[petsc-users] Combining DMDA and DMStag

[Carl-Johan Thore]

Thanks for the quick replies!

Using dmstag for the temperature, or even for both fields is a possibility of course. Preferably however I'd like to keep the temp. code as it is and be able to quickly switch between different methods for the flow, based on dmda, dmstag and so on, so I'll try a bit more with the dmda-dmstag approach first.





________________________________
From: Patrick Sanan <patrick.sanan at gmail.com>
Sent: 22 April 2022 17:04
To: Barry Smith <bsmith at petsc.dev>
Cc: Carl-Johan Thore <carl-johan.thore at liu.se>; petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Subject: Re: [petsc-users] Combining DMDA and DMStag



Am Fr., 22. Apr. 2022 um 16:04 Uhr schrieb Barry Smith <bsmith at petsc.dev<mailto:bsmith at petsc.dev>>:

   We would need more details as to exactly what goes wrong to determine any kind of fix; my guess would be that the layout of the velocity vectors and temperature vectors is slightly different since the DMDA uses nelx+1 while the stag uses nelx and may split things up slightly differently in parallel. You could try very small problems with say 2 or 4 ranks and put known values into the vectors and look at the ghost point update locations and exact locations in the local and global vectors to make sure everything is where you expect it to be.

  There is also the possibility of using a DMStag for the temperature instead of DMDA since DMDA provides essentially a subset of the functionality of DMDA to get a more consistent layout of the unknowns in the two vectors.


This was going to be my first suggestion as well - one way to ensure compatibility would be to use DMStag for everything. E.g. you could create your temperature DMStag with only vertex/corner (dof30 degrees of freedom, and then create one or more a "compatible" DMStags (same elements on each MPI rank) for your other fields, using DMStagCreateCompatibleDMStag() .

  Finally, one could use a single DMStag with all the unknowns but treat subvectors of the unknowns differently in your discretization and solve process. So assign velocity values (I guess) to the cell faces and temperature to the cell vertices, this will give consistent parallel decomposition of values but you will have to manage the fact that the unknowns are interlaced into a single vector so your solver portions of the code may need to "pull" out appropriate subvectors.

  Barry


On Apr 22, 2022, at 9:45 AM, Carl-Johan Thore <carl-johan.thore at liu.se<mailto:carl-johan.thore at liu.se>> wrote:

Hi!

I'm working on a convection-diffusion heat transfer problem. The temperature
is discretized using standard Q1 elements and a DMDA. The flow is modelled
using a stabilized Q1-Q0 method for which DMStag seemed like a good choice. The codes for the temperature
and flow work fine separately (both in serial and parallel), but when combined and running
in parallel, a problem sometimes arises in the assembly of the thermal system matrix.
Here’s a rough sketch of the combined code:

// Create dmda for thermal problem and dmstag for flow problem
DMDACreate3d(PETSC_COMM_WORLD, bx, by, bz, DMDA_STENCIL_BOX, nelx+1, nely+1, nelz+1, PETSC_DECIDE, PETSC_DECIDE, PETSC_DECIDE,
                1, stencilWidth, 0, 0, 0, &dmthermal);
…
// A bit of code to adjust Lx,Ly,Lz so that dmthermal and dmflow are compatible in the sense of having the same
// local elements
…
DMStagCreate3d(PETSC_COMM_WORLD, bx, by, bz, nelx, nely, nelz, md,nd,pd,
3,0,0,0,DMSTAG_STENCIL_BOX,stencilWidth,Lx,Ly,Lz,&dmflow);

PetscInt edofT[8];          // 8-noded element with 1 temp DOF per node
DMStagStencil edofF[24];       // 8 nodes with 3 velocity DOFs each

// Assemble thermal system matrix K
for (PetscInt e=0 ...)   // Loop over local elements
{
// Populate edofF, edofT
                             // Get element velocities in ue from local velocity vector uloc
                             DMStagVecGetValuesStencil(dmflow,uloc,24,edof,ue);
                                                          ...
                             Ke = Ke_diffusion + Ke_convection(ue)
                                                          ...
                             MatSetValuesLocal(K, 8, edofT, 8, edofT, Ke, ADD_VALUES);
}

This always works fine in serial, but depending on the mesh and the number of ranks,
we don't always get the correct values in the element velocity vector ue. I suspect
this has something to do with the ordering of the elements and/or the DOFs, because
the elements in the global velocity vector are always the same but their order may change
(judging from the output of VecView at least).

Is it possible to ensure compatibility between the dm:s, or find some kind of mapping
between them, so that something along the lines of the code above always works?

Kind regards,
Carl-Johan

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