[Nek5000-users] Outflow boundary conditions

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

Hi,
indeed, we typically use the stabilised outflow condition based on the 
paper by Dong et al. (2014, JCP), and have experimented with convective 
outflow conditions as well. The former is very easy as there are not 
many parameters to be chosen; for the latter one needs to have an 
estimate for the convection velocity which can be obtained in different 
ways.

Perhaps one should collect all options at some point and perhaps do a 
comparative study for a few canonical cases. That could be helpful.

Best regards,
Philipp

On 2018-03-06 14:21, nek5000-users at lists.mcs.anl.gov wrote:
> 
> Dear Dante,
> 
> 
> I typically use the turb_outflow boundary condition that is in navier5.f 
> (see below). It will guarantee no inflow.
> 
> 
> Sometimes you need to set "uin" to get the correct scale if your inlet 
> and outlet areas are disparate in one way or another, but I've rarely 
> found a case where this does not work.  Other groups use some form of 
> convective outflow treatment.  There were comments on this just last week.
> 
> 
> hth,
> 
> 
> Paul
> 
> 
> 
> 
> c     . Set div U > 0 in elements with 'O  ' bc.
> 
> c
> 
> c     . rq is nominally the ratio of Qout/Qin and is typically 1.5
> 
> c
> 
> c     . uin is normally zero, unless your flow is zero everywhere
> 
> c
> 
> c     . d and m1 are work arrays of size (lx1,ly1,lz1,lelt), assumed 
> persistant
> 
> c
> 
> c     This routine may or may not work with multiple outlets --- it has
> 
> c     not been tested for this case.
> 
> c
> 
> c
> 
> c     TYPICAL USAGE -- ADD THESE LINES TO userchk() in your .usr file:
> 
> c                      (uncommented)
> 
> c
> 
> c     common /myoutflow/ d(lx1,ly1,lz1,lelt),m1(lx1*ly1*lz1,lelt)
> 
> c     real m1
> 
> c     rq  = 2.
> 
> c     uin = 0.
> 
> c     call turb_outflow(d,m1,rq,uin)
> 
> c
> 
> 
> ------------------------------------------------------------------------
> *From:* Nek5000-users <nek5000-users-bounces at lists.mcs.anl.gov> on 
> behalf of nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
> *Sent:* Tuesday, March 6, 2018 1:33:29 AM
> *To:* nek5000-users at lists.mcs.anl.gov
> *Subject:* [Nek5000-users] Outflow boundary conditions
> Dear all,
> 
> are outflow advective boundary conditions available in nek5000?
> 
> Or is there any work around in order to impose outflow boundary conditions
> (for the velocity and the passive scalars) that can grantee that there 
> is no flow coming
> back from the outflow boundary?
> 
> Thanks,
> Dante
> 
> 
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