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 <[email protected]> wrote: > On Sat, Mar 6, 2021 at 1:58 PM Gabriele Bozzola > <[email protected]> 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 <[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/ > > > > -- > Erik Schnetter <[email protected]> > http://www.perimeterinstitute.ca/personal/eschnetter/ >
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