jan olieslagers wrote:
It's been puzzling me for a long while and there it is again popping up
in the "WIG airfoils" thread: what is this sacred Reynolds number?
I tried our alther friend en.wikipedia but its theory was quite beyond
my level of education* and its examples of oil in a pipe were not really
illuminating - not to mention the spermatozoa and the Major League
Baseball.

Is it a property of the wing, or of the whole plane, or do the fuselage
and wing and empennage &C each have their own Reynolds number?
I seem to understand this figure is a measure of aerodymanic quality?
Given a plane's weight and engine power, will it be faster (or slower)
for a higher Reynolds number?

Excuse my stupidity,
KA


*I am only a modest Solaris sysadmin, never went to university...

Detailed explanation at:
http://www.aerodrag.com/Articles/ReynoldsNumber.htm

Here's a simple example for a wing with a 10 feet chord at 100 mph
flying speed, at Sea Level and "Standard Day" conditions.

Re = 9346 x 100 x 10 = 9346 x 1000 = 9,346,000.


Reynold's Magic Number basically shows the ratio between inertial
forces and viscous forces in a fluid.

Think of it as (how fast it's moving) / (how sticky it is).

At low R, viscous forces predominate. (and generally laminar flow)
At high R, is dominated by inertial forces. (resulting in higher sheer
forces and turbulence)


Straight from wiki...

If an airplane wing needs testing, one can make a scaled down model of the wing
and test it in a wind tunnel using the same Reynolds number that the actual
airplane is subjected to. If for example the scale model has linear dimensions
one quarter of full size, the flow velocity would have to be increased four
times to obtain similar flow behaviour.

Alternatively, tests could be conducted in a water tank instead of in air
(provided the compressibility effects of air are not significant). As the
kinematic viscosity of water is around 13 times less than that of air at 15 °C,
in this case the scale model would need to be about one thirteenth the size in
all dimensions to maintain the same Reynolds number, assuming the full-scale
flow velocity was used.

The results of the laboratory model will be similar to those of the actual plane
wing results. Thus there is no need to bring a full scale plane into the lab and
actually test it. This is an example of "dynamic similarity".

Reynolds number is important in the calculation of a body's drag
characteristics. A notable example is that of the flow around a cylinder. Above
roughly 3×106 Re the drag coefficient drops considerably. This is important when
calculating the optimal cruise speeds for low drag (and therefore long range)
profiles for airplanes.