[Nek5000-users] Calculating local skin friction coefficient

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

Hi Harish,

It seems to me you should just be able to use the torque_calc
routine, applied to an object which is defined as a collection
of faces.   All that you have to do is to identify the faces.
I usually write a routine "set_obj" to do this --- two examples
of which can be found in

   ext_cyl

   turbChannel

in /examples directory.

Hope this helps.

Paul


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

> 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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