[FLASH-USERS] Hydrostatic BC, etc: How to make it work in FLASH 3.0

[Klaus Weide]

Summarizing from recent exchanges with Mateusz <mateuszr at umich.edu>:


1) To make the hydrostatic boundary conditions included with FLASH3 (or 
any user-supplied alternative implementation) work correctly, the 
following should be added to a simulations's flash.par file:

   enableMaskedGCFill = .FALSE.

This was an oversight; this flag probably should default to FALSE anyway.

Background:

The runtime parameter enableMaskedGCFill turns "masked guard cell filling" 
on, for improved performance (not all variables need to have guard cells 
filled all the time when Grid_fillGuardCells is called). We have tested 
the masked guard cell filling before the FLASH3 release quite thoroughly, 
and don't know of any problems with it except in connection with 
hydrostatic BCs.

The Grid unit includes logic that determines the set of variables for 
which guard cells need to be communicated (based on requirements specified 
by the caller of Grid_fillGuardCells). This logic does not take into 
account that some variables are always required as input for hydrostatic 
BC implementations (in particular, pressure).

This will be fixed in a future FLASH release, by making enableMaskedGCFill 
default to .FALSE. and/or by considering such requirements in the guard 
cell masking logic.



2) To make hydrostatic boundary conditions work with the staggered mesh 
MHD solver - or, more generally, with any setup that uses face variables 
[FACEVAR keyword in a Config file]:


Handling for some overlooked cases (filling of guard cells for face 
variables) needs to be added to the code.  Probably the simples way
is the following:

In source/Grid/GridBoundaryConditions/OneRow/Grid_applyBCEdge.F90,
in order to have guard cells for a face variable at domain boundaries
of type "hydrostatic+nvrefl" filled as if this was a "reflecting"
boundary, change
      case(REFLECTING)
to
      case(REFLECTING,HYDROSTATIC_F2_NVREFL)

Similarly, to have guard cells for a face variable at domain boundaries
of type "hydrostatic+nvout" filled as if this was an "outflow"
boundary, change
      case(OUTFLOW)
to
      case(OUTFLOW,HYDROSTATIC_F2_NVOUT)

and to have guard cells for a face variable at domain boundaries
of type "hydrostatic+nvdiode" filled as if this was a "diode"
boundary, change
      case(DIODE)
to
      case(DIODE,HYDROSTATIC_F2_NVDIODE)


Background:

For simple boundary conditions like "outflow", "reflecting", and "diode", 
guard cells for face variables are generally filled at domain boundaries 
by using the same rules as for cell-centered variables: by copying values 
from the last interior cell for "outflow", or copying values from the last 
few cell layers for "reflecting" (with some special handling for 
velocities), etc.  The above changes let the code handle face variables 
for hydrostatic-variant BCs in similar ways.  Whether this handling
is appropriate depends on the problem, of course.

The staggered mesh MHD implementation uses face variables for
divergence-free magnetic fields.



3) Finally, Mateusz found some errors in the code that actually implements 
the FLASH2-style hydrostatic boundary conditions, in
../source/Grid/GridBoundaryConditions/Flash2HSE/gr_applyFlash2HSEBC.F90.
The errors will eventually be corrected, but they are in code that can 
only be enabled by editing the source file.  Basically, be warned if
you are editing gr_applyFlash2HSEBC.F90, don't assume that the code
that is disabled is correct.




Klaus Weide