Here is another old message on water verses air.

Howard Wilkinson wrote:

Paul:
    Using water tunnel observations to predict air flow is a highly
questionable practice in my mind at least.  While air and water are both
fluids, one is compressible, and the other is not.  This compressability
results in considerably different behaviour between the two fluids in many
cases.  I suspect that this is one of them.  As the air enters the chamber
of increased area, it expands, pressure drops and velocity drops.  The
eddies shown in the illustration could well not exist at all in air moving
through this same structure.

         H.W.

Jeff Spitzer wrote:

For the purposes of subsonic aerodynamics, air is considered
incompressable.  The viscosity and density differences are more significant
than compressability.  Results with water are quite representative for
qualitative flow issues like separation patterns.

Jeff Spitzer

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