Bill Freeman wrote:

Paul,

The NACA duct is unlikely to work backwards if you think
how it works "frontwards".   The sharp, curved edge of
the duct causes the airflow to "fall" into the duct in a
non-uniform way, causing a vortex on each side which is
rotating to pull air into the duct.  Since the geometry is
designed to create a pair of trapped vortecies, I rather
doubt that it would be able to work in reverse, since there
would be no vortecies..

On the other hand, an normal ramp type of decreasing
depth exit may work to accelerate the air back to speed.

Probably still best to try to accelerate the air in the duct
before dumping, to minimize drag.

Bill

 
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