Hi Rob,
Paul Lamar here.
Here is some info I promised on the proper shape
for rad ducts. Unfortunately some one made a PDF
out of sideways scans so it is a bit hard to read.
Kuchemann & Weber wrote the bible on this subject way
back in 1955. The Aerodynamics Of Propulsion book is
available on the internet but it is real expensive.

This is the most important chapter.

http://www.rotaryeng.net/K_M_CHAPT12.PDF

There is a lot more on my web site and also in my
book How to Cool Your Wankel.

If you do any CFD's on this I would love to see them
as I don't have open foam working plus I would have to get up to
speed on CFD.

Paul Lamar