jan olieslagers wrote:
Or why is this Reynolds factor important, and where does one apply it?

My rough understanding:

It is useful to aircraft designers who want to first build small models.
The flow over small models will generally remain laminar over a larger
portion of the small model than the full-sized aircraft. Reynolds number
can be used as an _estimate_ of the amount that turbulent flow contributes
to aspects like drag. (The equation, Re = V*L/nu, doesn't even include
surface roughness; hence the approximation aspect of its nature.)

One could use the Reynolds number equation to estimate when the flow goes
from laminar to turbulent. In fact since length L is in the equation, there
are an infinite number of Reynolds numbers for a body! For example, at the
leading edge of a wing, the length L starts at 0, so Re = 0. That indicats
laminar flow at that point (in theory!) Then Re gets larger as the flow
moves along the wing because the L in the equation gets larger. If one
could factor in wing surface roughness and how much the fluid is already
edging toward turbulence before it even reached the leading edge of the
wing, then one could presumably estimate the value of L when the flow
transitions to turbulent flow (or laminar separation from the surface) for
a given V.

So when you see some publication saying that Re is, for example, 1,000,000
for a wing of length L in a fluid moving at speed V, they mean that is the
value Re reaches at the trailing edge of the wing. Roughly halfway along
that wing Re would be about 500,000 for the same V.

I can't think of anyone other than aircraft designers and testers needing
an understanding of the number.