I've had the grey matter looking at past attempts at indoor windsurfing and think they've been very wasteful. They just blow across the pool with no re-use of the breeze they generate. I started thinking of a rounded building, then thought why not wrap it around into a donut. The poloidal shaped wind current (new word for me too) would mean a 90 degree breeze at all times. Just imagine a permanent bear-away !!
Hopefully the picture explains it a bit better. Note that the fans create a current that goes from the inside to the outside of the donut and back, not around the path of the donut.
An inflatable type building made sense too, considering you already have fans.
Thanks in advance for any CONSTRUCTIVE feedback !!!

Jetlag