Simple question for friction velocity
 

Hi.. I have a simple question..

How to calculate the wall friction velocity (Utau) in three dimensional case?

In 2-d, Utau = sqrt((Mu*(du/dy))/rho).. but.. in case of 3-d, i think Utau = sqrt((Mu*(du/dy+ add something))/rho) or.. how can I express the Utau or thw wall shear stress in 3-D?

please answer for me.. when it is in x-y-z coordinate, x is streamwise, y is the normal direction, z is the spanwise..

Thank you..