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

To All:

I've always appreciated my father's explanation, which some of you may
find suitable if you have a nine-year old who is just getting into
free-flight.

Model competitions were quite popular up until WWII but never regained
their past glory after. My dad and several of his friends were
discussing a new wing for an existing fuselage and Reynold's number
was mentioned several times. When I asked what it was his friends
started to grin; a couple even laughed but he told me there were
somethings that we couldn't scale, such as the size of a molecule of
air. so a fellow named Reynold came up with a way of calculating a
number that could be used as a kind of artifical scaling factor.

-R.S.Hoover

(Do they still use Banana Oil?)

First, thanks to Oliver for a much better explanation.


It's not about the size of a molecule, but how they interact
with each other and surfaces at different energy levels.



These links are a few animations and film clips that depict air flow
and increasingly higher Reynolds numbers.



This first one is at VERY LOW Reynolds and displays a nearly Newtonian
reaction. I suspect that if people think about air flow, this is how
they would expect it to behave.
http://www.youtube.com/watch?v=rbMx2...eature=related



But kick the velocity up to flying speeds and see what actually happens...
http://www.youtube.com/watch?v=A4taH...eature=related
http://www.youtube.com/watch?v=vQHXI...eature=related



Transonic range...
http://www.youtube.com/watch?v=ZMEQJhiebu4
And a standing shock wave on a sub sonic airliner wing!
http://www.youtube.com/watch?v=duCHF...eature=related



Actual photographs of supersonic shock waves (an interesting film
about supersonic flight too)
http://www.youtube.com/watch?v=atItR...eature=related



So, what about an actual airfoil?
Department of Engineering, University of Cambridge, multimedia video from
Physics Education, 2003, by Holger Babinsky.
The smoke tunnel really lays it out clearly.
And the pulsed smoke flow can start a lot of arguments!
http://www.youtube.com/watch?v=6UlsA...eature=related



This one I like because it shows a "long bubble" developing on the top
surface of the wing.
It's not what they were after, but it clearly shows the bubble development.
http://www.youtube.com/watch?v=J-xxC...eature=related