[Nek5000-users] Simulation blows up after running for approx. 53500 time steps!!

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

Thanks Stefan I will try these suggestions and check the turbJet example.

Harish.

On Mon, 19 Apr 2010 nek5000-users at lists.mcs.anl.gov wrote:

> Try to add some divergence to the outflow boundary to accelerate the flow. You'll find an example in the turbJet example (see accl_outflow). This will work well if you're right and the problem is related to a significant backflow at the outflow boundary.
>
> Some solver performance suggestions:
>
> - lower your pressure tolerance (e.g 1e-5 is typically strict enough)
> - your timestep seems to be quite low (check the CFL-number). If the field is fully turbulent and you use IFCHAR=.false. you're stable up to CFL ~ 0.5. If you use the characteristics scheme try to use a dt such that  CFL = 2 (make sure you set COURANT in the .rea file correct).
>
>
> Stefan
>
>
> On Apr 19, 2010, at 2:02 PM, nek5000-users at lists.mcs.anl.gov wrote:
>
> > Hello,
> >
> > I am running a DNS simulation of backward facing simulation (BFS) at
> > Re=3000. The Reynolds # is based on step height (h=1) & free stream
> > velocity (ux = 1.0). The BFS geometry has 5184 elements and lx1=ly1=lz1=8.
> > I have attached the *.rea and *.usr files. I am running the case on 512
> > processors.
> >
> > The case blows up after approx. 53500 time steps with dt=5e-4. After
> > analyzing the case in VIsit, it looks as if the simulation blows up as the
> > vortex reaches the outflow boundary for the first time. I think, the
> > normal outflow boundary condition is not sufficient. I have attached a
> > shortened version of the logfile for reference.
> >
> > The movie for the simulation can be seen at the following link.
> >
> > http://www2.uic.edu/~hkanch1/movie.mpeg
> >
> > Any suggestions on how to fix the problem would be helpful.
> >
> > Thanks for all the help.
> >
> > Harish.<bfs3d.usr><logfile1>_______________________________________________
> > Nek5000-users mailing list
> > Nek5000-users at lists.mcs.anl.gov
> > https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>