[Users] change default value of CarpetRegrid2's min_distance

[Roland Haas]

Hello all,

I would like to propose change the default value of 
CarpetRegrid2::min_distance parameter from 4 (grid points) to 0.

Currently:
--8<--
CCTK_INT min_distance "Minimum distance (in grid points) between coarse 
and fine grid boundaries" STEERABLE=always
{
   0:* :: ""
} 4
--8<--

and min_distance is used in enlarged_fine_grid to enforce a minimum 
distance between grid levels. A distance of 0 means that the end of the 
buffer+ghost zone region of the fine grid touches the end of the active 
(ie. non-buffer) region of the coarse grid.

This can have unexpected effects when specifying "tight" boxes via the 
radius_n[] parameters when trying to translate old CarpetRegrid 
parameter files, since the radius_n[] parameters are no longer what 
actually specifies the final grid size. It also (I think) makes 
CarpetRegrid2 with boxes create a different grid than the equivalent 
manual-coordinates description in CarpetRegrid.

Making min_distance's default value zero reduces the number of 
parameters the affect the grid structure in the default setting and 
makes it the user's responsibility to choose a decent (maybe 4) value 
for the emergency distance parameter min_distance.

For the amr implementation in CarpetRegrid2 min_distance is also used 
and might be much more useful then for the nested boxes currently used. 
There a default of 4 is probably more sensible.

Any comments on this?

Yours,
Roland

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