Skin friction coefficient
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: C_f = 2(d theta)/ (d x)
An empirical relation you may use for comparison is: C_f = 0.0583/(Re )^0.2
Someone should add some correlations and references for them here