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? Thanks again for your help, Gabriele On Sat, Mar 6, 2021 at 7:45 AM Erik Schnetter <[email protected]> wrote: > On Fri, Mar 5, 2021 at 8:01 PM Gabriele Bozzola > <[email protected]> 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 <[email protected]> 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 > > > <[email protected]> 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 > > >> [email protected] > > >> http://lists.einsteintoolkit.org/mailman/listinfo/users > > > > -- > Erik Schnetter <[email protected]> > http://www.perimeterinstitute.ca/personal/eschnetter/ >
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