[pasadena_commut]

Covered rear wheels help with the airflow along the body. I wonder what they do to airflow within the wheel well.

The boundary conditions for the moving air give it zero velocity relative to the spinning wheel at the surface of the wheel, and zero velocity relative to the inside surface of the wheel well (including the added wheel cover). However, the outside edge of the tire is spinning at the vehicle's speed relative to the car, which makes me wonder how much new drag is introduced by the airflow shear between the fixed wheel cover and the surface of the spinning wheel. The distance from the inside of the wheel to the wheel well wall parallel to it tends to be at least 6-8". Presumably that is far enough that drag from shear is minimized. However, rear wheel covers in these forums appear to be significantly closer than that, some appear to be only an inch or so from the wheel. Are these covers so close that the air shear between the wheel and the wheel cover produces measurable drag?

It would be good to know what the minimum separation must be in order to avoid inducing this sort of drag.