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Originally Posted by Lil bastard
Well, yes and no. Laminar Flow of air does exist generally on several points on the top of a car. It is disrupted by such things as the windscreen and such, but often re-established on the rear deck.
As you know, laminar flow really gets the air moving and so it is at these points where the greatest low pressure araeas exist.
Generally (and I'm being general because not all cars are the same), laminar flow is re-established on the rear panel of the car (trunk area behind the windscreen) and so it is an area of particular low pressure. This is where the spoiler comes into it's own, breaking up this clean laminar flow and restoring high pressure to that area. It also has the by-product of moving the very turbulent, drag producing air which accumulates right at the rear bumper further back, beyond the car's bodywork.
In essence, I really think we're saying the same thing, but from a different perspective.
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Laminar is not really the word you're looking for. Laminar flow is a type of airflow where small eddy disturbances are non-existant. The bulk flow over a body can and will be both laminar and turbulent in different areas, but it's actually mostly turbulent in the case of a car. The terms you really want to use are attached flow and separated flow.
If you have a wing moving through air that forces the air to take a longer path above it than beneath, this will will produce lift (negative downforce), if you turn the wing up side down and move through the air at exactly the same speed, it will produce negative lift (downforce) in the same amount but in the opposite direction. The wing didn't change dimensionally, and neither did the force, just it's direction, which is the only difference between downforce and lift.
Now, depending on how fast that wing is moving, the air flowing over it will either be laminar or mostly turbulent.