I also can learn things and develop insights based on these little discussions.  I hadn't given much thought to the deflection of the air by propellers, fans, or airplane wings.  It just didn't seem to have any relevance to the behavior of wind mill blades.  Since you thought that it was important, I conceived of a specialized experiment consisting of a long tube shaped wind tunnel with the windmill occupying essentially the entire diameter of the tube.  This eliminates any vortices off of the tip.  There are two possible directions for the air flow behind the blades; straight back, or tangential to the side of the tube.  The latter flow will constitute a vortex and will quickly dissipate against the side of the tube.  The problem is, that it may not exist.  Just because something could be there doesn't mean it has to be there.  Nature has a way of confounding our expectations.  If it doesn't exist your theory, whatever its merits, will have no relevance to the behavior of a horizontal axis windmill.  If it does, the resulting expanding vortex will affect the placement of wind mills in a wind farm.  Perhaps your first proposal should deal with this issue.  Maybe I'll do it myself.  What is the name of the the foundation involved?  I could use a wind tunnel.

GeoM
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