On Nov 19, 2025, at 10:26, Steven Brandt via Users <[email protected]> wrote: > > 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? > Output doesn't affect time evolution, so it doesn't matter which way you output things. Of course, if you use second-order accurate interpolation to output a quantity you cannot expect 4th order convergence for these quantities. If you output time-interpolated values of e.g. the lapse then you should check convergence only for the fine grid values of the lapse there, not for the interpolated coarse grid values.
-erik -- Erik Schnetter <[email protected]> http://www.perimeterinstitute.ca/personal/eschnetter/
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