[petsc-dev] DM field interfaces

[Jed Brown]

[migrating from petsc-maint]

On Sat, Feb 25, 2012 at 11:45, Barry Smith <bsmith at mcs.anl.gov> wrote:

> > I agree, but it would be quite helpful to refactor DM_DA have a section
> > that holds topology and geometry, plus the (very small) extra information
> > that depends on the number of dofs per point. Then the common part could
> be
> > shared, making all the DMDAs coming out of the split easier to work with.
> >
>
>    this is true but completely irrelevent here
>

My understanding is that much of the reason Matt wants to use PetscSection
in places is because he feels like the DM implementation is quite heavy, so
if you want to address a subset, making a new DM would involve heavy
copies. I don't want PetscSection because I think the algebraic components
should have access to the richer information in a DM, but I also don't want
full copies of heavy DMs. If we can implement topology/geometry sharing
without much complexity, we shouldn't need to use PetscSection.


>
> > What interface are we going to use for splitting out a DM for a vector
> > field in a larger system? How do we get the bs=2 velocity DM out of the
> > lid-driven cavity's bs=4 velocity, vorticity, and temperature DM?
> >
>
>   again this is irrelevent here because the same issue is true for the ISs.
>
>  But
>
>  I explained this in my previous email:  DMPushSubDMType(DM,"name of a
> type of splitting like u,v,p or velocity,p"); DMPopSubDMType();  it is not
> perfect but a start. See that email for more details.


// Conservative gas dynamics with fields ordered as [rho, rho*u, rho*v,
rho*w, E]
DMGetFieldSplitting(dm,"conservative-vector",&fscons); // creates if it
doesn't exist
DMFieldSplittingAddField(dm,fscons,"rho",{0});
DMFieldSplittingAddField(dm,fscons,"momentum",{1,2,3});
DMFieldSplittingAddField(dm,fscons,"Energy",{4});

DMGetFieldSplitting(dm,"primitive-vector",&fsprim);
DMFieldSplittingSetPF(dm,fsprim,UserEquationOfStatePF,UserEquationOfStateInversePF);
DMFieldSplittingAddField(dm,fsprim,"velocity",{0,1,2});
DMFieldSplittingAddField(dm,fsprim,"pressure",{3});
DMFieldSplittingAddField(dm,fsprim,"Temperature",{4});

If a transform is not specified, the indices would address the original
state variables.

Then PCFieldSplit and eventual nonlinear and time-stepping splitting
methods would call

DMGetFieldSplitting(dm,&nsplits,&splits);
// choose split number to use
split = splits[splitidx];
DMFieldSplittingGetName(dm,split,&splitname);
DMFieldSplittingGetFields(dm,split,&pf,&pfinv,&nfields,&isfields,&dmfields);

If pf is not NULL, apply PF to the state variables, then isfields[i]
denotes what to extract in order to live in dmfields[i], pfinv is needed to
put a contribution back into the coupled dm.
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