[petsc-dev] DMDA_BOUNDARY_GHOSTED

[Barry Smith]

  For vertex centered finite differences it is easy, one simply does not include the boundary nodes in the DMDA and then puts the actual Dirichlet values into the ghost slots.

  For cell centered since the boundary value is the average of the ghost cell and the first cell I don't know how to do this because I don't know what one should put into the ghost location. Maybe include the "ghost" cell as a real point (don't use boundary_ghosted) and make the equation for that last cell force the average of that slot and its neighbor to equal the Dirichlet value.


   Barry

On Sep 26, 2013, at 11:28 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Sep 26, 2013 at 9:25 AM, Mark F. Adams <mfadams at lbl.gov> wrote:
> I am interested in making a cell centered test problem with Dirichlet BCs.  I want to go high(ish) order and doing explicit stencils (preferred) looks rather messy and I am interest in using ghost cells.  I assume this would entail using DMDA_BOUNDARY_GHOSTED.  I do not see any examples or documentation on using this other than just creating it.  Any suggestions?  I would think that there would have to be callback function for the user to set ghost values but I am not seeing anything like this around.  Perhaps I am not understand the intent of DMDA_BOUNDARY_GHOSTED.
> 
> My understanding is that it adds a single layer of ghost cells around the domain boundary. Then these show up in the gxm, etc. so that
> you can put values there if you want.
> 
>   Matt
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> Mark
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> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener