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<div class="">On 8 Apr 2019, at 18:03, Erik Schnetter <<a href="mailto:eschnetter@perimeterinstitute.ca" class="">eschnetter@perimeterinstitute.ca</a>> wrote:</div>
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<div class="">I am trying to set up a binary black hole simulation that uses 8th<br class="">
order accurate finite differencing. (I need the black holes to be<br class="">
highly resolved, and I don't need the wave zone, thus I am not use<br class="">
AMR.) On the other hand, I am using Cartoon2D, and I extended it to<br class="">
support 8th order interpolation.<br class="">
<br class="">
My problem is that I cannot get the simulation started well. After a<br class="">
few (about 70) time steps, the code aborts due to nans. Of course,<br class="">
having modified thorn Cartoon2D, I cannot exclude that I made an error<br class="">
there. On the other hand, I did find some pre-existing errors that I<br class="">
corrected.<br class="">
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I wonder: What are good settings for numerical dissipation in this<br class="">
case (ML_BSSN with fdOrder=8)? I see a sample parameter file that uses<br class="">
epsDiss = 0.1; should I expect this to work? Would puncture initial<br class="">
conditions need any kind of special treatment? (I'm using<br class="">
TwoPunctures.)<br class="">
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<div>Hi Erik,</div>
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<div>I did experiments with this a while ago with a production BBH simulation (multipatch), and the limit seemed to be about 0.08 in my tests, and I use 0.05 for safety. I don't have access to the data right now (though can dig it out if you want), but I think
it was for a Courant factor of 0.45, though it might have been 0.5. It is strongly dependent on the Courant factor. You can probably get away with larger dissipation strengths for a short time or for low resolution, or if you don't have very sharp features
(e.g. junk radiation from high spin or high q configurations), but you probably don't want to rely on this.</div>
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<div>For reference, the parameters I use are below.</div>
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<div>With Cartoon, I think you will have an effective very small grid spacing for small r, which means that to maintain the Courant condition, you would need a smaller timestep. Is that right? I've never used Cartoon, but it seems like this is something that
should be taken into account.</div>
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<div>Puncture initial conditions shouldn't need any special treatment. Without Cartoon, a resolution of something like 20 grid cells across the radius of the settled AH (which could be a factor of 2 larger than the initial radius) is probably a good minimum
to use.</div>
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<pre style="word-wrap: break-word; white-space: pre-wrap;" class="">################################################################################
# Spatial finite differencing
################################################################################
SummationByParts::order = 8
# Drop order instead of using upwinded stencils, only for advection derivatives
SummationByParts::sbp_upwind_deriv = no
SummationByParts::sbp_1st_deriv = yes
SummationByParts::sbp_2nd_deriv = no
SummationByParts::onesided_interpatch_boundaries = no
SummationByParts::onesided_outer_boundaries = yes
SummationByParts::use_dissipation = no
GlobalDerivative::use_dissipation = yes
SummationByParts::scale_with_h = yes
SummationByParts::dissipation_type = "Kreiss-Oliger"
# Stability limit seems to be about 0.08
SummationByParts::epsdis = 0.05
# Variables for dissipation
SummationByParts::vars = "
ML_BSSN::ML_log_confac
ML_BSSN::ML_metric
ML_BSSN::ML_trace_curv
ML_BSSN::ML_curv
ML_BSSN::ML_Gamma
ML_BSSN::ML_lapse
ML_BSSN::ML_shift
ML_BSSN::ML_dtlapse
ML_BSSN::ML_dtshift
"</pre>
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Ian Hinder</div>
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Research Software Engineer</div>
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University of Manchester, UK</div>
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