Skin friction coefficient 2(u*/uref)2

or the nondimensional local surface shear stress coefficient. The engineering “skin friction coefficient,” cf, is traditionally tabulated in engineering texts. It is sometimes given as a function of surface type and ratio of roughness element height to pipe or boundary layer thickness. Often it is implicitly presented through the classic Moody diagram (Schlichting, 1955) which covers aerodynamically smooth and rough surfaces over a comprehensive range of Reynolds numbers including laminar, transitional, and turbulent flows