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.

I'm also wondering if you want to try using PreSync, even if boundary conditions and syncs aren't your major problem.

--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

On 8 Apr 2019, at 18:03, Erik Schnetter <eschnetter@perimeterinstitute.camailto:eschnetter@perimeterinstitute.ca> 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.)

Hi Erik,

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.

For reference, the parameters I use are below.

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.

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.

################################################################################ # 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 "

-- Ian Hinder Research Software Engineer University of Manchester, UK