M840TR wrote: ↑
Sat Apr 14, 2018 8:03 pm
PhillipM wrote: ↑
Sat Apr 14, 2018 7:24 pm
In itself, it adds drag, but by cleaning up the boundary layer it can reduce drag downstream.
What do you mean by boundary layer?
In this context its the specific layer of air that is in contact with the car. As the air comes up over the nose/similar part of the car, the boundary layer can detach from the car. This in essence would leave what is essentially a very small vacuum / disturbed air and in the end adds to drag. The S-duct can help control this boundary layer a little better.
EDIT: It helps control the boundary layer because the air coming out of the S-duct is pretty much guaranteed to be laminar rather than turbulent. So they can basically "clean up" the boundary layer as Phillip quite rightly says.