Very nice read cannga, I always like technical articles that make hard stuff easy to understand, or at least give you the feeling that you understand it partially.

I'd like to make two additions:

In this picture:

 

You see the effect of the turbulence behind the rear window. This is because a low air pressur exists behind the rear window, and the airflow is curved toward that low air pressure. The same phenomenon also takes place behind the rear of the car. To me, it seems that the cougar and the Audi TT try to solve that problem by adding the tiny wing at the aft edge of the car: the little wing forces the air to stop trying to follow the shape of the car.

Additionally, it is mentioned that a venturi for the ground effect doesn't add to the drag... This is not entirely true: you create a vacuum (relatively low air pressure) at the aft underside of the car. As much as the car is "sucked down" by this vacuum, it is also being "sucked backward", because it increases the vacuum behind the car.

Often, aerodynamics may be illustrated by comparing it to hydrodynamics: Air tends to behave like a fluid in many cases. Picture a heavy, square boat travelling pretty fast. You'll see all those whirls and currents behind the hull, slowing the boat down dramatically. Same goes for cars with a very square/high rear shape.


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