# Skin friction coefficient

The skin friction coefficient, $C_f$, is defined by:

$C_f \equiv \frac{\tau_w}{\frac{1}{2} \, \rho \, U_\infty^2}$

Where $\tau_w$ is the local wall shear stress, $\rho$ is the fluid density and $U_\infty$ 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