[Users] GRHydro::Con2Prim question

Haas, Roland rhaas at illinois.edu
Thu Nov 14 11:22:41 CST 2019

Hello Giulia,

this has to do with the order of timesteps when doing subcycling in
time (which is what Carpet does).

cctk_iteration counts RHS evaluations, and is not directly related to
cctk_time ie cctk_time != cctk_iteration * DELTA_TIME.

Basically the coarsest reflevel has to step first, so that in iteration
1 (evaluating the RHS for the first time) all reflevels 0..7 step
forward and evaluate the RHS in the EVOL bin.

In the ANALYSIS bin code only runs once data has been properly
restricted from the finer ones etc. Which happens at slightly different
times than EVOL.

The net result is that in *EVOL* a function runs in refinement levels
whenever (cctk_iteration - 1) % reflevel_every == 0 while in *ANALYSIS*
it is cctk_iteration % reflevel_every == 0. The strange rule in EVOL
comes from asking "was the last iteration one where the timelevel was
aligned so that I now need to compute a new RHS"?

Thus 473 - 1 = 472 which is evenly divided by 8. NB you say reflevel 4,
but do you mean the fourth reflevel (which is level "3" since they
count from "0")?


> Hi all,
> I am using GRHydro in running a simulation, and I get a crash in
> Con2Prim at iteration 473 on reflevel 4. Given that I have set 8
> refinement levels, I'm wondering why Con2Prim is being executed at
> iteration 473 on this reflevel: shouldn't it be executed only at an
> iteration which is a multiple of 8? Maybe I'm missing something
> fundamental. Thank you,
> Cheers,
> Giulia Crotti

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