Question about momentum thickness Reynolds number
 

Hi,

How is 'momentum thickness Reynolds number' defined?

For pipe, diameter is always used to compute the Reynolds number.

But 'momentum thickness Reynolds number' is computed by the momentum thickness.

So shall I suppose the 'momentum thickness Reynolds number' of a pipe is much smaller than the pipe Reynolds number computed by the diameter?

Why the 'transition onset Reynolds number' is always expressed by the 'momentum thickness Reynolds number'? How is 'momentum thickness Reynolds number' directly related to the flow regime transition?

Thanks!

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