[siw]

Hi,

It's well known that non-dimensional parameters should match between a model (e.g. in a wind tunnel or water tank) and the full scale application. Which non-dimensional parameters are considered is dependent upon the scenario.

I have a compressible aerodynamics test to conduct on a model but due to the equipment I cannot get the model to full scale similarity for both the Mach number and the Reynolds number. So I have to choose only one of those, either or , and let the other not match. The question is which one to match. Since the goal of the test is to determine the rotation of an object which is driven by the airflow I am favoring the Reynolds number, since that is the ratio of inertial to viscous forces. Therefore, my test Mach number will not match the full scale application Mach number.

I'd appreciate any comments if this is right or wrong. Thanks.

[agd]

The answer to your question depends on what you are testing. If you are looking at ground vehicle aerodynamics then the Re is the best choice to match, since the Ma will generally be small enough to assume essentially incompressible flow. On the other hand, most flight testing (except for small planes at low speeds) seek to match the Mach number, since compressibility effects become important, and shock development can play an important role. Very few wind tunnels can match both parameters, and your situation is very common. Unless your situation involves a freestream Mach number above 0.3, your choice of Reynolds number sounds reasonable to me.