I stumbled over this paper today
http://www.ansys.com/-/media/ansys/c...ormula-car.pdf
they do CFD on various ducting for the heat exchanger on an FSAE car. I think people will find it has some useful information. I had suspected that it was possible to basically stall the air in the duct to where it spills out around the duct opening instead of continuing to flow through and was pleased to see that confirmed in the paper.
cheers,
Michael