On 11/19/2025 8:24 AM, Erik Schnetter wrote:
Miguel
If I recall correctly, Ian Hinder studied convergence of black hole
simulations with subcycling in time in great detail. The Einstein
Toolkit gallery example for GW150914 contains the respective distilled
knowledge. https://einsteintoolkit.org/gallery/bbh/index.html
Some important details that I recall:
- You can regrid only when the fine and coarse grids are aligned
- You cannot use time interpolation at all. You need to use enough
buffer zones for all the RK substeps for all the fine timesteps for
each coarse time step. With 3 ghost zones and RK4 you need 21 buffer
zones.
Does no time interpolation mean no dense output? That didn't exist when
Ian did these tests, right?
--Steve
You likely do not need to evolve for a long time to see loss of
convergence. It might suffice to run for just a few time steps (a few
coarse grid steps) and study convergence of one of the evolved
variables (e.g. the lapse) in great detail over a wide range of
resolutions. For this you do not need a large domain, nor do you need
many refinement levels, nor do you need many grid points. For example,
studying convergence near the point (1,0,0) requires probably about
100^3 points for each resolution in a two-level setup.
-erik
On Nov 19, 2025, at 06:17, Miguel Zilhão <[email protected]> wrote:
hi all,
we've recently added the option to compute the Gauss constraint in
the ProcaEvolve thorn
(https://bitbucket.org/canuda/proca/src/experimental_miguel/ProcaEvolve/)
and we were surprised to see that, even for very small evolutions of
a simple (unperturbed) charged black hole, this constraint violation
does not converge. of course everything is converging just fine at
t=0, but once the evolution starts, convergence is lost really quickly.
after experimenting with different things, i think i've narrowed the
issue down to the subcycling in time. i've attached some plots as
well as the corresponding parameter files. these plots are not
convergence tests, it's just what i'm observing with and without
subcycling in time (everything else is the same as you can see from
the parfiles).
these are evolutions for a single charged BH, with Q=0.2.
in one of the parfiles (LeanBSSN_RN_Q0.2_nosubcycl_hf40_202509.par)
all timelevels update at the same rate with
Carpet::time_refinement_factors = "[1, 1, 1, 1, 1, 1, 1]"
for the other parfile (LeanBSSN_RN_Q0.2_hf40_202509.par), all
timelevels update at the same rate *except* the two inner ones. for
the "base" grid functions, i see no noticeable difference insofar as
i've checked, but for the Gauss constraint violation the results are
very different as you can see from the plots attached:
- in the gc_x_RN_nosubcycl.pdf plot, everything looks fine
- in the gc_x_RN_subcycl.pdf case, notice how a lot of noise
propagates from the two inner refinements levels -- the ones that
updated more frequently than the rest.
the problem is that this noise is not contained in the buffer region
(if it was, we'd be fine), but propagates out, contaminates the rest
of the grid, and totally ruins any convergence study.
actually a similar thing also happens for the hamiltonian constraint
-- plots also attached (hc_x_RN_nosubcycl.pdf, hc_x_RN_subcycl.pdf)
-- but in this case the effect is smaller and it doesn't spoil
convergence studies.
so i was wondering whether this is at all expected and/or if there
are some parameters or setting that we may have overlooked?
thanks,
Miguel
<hc_x_RN_nosubcycl.pdf><hc_x_RN_subcycl.pdf><gc_x_RN_subcycl.pdf><gc_x_RN_nosubcycl.pdf><LeanBSSN_RN_Q0.2_hf40_202509.par><LeanBSSN_RN_Q0.2_nosubcycl_hf40_202509.par>_______________________________________________
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