[Users] BSSN, 8th order accurate -- how much dissipation?

[Erik Schnetter]

On Mon, Apr 8, 2019 at 4:46 PM Peter Diener <diener at cct.lsu.edu> wrote:
>
> Hi Erik,
>
> I do believe that epsDiss=0.1 should be fine for 8th order dissipation.
> The dissipation operators should be multiplied by suitable factors so
> that the range of good dissipation strength values should be the same
> regardless of dissipation order.
>
> Regarding you second question about whether puncture initial data would
> need any kind of special treatment, my answer is: I don't think so.
>
> What kind of resolutions are you trying to use?

M/128 isn't quite good enough, so I'm starting from M/256 up to M/768,
maybe higher if necessary.

> Would it be possible to run without Cartoon2D for a short time at the
> same resolution and check if those issues disappear?

I totally should have done that. The problems appear quickly enough.
Thanks for the reminder.

-erik

> Cheers,
>
>    Peter
>
> On Monday 2019-04-08 15:01, Erik Schnetter wrote:
>
> >Date: Mon, 8 Apr 2019 15:01:46
> >From: Erik Schnetter <schnetter at cct.lsu.edu>
> >To: Steven R. Brandt <sbrandt at cct.lsu.edu>
> >Cc: Einstein Toolkit Users <users at einsteintoolkit.org>
> >Subject: Re: [Users] BSSN, 8th order accurate -- how much dissipation?
> >
> > On Mon, Apr 8, 2019 at 3:41 PM Steven R. Brandt <sbrandt at cct.lsu.edu> wrote:
> >>
> >> I don't know how to answer your question, but I hope to see the
> >> extensions to Cartoon2D etc. to come back to the toolkit.
> >
> > They certainly will.
> >
> >> I'm also wondering if you want to try using PreSync, even if boundary
> >> conditions and syncs aren't your major problem.
> >
> > You may or may not be aware that Cartoon2D syncs both before and after
> > applying boundary conditions for each group, and that it does not work
> > well with OpenMP. So I am using many MPI processes and have many syncs
> > in my code.
> >
> > -erik
> >
> >> --Steve
> >>
> >> On 4/8/2019 12:03 PM, Erik Schnetter wrote:
> >>> I am trying to set up a binary black hole simulation that uses 8th
> >>> order accurate finite differencing. (I need the black holes to be
> >>> highly resolved, and I don't need the wave zone, thus I am not use
> >>> AMR.) On the other hand, I am using Cartoon2D, and I extended it to
> >>> support 8th order interpolation.
> >>>
> >>> My problem is that I cannot get the simulation started well. After a
> >>> few (about 70) time steps, the code aborts due to nans. Of course,
> >>> having modified thorn Cartoon2D, I cannot exclude that I made an error
> >>> there. On the other hand, I did find some pre-existing errors that I
> >>> corrected.
> >>>
> >>> I wonder: What are good settings for numerical dissipation in this
> >>> case (ML_BSSN with fdOrder=8)? I see a sample parameter file that uses
> >>> epsDiss = 0.1; should I expect this to work? Would puncture initial
> >>> conditions need any kind of special treatment? (I'm using
> >>> TwoPunctures.)
> >>>
> >>> Thanks,
> >>> -erik
> >>>
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> >
> >
> >
> > --
> > Erik Schnetter <schnetter at cct.lsu.edu>
> > http://www.perimeterinstitute.ca/personal/eschnetter/
> > _______________________________________________
> > Users mailing list
> > Users at einsteintoolkit.org
> > http://lists.einsteintoolkit.org/mailman/listinfo/users
> >
>


-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/