[petsc-users] About the interpolation and restriction matrix for cell-centered multigrid.

[Xiaodong Liu]

Thanks, Mark and Jed. It is very helpful.
So, for present, Petsc doesn't support Q1 interperpolation for
cell-centered multigrid, right?

Xiaodong Liu, PhD
X: Computational Physics Division
Los Alamos National Laboratory
P.O. Box 1663,
Los Alamos, NM 87544
505-709-0534


On Tue, Mar 24, 2020 at 8:22 AM Jed Brown <jed at jedbrown.org> wrote:

> Mark Adams <mfadams at lbl.gov> writes:
>
> > Good question. It does look like there is Q1:
> >
> > src/dm/impls/da/da.c:-  ctype - DMDA_Q1 and DMDA_Q0 are currently the
> only
> > supported forms
> >
> > And in looking at a cell centered
> > example src/snes/examples/tutorials/ex20.c, it looks like only DMDA_Q1
> > works. I get an error when I set it to DMDA_Q0 (DMDA_Q1 is the default).
> > This is puzzling, Q0 is natural in cell centered.
>
> The comments in those examples are kinda wrong -- they never told the DM
> it was cell-centered so it uses a multigrid that isn't compatible with
> the boundary conditions.  The interpolation is Q1 on the dual grid, not
> conservative Q1 on cells.
>
> > I am not familiar with DMDA and I don't understand why, from ex20, that
> you
> > have an odd number of points on a cell centered grid and an even number
> for
> > vertex centered (eg, ex14). I would think that it should be the opposite.
>
> The example is bad.
>
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