[Nek5000-users] problem with applying boundary conditions

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

Hi,
Not sure if I understand your question well, but I guess what happens 
during the first time step is that through the continuity equation and 
the incompressibility constraint you will have velocity about one 
everywhere. This is just to maintain mass conservation.

Philipp

On 2017-09-14 18:29, nek5000-users at lists.mcs.anl.gov wrote:
> Dear Neks,
> 
> I am trying to run a simulation, replicating the Nek5000 run from this 
> youtube video:
> https://www.youtube.com/watch?v=RmmN-KFSvAY
> 
> The inlet BC is constant velocity (which is set to 1.0), and as can be 
> seen from the video, the velocity magnitude reasonably away from the 
> inlet is zero at starting times. However, in all of my simulations, I 
> see the velocity magnitude as one at all the interior points away from 
> the inlet from the beginning (the inlet BC ux=1.0 I am applying is 
> somehow propagated to all interior elements). Am I doing something wrong 
> somewhere, or is this the expected behavior? Please see my userbc() and 
> useric() from my .usr file below:
> 
>        subroutine userbc (ix,iy,iz,iside,eg)
>        include 'SIZE'
>        include 'TOTAL'
>        include 'NEKUSE'
>        integer ix,iy,iz,eg,e,i,n
> 
> CC "My inlet is at x=0"
> 
>        if (x.le.1e-4) then
>          ux=1.0
>        endif
> 
>        return
>        end
> 
>        subroutine useric (ix,iy,iz,eg)
>        include 'SIZE'
>        include 'TOTAL'
>        include 'NEKUSE'
> 
>        ux=0
>        uy=0
> 
>        return
>        end
> 
> I believe userbc() is executed for only 'v' in my case. For the other 
> constraints on the elements (like 'W', 'O', 'E'),  I think userbc() 
> would not run (please correct me if I am wrong, I am learning here). 
> Since the above functions wouldn't work as expected, I changed userchk() 
> and removed the above if statements in userbc(). My userchk() is as follows:
> 
>        subroutine userchk
>        include 'SIZE'
>        include 'TOTAL'
>        integer i,n
> 
>        n = nx1*ny1*nz1*nelv
> 
>        if (istep.eq.0) then
>          do i=1,n
>            if (xm1(i,1,1,1).le.1e-4) then
>              vx(i,1,1,1)=1
>            endif
>          enddo
>        endif
> 
>        return
>        end
> 
> This didn't work either. Can somebody point me in the right direction?
> 
> Thanks,
> Ajay
> 
> 
> 
> 
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