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Originally Posted by hat_man
As the pressure travels rearwards along the teardrop (and comes back together) does it's cumulative effect "squeeze" the tail of the teardrop causing theoretical forward movement? Like pinching a watermelon seed between your thumb and forefinger and shooting it at your wife?
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No--that would be impossible, since you would be getting more energy out of the system than you put into it. But yes, after the flow goes over the top of the car, it accelerates over the shoulder (the high point of the roof or the windshield/roof junction, depending how the car was designed) thus lowering its pressure. If the body behind that point curves away gently, the pressure is gradually recovered as the flow decelerates back to the free stream velocity; if it drops too fast, the flow in the boundary layer actually starts to stall or reverse and it detaches from the surface. However, you can't recover more pressure in the rear than the body's movement generated at the stagnation point in the front. If you taper the tail all the way to a point, with attached flow all the way back and pressure drag = 0, you will still have drag from the increased friction drag. There's no free lunch.
You mentioned you have a copy of
Aerodynamics of Road Vehicles; you might find a lot of useful information reading Chapter 2 ("Some Fundamentals of Fluid Mechanics") all the way through.
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So another question.....in my mind I'm seeing a half teardrop with lines representing the (laminar?) flow along it's surface.
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ETA: Also, no--not laminar, and that's a good thing. The transition from laminar to turbulent flow is determined by the Reynolds number, which = (free stream velocity * length) / (kinematic viscosity of the fluid). At critical Re = 5 x 10^5 or thereabouts, the flow transitions from laminar to turbulent--and the drag decreases. Turbulent flow also will stay attached to surfaces that are too steep for laminar flow to stay attached. Turbulent flow has higher friction drag, but the potential lower pressure drag far outweighs that.