how to determine friction velocity?
 

Hello!
What is the correct way to determine u_tau during computations (I need it to find y^+, to compare with law-of-the-wall, etc.):
1) u_tau=sqrt(mu*(dv/dy)/rho) (mu is molecular viscosity)
2) u_tau=sqrt((mu+mu_turb)*(dv/dy)/rho)
3) u_tau=karm*y*(dv/dy)
4) any other suggestions?