Truck aerodynamics: zero shear BC
 

Hi everyone,

I am trying to model a truck-trailer with a moving cylinder on the rear edge that would freely rotate and reduce skin friction and energise the boundary layer, reducing pressure drag. I have modeled a rectangular prism with a curved rear edge and set the BC for the curved rear edge to zero shear. Any ideas why this would be increasing the total drag?
I'm also trying to play around with a velocity equation at this surface but am not sure how to calculate it.

Many thanks :)