[Nek5000-users] problem with applying boundary conditions

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

Hi,
if you have unit velocity at the inflow, then due to mass conservation 
each cross section in the channel needs to have the same mass flux. I am 
not sure what boundary conditions were used in this youtube movie; it 
appears it was not simply one everywhere, but perhaps slowly ramped up 
from zero. In this way your flow would slowly accelerate.

Best,
Philipp

On 2017-09-14 19:21, nek5000-users at lists.mcs.anl.gov wrote:
> Hi JC, I double-checked and am using v (rather than V) only.
> 
> Hi Philipp, Thanks for your answer. Can someone else confirm this too? 
> Also, is there a way to have zero velocities in the interior, just like 
> in the youtube simulation?
> 
> Thanks,
> Ajay
> 
> 
> Could it be that he uses V instead of v in the rea file?
> 
> ++
> JC
> 
> Le 14 sept. 2017 18:41, <nek5000-users at lists.mcs.anl.gov 
> <mailto:nek5000-users at lists.mcs.anl.gov>> a ?crit :
> 
>  > 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 
> <mailto: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 
> <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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