[Nek5000-users] Calculating local skin friction coefficient

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

Hi Harish,

I have not used comp_sij routine myself but from what I see in the source 
(navier5.f:2266)

          sij(i,4,e) = j*  ! du/dy + dv/dx
...
which is different from your goal of du/dy unless v is not a function of 
x.

I am assuming you set your j1,j2 correctly for your boundary element 
surfaces and you solve for non-dimentional values.  If you have constant 
properties then you do not need to fill up the array visc -- you can just 
use param(2) instead.

It may be a good idea as usual to set a test problem with analytical 
non-trivial initial conditions and run it w/ nsteps=0 just to make sure 
you get the answer you want...

Best,
Aleks





On Wed, 2 Mar 2011, nek5000-users at lists.mcs.anl.gov wrote:

> 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.
>
>
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