[petsc-users] Fwd: Interpolation in staggered grid

[Anton Popov]

Hi Manuel,

you might consider useful taking a look at this code 
https://bitbucket.org/bkaus/lamem, which is a multi-DMDA staggered grid 
implementation.

Also, as Matt pointed out, there is a direct PETSc support for staggered 
grids (DMSTAG), recently contributed by Patrick Sanan.

Thanks,

Anton


On 15.10.2018 23:22, Manuel Valera wrote:
>
> Thanks Matthew, i have made some progress but i am still unsure on how 
> to proceed to make the DMDAs work as intended, i will try to lay out 
> what i am trying now:
>
> I was able to port the interpolation into the DMDA model but it works 
> only in serial runs, and it becomes unstable for more than one 
> processor running,
>
> What i do is roughly this:
>
>   * Calculate density at a cell-centered DMDA object, using a local
>     vector so i can access the ghost rows (indices -1 and max+1)
>   * Interpolate into a face-centered DMDA object for velocities, using
>     also a local vector.
>
> Doing this i get the right results using the same interpolation i used 
> for my non-petsc implementation of the model, as long as i use only 
> one processor but the doubts i have are:
>
>   * How to use the local vectors properly: Is operating on them the
>     recommended course in this case?
>   * How can i access the ghost indices in the global vector so i can
>     then communicate GlobalToLocal? would this be a better strategy?
>   * I feel is wrong to interpolate a cell-centered based vector into a
>     face-centered based vector, using the indices of the latter, what
>     strategy would work best in this case?
>
> I also tried opening a global vector with a different DA layout than 
> the one created (opening global density on the velocities DMDA layout) 
> and this introduced an error in the GlobalToLocal, LocalToLocal, 
> LocalToGlobal communication, something didn't happen if i used the 
> local versions of these vectors instead,
>
> Thanks for your help,
>
> Manuel
>
>
>
>
>
> On Sat, Oct 6, 2018 at 4:45 AM Matthew Knepley <knepley at gmail.com 
> <mailto:knepley at gmail.com>> wrote:
>
>     On Fri, Oct 5, 2018 at 6:49 PM Manuel Valera <mvalera-w at sdsu.edu
>     <mailto:mvalera-w at sdsu.edu>> wrote:
>
>         Hello,
>
>         I'm trying to do a simple variable interpolation, from a cell
>         center to a face in a staggered grid, my model data management
>         is done with DMDAs, with two different DMs one for each cell
>         position,
>
>         I already did this task on a fortran only version of the model
>         using the 4 closest neighbors of the scalars (cell center) to
>         be interpolated at the velocity site (cell face), i did it
>         using a loop over the domain, somehow this easy task is not
>         translating into the DMDA framework,
>
>
>     Its not clear to me what problem you are having. We have done this
>     before. For example,
>
>     https://www.mcs.anl.gov/petsc/petsc-current/src/snes/examples/tutorials/ex30.c.html
>
>     I would note that the development version of PETSc now has DMStag
>     which supports staggered grid discretizations directly.
>
>       Thanks,
>
>         Matt
>
>         I'm not sure what I'm doing wrong or not aware of a easier way
>         Petsc may have for this task, if you could point out an easier
>         strategy or an example I would be grateful,
>
>         Thanks,
>
>
>
>
>
>
>     -- 
>     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
>
>     https://www.cse.buffalo.edu/~knepley/
>     <http://www.cse.buffalo.edu/%7Eknepley/>
>

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