Check out google for "reverse NACA duct"
Bring back the NACA duct..... - PistonHeads
NACA can work both ways. I've seen NACA-like outlet working as an exhaust tip, but can't find a photo right now to link it. It was just build as reverse of NACA inlet. So it was wider at the very end of the outlet and it was tapering to the front and the actual exhaust pipe. I'm hoping you get the idea from my description.