[Users] Loss of convergence with subcycling in time

[Miguel Zilhão]

hi Erik, all,

thanks for your input. for this specific example the grid is not moving 
at all, so i was expecting it to be a simpler setting than the case with 
binary BHs...

is it then expected to see all that noise propagating out of the buffer 
region and contaminating the whole grid in such a short amount of time 
(plot attached)? this is a bit uncomfortable, since the convergence on 
the electric constraint violation is completely lost... if it were a 
matter of dropping from 4th to 2nd order (as is the case in the L2 norm 
of the Hamiltonian constraint) i'd be fine with it, but a complete lack 
of convergence is difficult to justify...

thanks,
Miguel


On 19/11/2025 16:23, Erik Schnetter wrote:
> 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/
> 
> 
> _______________________________________________
> Users mailing list
> Users at einsteintoolkit.org
> http://lists.einsteintoolkit.org/mailman/listinfo/users
-------------- next part --------------
A non-text attachment was scrubbed...
Name: gc_x_RN_subcycl.pdf
Type: application/pdf
Size: 13022 bytes
Desc: gc_x_RN_subcycl.pdf
URL: <http://lists.einsteintoolkit.org/pipermail/users/attachments/20251119/86baf2c2/attachment.pdf>