Wall function calculation of tau_wall

KRK

Does anyone know how wall shear stress is calculated in order to apply the enhanced wall functions??? I know for equilibrium wall functions, you can relate the tke at the center of the wall adjacent cell to tau_wall.

But for the enhanced wall functions, you need to be able to compute tau_wall whether you are in viscous sublayer, buffer region, or log region.

Also, I'm a bit confused as to why the k equation (for the k-omega RANS model) uses a boundary condition of zero flux at the walls. Shouldn't the value of k be zero at the walls?