# [Users] Question about iterations in ETK

Roland Haas rhaas at illinois.edu
Tue Aug 4 07:28:12 CDT 2020

```Hello Yu Liu,

> CarpetRegrid2 only sets two levels on the grid structure.
>
> INFO (Carpet): Grid structure (superregions, coordinates):
>    [0][0][0]   exterior: [-6,-6,-6] : [28,28,28] : [2,2,2]
>    [1][0][0]   exterior: [-3,-3,-3] : [26,26,26] : [1,1,1]
>
>
> Why the course level evolves in time every 512 steps.
The reason lies in the Carpet::max_refinement_levels = 10 that you
asked about below. This setting says that, during the course of the
simulation, there may be up to 10 levels of mesh refinement, even if
not all of them are active at the beginning. Thus for 10 levels and a
factor of 2 between each level, the finest level steps every single
iteration (1 = 2^0), the 2nd finest every 2nd iteration (2 = 2^1),
then the 3rd finest every 4th iteration (4 = 2^2), until the 10th finest
(ie the coarsest) level steps every 2^9 = 512 steps.

If fewer levels are actually active (as in this simulation) then the
steps corresponding only to non-existing levels never execute.

> -------------------------------------------------
> Iteration      Time | *me_per_hour | *ROBASE::rho
>                     |              |      maximum
> -------------------------------------------------
>         0     0.000 |    0.0000000 |    0.0012800
>       512     0.500 | 2.545256e+03 |    0.0012770
>
>
> I noticed the parameter Carpet::max_refinement_levels    = 10, but it
> does not seem to affect the grid structure.
This parameter declares the maximum possible number of refinement
levels that can exist, not the actual refinement levels created. Those
are controlled by the regridding thorn CarpetRegrid2 in its num_levels
parameters.

> I think this is important for us to set I/O and CarpetRegrid2::regrid_every.
Correct. Though regrid_every is also usually driven by physical time
scales. Eg if you are tracking an object then regrid_every must be
short enough that, given a typical speed, the object has not moved out
of the refined box that is supposed to track it by the time the regrid
happens and the box location is adjusted. Note that regridding more
often then every coarse step is perfectly fine.

Yours,
Roland

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