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