[nikosb]

I am trying to calculate the shear stress on the surface of a golfball but I am having trouble figuring out how the wall shear stress is defined on 3D complex surfaces.
When I am on the main part of the golfball that it is similar to that of the sphere the calculation of the wall shear stress is straightforward. From the surface I move a distance "h" along the normal and compute the tangential velocity. In this case I choose the tangential velocity to be aligned with the azimuthal plane.
However when I am in the dimples, it is hard to choose a single tangential velocity as there are infinite number of tangents and not anyone of them is necessarily aligned with an azimuthal plane. How do I choose the tangential velocity in such a case?

Thanks

[idrama]

tau=mu*(grad U,n) if n is surface normal. By Boussinesq approx is
tau=(mu+mu_T)*(grad U,n)

(*,*) scalar product