Saw this on the EV digest ala google.
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Its refered to as the Magnus effect. You can buy dimpled tape and sheetswith this on it. Skin friction on autos has a small effect compared tofrontal area because of the speeds. The single largest drag item on a car isactually the road - under body interface. That's the highest pressure and normally byfar the dirtiest place aerodynamically on a vehicle.
About the only effects you could expect to see with the dimple tape on a caris having a nifty conversation starter and making the guy who sold you thestuff a wee bit richer. ;-]
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If they did work, you'd need
a lot of them. I remember seeing some lexus commercial about how dimpled underbody panels reduced drag, but I can't find anything on it.
Here's some more. VGs supposedly work because the drag they induce in order to fill in low pressure areas in the rear with air (and have the flow seperate farther down stream) is less than having the flow seperate immediately at the rear of the vehicle. Ideally uinderbody panels should smooth flow enough to where these aren't needed, of course the ground may not be as smooth as the underbody of your car, so who knows?
Another quote
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If your car had the same profile as a golf ball dimples would help, but cars don't. If the golf ball was smooth the air flow would separate at around 0 and 180 degrees and then cause "pressure drag" which is suction. The dimples disrupt the boudary layer, causing the flow separation to happen over a smaller area, thus reducing drag.
Laminar flow is a good thing, flow separation is a bad thing. Eventhough laminar flow is "sticky" the boundary layer acts like an air bearing, reducing overall drag. Zero flow separation would get you close to zero drag, but there will alway be some drag due to the air "sticking" to the surface. Well unless you are in a vacuum.
For a very nice, low drag, shape, look at a dolphin or a shark. They have it made, low drag in a fluid much more vicous than air (yes, air is a fluid aerodynamically speaking).
Anyway, that is the way I remember it from my sub-sonic aerodynamics class.
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