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