[Users] Boundary conditions in diagnostics with compute_every

[Gabriele Bozzola]

Hi Erik,

In this case, you can declare the grid function to use 0-th order time
> interpolation, and allocate only a single time level. This would do
> the same thing. I think the respective time prolongtation operator is
> called "copy".


Do I achieve this by setting the tags prolongation="copy"
and InterpNumTimelevels = 1
in the declaration of the grid function in the interface.ccl?

Thanks again,
Gabriele

On Sun, Mar 7, 2021 at 10:05 AM Erik Schnetter <schnetter at cct.lsu.edu>
wrote:

> On Sat, Mar 6, 2021 at 1:58 PM Gabriele Bozzola
> <bozzola.gabriele at gmail.com> wrote:
> >
> > Hi Erik,
> >
> > thanks for your response: it is very useful.
> >
> > > > Also, is it a problem if I don't worry about the boundaries If I
> > > > want to interpolate the constraints onto a sphere?
> > >
> > > Yes it is. Interpolation requires a stencil, which requires boundaries.
> >
> > I suspected so. Then, going back to the question in the first email, you
> > said that I am essentially forced to compute the diagnostic at
> > each timestep. The diagnostic I want to compute is very expensive,
> > and it would slow down dramatically the evolution, so I really want to
> > compute it only when I am going to output it. What I had in mind was
> > to copy grid function to the previous timelevels by setting _p and _p_p.
> > If I copy the same values as the one at the current time, this would
> > essentially disable time interpolation. But, if I output only when all
> > the refinement levels at the same time, this should not be a problem,
> > because there shouldn't be a need for time prolongation, right?
>
> In this case, you can declare the grid function to use 0-th order time
> interpolation, and allocate only a single time level. This would do
> the same thing. I think the respective time prolongtation operator is
> called "copy".
>
> -erik
>
> > Thanks again for your help,
> > Gabriele
> >
> > On Sat, Mar 6, 2021 at 7:45 AM Erik Schnetter <schnetter at cct.lsu.edu>
> wrote:
> >>
> >> On Fri, Mar 5, 2021 at 8:01 PM Gabriele Bozzola
> >> <bozzola.gabriele at gmail.com> wrote:
> >> >
> >> > Hi Erik,
> >> >
> >> > thank you very much for your answer.
> >> >
> >> > Just a clarification: what is 'boundary' exactly in this context?
> >>
> >> "Boundary" in the context are all grid points where the constraints
> >> cannot be calculated directly, i.e. by evaluating finite differences.
> >>
> >> > Also, is it a problem if I don't worry about the boundaries If I
> >> > want to interpolate the constraints onto a sphere?
> >>
> >> Yes it is. Interpolation requires a stencil, which requires boundaries.
> >>
> >> Cactus interpolation supports taking derivatives during interpolation.
> >> You can thus interpolate the ADM variables and their derivatives onto
> >> a sphere, and calculate the constraints there. You won't need to take
> >> derivatives on the sphere since you interpolated all derivatives, so
> >> evaluating the constraints on points on a sphere is then a point-wise
> >> operation. The horizon finder does this (calculating the expansion,
> >> not the constraints, but both have equivalent requirements).
> >>
> >> -erik
> >>
> >> > Thanks,
> >> > Gabriele
> >> >
> >> > Erik Schnetter <schnetter at cct.lsu.edu> writes:
> >> >
> >> > > 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
> >> > >>
> >> > >>
> >> > >> _______________________________________________
> >> > >> Users mailing list
> >> > >> Users at einsteintoolkit.org
> >> > >> http://lists.einsteintoolkit.org/mailman/listinfo/users
> >>
> >>
> >>
> >> --
> >> Erik Schnetter <schnetter at cct.lsu.edu>
> >> http://www.perimeterinstitute.ca/personal/eschnetter/
>
>
>
> --
> Erik Schnetter <schnetter at cct.lsu.edu>
> http://www.perimeterinstitute.ca/personal/eschnetter/
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.einsteintoolkit.org/pipermail/users/attachments/20210308/a4d39adf/attachment-0001.html