reynolds number

Thanks for your post Oliver, you are of course absolutely correct in
everthing you've said. I was regretting my second post with the
'thought exercise' as I hit the button, the intent was to demonstrate
the relationship of Re to density, and I think I missed the mark.

My intent for posting was not to provide an accurate engineering
description of Reynolds number, but to help a non-engineer understand
the basic concept. As you've provided, Re = v * L / nu. If we
consider air viscocity / density / stickyness to be constant, then it
holds that an airfoil operating at half the speed and twice the length
of another will calculate out to the same Reynolds number, and both
will essentially behave the same.

I will submit for your consideration that giving a fully accurate and
complete engineering description of the effects of Re that is gold for
we engineers will do little or nothing to help the layman understand
the underlying concept. (I'm a mechanical engineer, but not an
aerodynamicist) I will contend that the terms kinematic and dynamic
viscosity, drag polar, laminar and turbulent flow, laminar bucket,
lift coefficient, while joyful for us engineers to bat around in
aircraft design, they are abolutely meaningless to the non-engineer.
(You did mean kinematic and not 'cinematic' didn't you? Sorry, just a
friendly dig there. ;^)

I'll try to restate: Reynolds number is essentially a count of how
much air is acting on a wing in a unit of time. If a given length of
airfoil traveling at a given speed calculates to a certain Re, then
the same airfoil shape in a smaller size will have to go faster to
have the same quantity of air working on it.

Would that statement pass the accuracy test for you?

Jan, is the concept starting to come together for you?

Best regards all
Gerry