If you want a good explanation of secondary flow there is a cool video on youtube. It cuts off the more interesting parts that occur later, that you can find on MIT's OCW
It's possible to used vanned diffusers to reorganize the flow without introducing turbulence, which the video delves into later if you go beyond the clip on youtube. For cars the vortex generators are about closing the flow on the pressure differential down, not actually assisting the flow. That is the turbulent flow interrupts the pressure differences caused by the sudden termination of the body. This is done by adding a swirl component; this is abstracted by the Euler-N equation and others. The swirl causes a theta differential in the flow direction that disrupts laminar flow that causes the wake.
My graduate studies are focused on fluids, and more specifically turbomachinery right now. I fully intend to concentrate on heat transfer in the future, so this isn't quite my first love but I should be able to help.