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

[Peter Diener]

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?

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

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/
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