[Users] Boundary conditions in diagnostics with compute_every

[Erik Schnetter]

Gabriele

If you do not use the constraints, then you do not need to set the
boundaries. That would simplify many things; for example, you can
calculate them at any time, and you do not need to worry about time
levels. However, you then need to be careful about visualization and
reductions: You need to ensure that you don't accidentally visualize
the boundaries, and you cannot perform vertex-centred reductions in
Carpet because they need some boundary values.

If you do need boundaries, then you need three time levels to allow
prolongation on boundaries, and you are essentially forced to evaluate
the constraints at every iteration. I recommend the schedule bin
"MoL_PseudoEvolution" for this, which runs once per time step, after
MoL's loop, at the right time (i.e. before restriction).

-erik

On Fri, Mar 5, 2021 at 11:01 AM Gabriele Bozzola
<bozzola.gabriele at gmail.com> wrote:
>
> Hello,
>
> suppose (for clarity) that I want to write a thorn that computes the constraint violations
> as grid functions. Since this is a diagnostic, I don't need to compute it at every iteration,
> so I will add a parameter "compute every" and I will schedule the computations in
> CCTK_ANALYSIS. Then, I will be careful and make sure that compute_every is a
> multiple of when all the refinement levels are synced up.
>
> How are boundary conditions handled in this case?
>
> I can call Boundary_SelectGroupForBC every "compute_every" and schedule the
> corresponding functions in the scheduler. But, do I need to (1) allocate multiple timelevels
> for my grid functions, (2) do anything about filling previous timelevels?
>
> I am looking at WeylScal4 as an example. The thorn has parameters "compute_every",
> the grid functions have 3 time levels, and Boundary_SelectGroupForBC is called
> every "compute_every", but nothing is done to fill the previous timelevels. How does this
> work?
>
> Assuming that the boundary conditions are 'flat', is there any way to just work with one
> timelevel?
>
> Thanks,
> Gabriele
>
>
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-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/