[Users] GRHydro Multipatch announcement

[Roland Haas]

Dear all,

as already mentioned in the meeting minutes and discussed in last week's
CIGR call, Christian Reisswig has been working on adding Llama
Multipatch compatibility to GRHydro. For those of you not familiar with
Llama, this is the description from the Llama website (www.llamacode.org):
--8<--
The Llama code is a 3-dimensional multiblock infrastructure with
adaptive mesh-refinement for Cactus based on Carpet. It provides
different patch systems that cover the simulation domain by a set of
overlapping patches. Each of these patches has local cooordinates with a
well defined relation to global Cartesian coordinates. However, all
computations are carried out using a global Cartesian tensor basis such
that involved tensor transformations between patch systems can be
avoided. Information between the different patches is communicated via
interpolation in the overlap zones.
--8<--

We would like to incorporate these changes into the public development
version of GRHydro. Right now multipatch support is functional for
pure hydro simulations but still incomplete (eg. only the HLLE solver
is supported, others will currently fail with multipatch enabled). No
(serious) attempt has been made to incorporate MP into MHD, though the
changes required should be straightforward. The proposed patch is
backward compatible in that old parameter files will continue to work
without change.

However the patch touches a large number of code lines since all
references to vector/tensor components have to be replaced with
references to tensor components in a local tensor basis. In practice
this means that all occurances of gxx, betax, vel[i] etc. have to be
replaced by gaa, betaa, lvlel[i]. On top of that, the requirement to
stay backwards compatible (and to not use extra memory) with old
parameter files when MP is not actually employed adds another layer so
that in the end gxx is replaced by g11 etc. This is all rather
straightforward, but unfortunately tedious and makes for a large
patch.

We have tested that the code runs the old GRHydro testsuites and passes
the same testsuites that a current vanilla checkout passes.

We will begin adding multipatch support during the day.

Yours,
Christian R., Christian O. and Roland

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