[Users] Loss of convergence with subcycling in time

[Erik Schnetter]

On Nov 19, 2025, at 10:26, Steven Brandt via Users <users at einsteintoolkit.org> 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 <schnetter at gmail.com>
http://www.perimeterinstitute.ca/personal/eschnetter/

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