[Nek5000-users] Calculating local skin friction coefficient

[nek5000-users at lists.mcs.anl.gov]

Hello,

I am trying to compute local skin friction coefficient (C_f) at bottom
wall in a backward facing step. The flow is simulated at Re=5000. I need
to figure this out to test the effect of time step size (dt) on transition
to turbulence.

Quantities used:
tau_w = wall shear stress
U_inf = reference velocity

definition of C_f := 2 * tau_w/ rho*U_inf

tau_w = -mu*du/dy ( at wall)

C_f = 2 * nu * du/dy

To compute C_f I am using use the following (code snippet):

call comp_sij(sij,nij,vx,vy,vz,ur,us,ut,vr,vs,vt,wr,ws,wt)

         v  = visc(j1,j2,1,e)        ! from subroutine drgtrq()
        s12 = sij(j1,j2,1,4,e)
        s22 = sij(j1,j2,1,2,e)
        s32 = sij(j1,j2,1,5,e)
        Cf  = 2*v*s12

Am I doing this right? Any help would be appreciated.

Thanks,

Harish.