Quote:
Originally Posted by transio
What they're asking for is optimal laminar air flow detached as close to the elevation of the roofline as possible. Laminar air flow means less drag (turbulence). The TT is a great example of a low-drag car that is basically like one big laminar airflow magnet. The problem with the original design is that its egg shape never detaches the airflow, and therefore when the car curves back down, the air sticks (laminar = laminated) to the car and pulls it up... the car becomes a giant airfoil (wing). A tapered back end will allow the laminar flow to detach with minimal turbulence. You don't want to just cut off and have a flat back end. That will create a void below the car, and that high pressure above and to the sides will collapse inwards and create swirls of air behind it. That's turbulence, which will also slow your car down, because it sucks the car backwards. It's like trying to pull a capsule out of a vacuum tube... you can pull it 1" no problem. 2" a little harder, but the further you pull, the harder it gets. Same with the back end of a car and top speed.
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It's not that simple, actually. There are two factors involved: laminar vs. turbulent flow, and separate from that, attached vs. detached flow.
Attached laminar flow is the lowest drag, but is very difficult to achieve. Next lowest is attached turbulent flow, followed by the detached flows.
Turbulent flow has the advantage of being able to stay attached to surfaces where laminar flow would detach - like the roof of most notchback sedans. As such, it is often desirable to use it when laminar flow is not achievable.
For example, the vortex generators on the wing of a B757 create turbulent flow that can stay attached to the ailerons over their entire range of motion. This is sufficiently desirable that the designers of the aircraft were willing to suffer the slight drag penalty when the ailerons aren't deflected.
http://img.photobucket.com/albums/v5...p06/BHX020.jpg
This pic shows the vortex generators out there - they are the black "dots" on the outboard section of the wing.
Vortex generators are also used on the rear roofline of (some?) Evos.
The "vacuum" effect you describe (difference in pressure) is more an effect of detached flow, rather than of laminar vs. turbulent flow.
ed