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Simply put in aerodynamics the more turbulent displacement you create, the more energy you impart into the air stream. Some of it is the whirls, eddys and vortexes created and some of it is from air displacement. That air drug along with the car due to skin friction and form drag.
If you look at an airfoil shape you will see a smooth rounded curve that creates even acceleration of the air up to about the 1/3 chord point (1/3 from leading edge). Then since the air has been smoothly accelerated out of the way it will tend to fall evenly back into line as long as there is a smooth transition (not too sharp an angle) without too much turbulence, back into the hole the car just made in the air. The air can only fall back in so much before there is seperation of flow. The process is called by some the pressure recovery section of the airfoil and it is the most critical area in reducing drag. The kind of drag we are dealing with in our boat tails is mostly pressure recovery. Just look at the solar racers and you can see the clean flow lines.
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