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I'm looking for a little help understanding Reynold's Number. I know that its the ratio of the inertial forces versus the viscous forces. With that being said, everywhere I look, the Reynold's Number is calculated for flow in a pipe or duct or across some sort of surface. Why can't the Reynold's Number be calculated for a random point? For example, if i was interested in the Reynold's Number of air after exiting an A/C duct how could one apply any of the general equations to that? The general equations all use some sort of characteristic length or diameter to make the calculations.

I assume air (since it is viscous) will have some effect on flow that is introduced into is (such as air exiting a duct), so I would assume there has to be a way to calculate the Reynold's Number for air (knowing the viscosity of the air and the velocity) that is not in any duct or tube or moving across any surface.

Any help understanding this would be appreciated.

Thanks.