[Nek5000-users] Calculating local skin friction coefficient

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

Hi Aleks,

Thanks for your reply.

I am solving for constant properties and in non-dimensional terms. I am
using a modified drgtrq subroutine, so my j1,j2's are right.

You are right that from navier5.f and my definition of s12,

       s12 =  s(i,4,e) = j * ! du/dy + dv/dx

But at wall dv/dx = 0, therefore

       s12 = du/dy

That is why I was using s12 in my computation. I this wrong then.

Thanks,

Harish.

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

> 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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