[petsc-users] Semi-coarsening for GMG using DMDA?

[Joe Alexandersen]

Great, thanks for the input so far. We will wait for Matt's response soonish.

Sincerely,

Joe Alexandersen
Associate Professor
DFF Sapere Aude Research Leader

The Faculty of Engineering
Institute of Mechanical and Electrical Engineering
SDU Mechanical Engineering

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________________________________
From: Mark Adams <mfadams at lbl.gov>
Sent: Thursday, March 20, 2025 5:00:31 pm
To: Barry Smith <bsmith at petsc.dev>
Cc: Joe Alexandersen <joal at sdu.dk>; petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Subject: Re: [petsc-users] Semi-coarsening for GMG using DMDA?

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We have worked on semi coarsening in DMPlex, but it is not finished and we are not working on it now.

I'm not sure about how easy it would be in DMDA, but Barry is suggesting that it is doable.

We need to wait for Matt and he is on travel so his response may be delayed.

Mark

On Thu, Mar 20, 2025 at 11:34 AM Barry Smith <bsmith at petsc.dev<mailto:bsmith at petsc.dev>> wrote:

   In theory you can do as you propose. In the context below uniform refinement" only means that the coordinates of the DMDA are ignored so each refinement. The interpolation is fine woth different refinements in the different coordinate directions.

  Barry


On Mar 20, 2025, at 5:56 AM, Joe Alexandersen via petsc-users <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>> wrote:

Dear PETSc developers,

We are working with a code that uses regular meshes (DMDA) and geometric multigrid. We would like to go from uniform coarsening/refinement to semi-coarsening/refinement, due to anisotropy in our underlying equations. We have tried to figure out if we can do this using built-in functions of PETSc, but it is unclear to us whether we can get it done relatively easily.

It seems that we can go from the coarsest grid and refine differently in each direction using DMDASetRefinementFactor and then use DMRefine to define the finer levels. However, from the doc page for DMCreateInterpolation, it states that it only works for "uniform refinement" which to me seems to indicate it will not work with different refinement in each direction. But on the other hand, it states that it should work if using DMRefine, which I assume used the information from DMDASetRefinementFactor upon creation?

So our questions are: is there are feasible and relatively simple way to do semi-coarsening/refinement of DMDAs for geometric multigrid hierarchies? Would the above work?

Thanks in advance!

Sincerely,
Joe Alexandersen
University of Southern Denmark


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