[Carpet] Boundary conditions and synchronising

[Erik Schnetter]

It is Cactus philosophy that boundary conditions can be applied  
either before or after sychronising; the results should be  
identical.  It just occurred to me that this is not the case any more  
in the presence of mesh refinement.

Boundary prolongation can not fill outer boundaries or symmetry  
boundaries, since there may not be enough grid points for the  
prolongation stencil.  It is therefore necessary to prolongate the  
mesh refinement boundaries first, and apply the symmetry boundary  
conditions later.  In this way, the edges and corners, which are  
defined by the combination of prolongation and symmetry, are defined.

Given the way in which boundary conditions in Cactus are first  
selected and later applied, I believe that this is already the case  
for most codes.  That is, it is likely that most codes are already  
correct.  Apart from old codes that apply symmetry conditions  
"manually" by calling CartGrid3D directly -- which is deprecated -- I  
am not aware of any places where there is a problem.  However,  
nevertheless, please keep this in mind.

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>

My email is as private as my paper mail.  I therefore support encrypting
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