What are some characteristic length scales used to calculate

What are some characteristic length scales used to calculate Reynolds number. (Sphere and Hemisphere)

Solution

The characteristic length scale used to calculate the Reynolds number is usually the size of the computational domain (say L)

for sphere :The characteristic dimension is the diameter of the sphere. The velocity is that of the sphere relative to the fluid some distance away from the sphere. It does not matter if the fluid is static and the sphere moving.

for hemisphere : L =D/2