[Users] Grid structure inconsistency & errors in surface mask locations

[Erik Schnetter]

On Fri, Apr 16, 2021 at 11:39 AM Seven Eight <seveneights7e8 at gmail.com> wrote:
>
> Hi, I was wondering if anybody could help me spot the problem(s) in (possibly) my parameter file?
>
> I am trying to run a BBH simulation with mass ratio = 3. The parameter b=3 and the punctures were set at +3 and -1 on the x axis (as I understand the setup requires, so that the center of mass is at 0). I set up the CarpetGrid refinement domains accordingly. The simulation runs for about 8000 steps and ends with an error concerning the grid, which I put here (error log and parameter file attached):
>
> https://gist.github.com/konrad-topolski/1f292fbb234232e68dd76e4fc784e481
>
> The differences between this parameter file and the one originally provided with Einstein Toolkit was that I had removed any reflectionsymmetry180 and I commented out the Carpet Mask variables concerning the surfaces, i.e I simulated without:
>
> CarpetMask::excluded_surface [0] = 0
> CarpetMask::excluded_surface_factor[0] = 1.0
> CarpetMask::excluded_surface [1] = 1
> CarpetMask::excluded_surface_factor[1] = 1.0
> CarpetMask::excluded_surface [2] = 2
> CarpetMask::excluded_surface_factor[2] = 1.0
>
> I did so because there was some weird behaviour with Surface mask location - it correctly localized the puncture 0 (at +3), but couldn't locate the puncture 1 (at -1) - it said it was at [e^273, ..., ...] etc.

I assume that you mean that the horizon finder was unable to find a
horizon in the initial conditions. This is worrisome, and may point to
a lack of resolution. If I recall correctly, a puncture has a
coordinate radius of M/2, or 0.125 for the smaller black hole in your
case. If I read your parameter file correctly, your resolution at the
punctures is 0.0234375, which means that you have only about 10 grid
points across the puncture. This is likely not enough. You can try
adding one or two more refinement levels near the smaller puncture.

The black hole horizon coordinate radius will grow initially. If you
are worried about performance, then you can try removing the finest
level once the horizons have grown a bit.

Your parameters for the initial guess for the horizon radius
(AHFinderDirect::initial_guess__coord_sphere__radius) are also off.
They should be 0.75/2 and 0.25/2, respectively.

I would begin by ensuring that the initial conditions are set up
correctly. In addition to locating horizons and having good masses and
spins, you can also look whether the RHS of the BSSN variables and the
constraints converge. They won't converge at the punctures themselves,
but they should look good at least at and outside of the horizons.

All the best! Don't hesitate to ask again. (And I hope that others
will correct me if my estimates are wrong.)

-erik

> Hence the question, how to set up the puncture tracker, AHFinderDirect and CarpetMask correctly?
> I also set up the target masses instead of the bare ones.
>
> I am currently playing around with some options, but in case I don't get anywhere, I would much appreciate any help.
>
> Best regards
> Konrad Topolski
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-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/