I'll elaborate on this slightly:
Bill Swingle wrote:
> Reynold's number is a very important parameter in fluid dynamics. Flows at
> the same Reynolds number are similar, which make it possible to use smaller
> models (length l) for tests at higher velocities (velocity v) to achieve the
> same Reynolds number as the full scale airplane.
Reynold's number (Re) is called a non dimensional parameter. Basically it takes
all the important dimensional parameters in a fluid dynamics situation, length,
fluid density, fluid viscosity, and fluid velocity, and combines them in such a
way so that all the units cancel out. There are other ways of combining
important parameters to cancel out the dimensions which yield other non
dimensional parameters named after other guys that characterize other
situations. But the Re number is by far the most often used and useful for
characterizing fluid flows.
The way that scale models can be used to obtain information about full scale
aircraft is by maintaining "dynamic similarity." Basically this means
preserving the Re number between the scale case and the full scale case. So for
example, if you take a 1/4 scale model and test it in the same fluid at the same
speed as the full scale version, your Re number will be 1/4 as large since the
length is 1/4 size, and other important parameters are identical (same air, same
density, same viscosity, same velocity). Testing a 1/4 scale aircraft in the
same fluid at the same speed does not maintain dynamic similarity. Any results
obtained will be useless in predicting the behavior of the full scale aircraft.
To maintain dynamic similarity between scale model and full scale when using the
same fluid, your velocity must be much higher to match the Re number. For
modeling scale aircraft in a wind tunnel, this means you need a high speed wind
tunnel to match the Re. But if you change the fluid, you change the viscosity
and density. So for example, it is common to test scale models of aircraft in a
"tow tank" in which the fluid is water. The scale of the model and the velocity
used are chosen so that the Re number is matched with the intended full scale
application. The velocity used in a water tow tank test for a 1/4 scale model
is somewhat slower than the full scale velocity application which is why simple
towing works (no need for elaborate high speed recirculating water systems).
-- James
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