On Wed, 28 Jan 2004 14:12:44 UTC, "W.J. \(Bill\) Dean \(U.K.\)."
wrote:

: I don't understand Reynolds numbers

They are simply a way of comparing flow patterns round gliders in
different situations. If two gliders have the same Reynolds number
then the flow round them will look the same - the stream lines are the
same shape - regardless of the velocity. It's given by density *
velocity * characteristic length / viscosity. So if you go to a high
altitude (- low density) you need a higher speed for the same flow
pattern [1]. Or if you make a 50% scale model (- smaller length) you
also need a higher speed.

But beware - Reynolds numbers can be used to compare smaller parts of
aerodyamics as well. For example, separation of a boundary layer
typically occurs at a particular Reynolds number based on distance
from the leading edge of the wing, frontal drag from the fuselage will
depend on the Reynolds number based on the mean diameter of the
fuselage and so on. This is one of the main reasons people find
Reynolds numbers confusing ...

Ian, with his Fluid Dynamics Lecturer's hat on.

[1] Yes, I know viscosity must change with pressure and density, but I
can't offhand remember how!
--