Skin friction coefficient

From CFD-Wiki

The skin friction coefficient, , is defined by:

Where is the local wall shear stress, is the fluid density and is the free-stream velocity (usually taken ouside of the boundary layer or at the inlet).

It is related to the momentum thickness as follows: Failed to parse (unknown function\diff): C_f \equiv 2\frac{\diff{theta}}{\diff{x}}

Someone should add some correlations and references for them here