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

[Mark Adams]

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.

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.

I'm puzzled,
Mark


On Mon, Mar 23, 2020 at 8:32 PM Xiaodong Liu <xliu29 at ncsu.edu> wrote:

> Hi, all,
>
> I want to confirm one thing about the interpolation and restrict matrix
> for the cell-centered multigrid.
>
> I am running ex32.c. The following is my understanding.
>
> For cell-centered multigrid, only DMDA_Q0 can be set as the interpolation
> type.
> For interpolation from the coarse mesh, the values for the 4 finer cells
> are set equal to that of coarse cell. Then the restriction matrix is the
> inverse of the interpolation one for Galerkin type.
>
> If I want to use the bilinear interpolation, I need to code the subrotuine
> myself, right?
>
> Please double check whether my understanding is right.
>
> Thanks A LOT.
> Xiaodong Liu, PhD
> X: Computational Physics Division
> Los Alamos National Laboratory
> P.O. Box 1663,
> Los Alamos, NM 87544
> 505-709-0534
>
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