions as the speed goes on. Don't say car designers don't have aerodynamic sense: every successful car company has wind tunnel testing facility.

Best Wishes!
 
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My understanding is that yes, if you just slap together a car without giving it much thought, it does tend to produce lift. When you start getting into high speeds you do need to keep this in mind when designing a car. The lift is likely not going to be even close to lifting the car off the ground (unless you start getting into 200mph+ speeds), but any lift is detrimental to a car's traction, performance, and stability...which coincidentally you need more of the faster you go.

So yes, sports cars makers DO look into to this phenomenon and go out of the way to try to at least reduce it (and of course, in some cases, even reverse it).
 

minger

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To clarify what boneh3ad mentioned, there is something called the Kutta condition which essentially sets the stagnation point at the trailing edge of an airfoil. It says that there must be a circulation strong enough to maintain the stagnation point at the trailing edge.

For blunt bodies, the stagnation point will not necessarily be the furthest aft point, and the flow will recirculate back up the blunt edge if allowed.

Now, the cars you showed often times have very little in common. As a CFD guy, I know a CFD guy who works in the automotive industry. While they do tweak, much of the car's design comes from a guy with a drawing tablet and a huge monitor. They can tweak certain things to avoid NVH issues, but much of what we see in cars is aesthetic. Yes, they wind tunnel test. However, you'll find much of the testing is again, comfort based. Nearly all the testing done is for vibratory issues, noise generated from side mirrors at certain speeds, moon roof cavity modes, etc etc.

Now, you'll notice that our high-mileage (e.g. Prius) cars look much different than a typical sedan. The goal behind this is to produce a low drag vehicle, while maintaining some usability and eye appeal. So, on many of these high MPG cars, you'll see a nice slowly sloping profile with often times a Kammback styled rear.

Now, F1 cars are completely different; they often times have plenty of power, so drag isn't as much as an issue as control and stability is. If you google/wiki it, you'll see that F1 cars have Cd values many times higher than production sedans. In these cases producing downforce comes at a cost of drag (when a fighter jet is in a high angle of attack, or high lift situation the drag on the plane can go way up).
 
How much would drag and lift change if the underside of the car was smooth ? Because if dimples can affect a golf ball's aerodynamics, I assume exhaust pipes, mufflers, and transmissions would affect a car's.
 
70
0
It's really glad to see your interests in aerodynamics of cars. Actually, the car designers mainly focused their body to create necessary downward force to make cars stable and
help improve turning performance. Note: Cars body are not designed for the lift but for down force. And the other major concerns for car designer is to decide how their windshield shape should be. This help the driver inside not to be so much exposed to the windy conditions as the speed goes on. Don't say car designers don't have aerodynamic sense: every successful car company has wind tunnel testing facility.

Best Wishes!
Actually the question has puzzled me since I was a kid in school when my science teacher first showed me how airplane flies. The first question I immediately appear in my mind was why a car does not fly like airfoil does, since the car looks just like an ugly airfoil (flat at the bottom, some curvy contour at the top). Unfortunately at that time none of the teacher able to give a satisfactory explanation.
 
70
0
My understanding is that yes, if you just slap together a car without giving it much thought, it does tend to produce lift. When you start getting into high speeds you do need to keep this in mind when designing a car. The lift is likely not going to be even close to lifting the car off the ground (unless you start getting into 200mph+ speeds), but any lift is detrimental to a car's traction, performance, and stability...which coincidentally you need more of the faster you go.

So yes, sports cars makers DO look into to this phenomenon and go out of the way to try to at least reduce it (and of course, in some cases, even reverse it).
Indeed, my intuition also feel that car, although not all, some of them do produce lift, even though just very little. But I was unable to verify this thought with scientific argument. That's why I asked the question at here.

I remember I once watched a youtube clip before, where a sport car is racing, going straight uphill (not very oblique though), then all of a sudden, the car just lift up! And roll in the sky before smashing on the ground. I am not sure what had happened but the car just act like an airplane for that split second. That car is looks like this:
[PLAIN]http://blogs.technet.com/photos/robert_hensings_photos/images/866708/original.aspx [Broken]
 
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70
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To clarify what boneh3ad mentioned, there is something called the Kutta condition which essentially sets the stagnation point at the trailing edge of an airfoil. It says that there must be a circulation strong enough to maintain the stagnation point at the trailing edge.

For blunt bodies, the stagnation point will not necessarily be the furthest aft point, and the flow will recirculate back up the blunt edge if allowed.

Now, the cars you showed often times have very little in common. As a CFD guy, I know a CFD guy who works in the automotive industry. While they do tweak, much of the car's design comes from a guy with a drawing tablet and a huge monitor. They can tweak certain things to avoid NVH issues, but much of what we see in cars is aesthetic. Yes, they wind tunnel test. However, you'll find much of the testing is again, comfort based. Nearly all the testing done is for vibratory issues, noise generated from side mirrors at certain speeds, moon roof cavity modes, etc etc.

Now, you'll notice that our high-mileage (e.g. Prius) cars look much different than a typical sedan. The goal behind this is to produce a low drag vehicle, while maintaining some usability and eye appeal. So, on many of these high MPG cars, you'll see a nice slowly sloping profile with often times a Kammback styled rear.

Now, F1 cars are completely different; they often times have plenty of power, so drag isn't as much as an issue as control and stability is. If you google/wiki it, you'll see that F1 cars have Cd values many times higher than production sedans. In these cases producing downforce comes at a cost of drag (when a fighter jet is in a high angle of attack, or high lift situation the drag on the plane can go way up).
Does your friend find out any lift component in his CFD on car?
Why the race car does not simply adopt the reverse airfoil design for entire car? It will pose high down force, and low drag, isn't that is what the race guy looking for? Although from aesthetic point of view, it is indeed a very ugly looking car. But we are talking about race here. Performance matters more.
 
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How much would drag and lift change if the underside of the car was smooth ? Because if dimples can affect a golf ball's aerodynamics, I assume exhaust pipes, mufflers, and transmissions would affect a car's.
Yah what you said make sense. I would imagine that the completely flat and smooth bottom car experience lesser drag compares to ordinary bottom car.
 

minger

Science Advisor
1,495
1
Actually the question has puzzled me since I was a kid in school when my science teacher first showed me how airplane flies. . .
I'm sorry, but 99% of high school teachers explain planes flying via the "equal transit" theory, which is completely incorrect.

How much would drag and lift change if the underside of the car was smooth ?
A lot actually. You'll notice high performance cars are designed such to maintain a smooth underbody. High velocity air is typically at lower pressures than slow moving air. So, any disturbances underneath the car will slow the air down, decrease velocity (increasing pressure), increasing lift and thus decreasing stability.

You really shouldn't get caught up too much on this "airfoil" shape thing. The whole high pressure, low pressure isn't where 90% of the airfoil's lift comes from. You can produce quite a bit of lift from a flat plate.
 

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