[Users] 3D Carpet component with one grid point along the z direction

[Erik Schnetter]

Gabriele

This is indeed a funny component shape. To my knowledge, certain
thorns will not handle it correctly, because they assume that the
width of the domain is at least the same as the number of ghost or
boundary points. (This assumes you're using more than one ghost and
boundary points.)

There is special code in Carpet to handle the case where a component
is only one point wide, and which sets the width to 0 in this case.
The reason is that Cactus does not store the grid spacing internally,
but only the lower and upper bound of the grid. Both are identical in
this case, so it is not possible to determine the grid spacing.

In your case, the proper solution would be to ensure that no such
component is created, as it is quite inefficient. Unfortunately, the
domain decomposition algorithm already has to satisfy many
constraints, and modifying it is thus difficult. One way would be to
set "Carpet::granularity" to e.g. 4, which forces all grids to have a
size that is a multiple of 4. This would also restrict the total
domain sizes you can have, so using this is not straightforward.

To correct the actual bug you noticed, one could apply a correction
that is local to OutputSlice.cc. The code there has access to the grid
spacing, which is the same for all components, and which could
therefore be calculated in the same place where coord_lower and
coord_upper are calculated (function "GetCoordinates"), and could then
be passed to the routine that outputs the "delta" attribute.

-erik

On Tue, Apr 13, 2021 at 11:04 AM Gabriele Bozzola
<bozzola.gabriele at gmail.com> wrote:
>
> Hello,
>
> one of my evolutions produced the attached file.
>
> What is interesting about this file is that refinement level 10 has a component
> that has shape (1, 53, 105). So, we have only one point along the z direction.
> I am not entirely sure how this came to be, but the problem is that the HDF5
> attribute 'delta' reports a cell width of 0 along the z direction:
>
> In [12]: with h5py.File("alp.xyz.file_0.h5") as f:
>     ...:         print(f['ADMBASE::alp it=55296 tl=0 rl=10 c=0'].shape)
> (1, 53, 105)
>
> In [13]: with h5py.File("alp.xyz.file_0.h5") as f:
>     ...:         print(f['ADMBASE::alp it=55296 tl=0 rl=10 c=0'].attrs['delta'])
> [0.072 0.072 0.   ]
>
> I would have expected the value of 0.072, even if the cell has only one grid point.
>
> Is this a bug?
>
> Thanks,
> Gabriele
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-- 
Erik Schnetter <schnetter at gmail.com>
http://www.perimeterinstitute.ca/personal/eschnetter/