ex18 has the same issue, but this aspect can probably be fixed by setting DMDA_Q0.
Patrick Sanan <patrick.sa...@gmail.com> writes: > This sort of feedback is great in terms of learning what can move out of > "tutorials" and into "tests". > https://gitlab.com/petsc/petsc/-/merge_requests/2629 > > > Am Di., 24. März 2020 um 16:22 Uhr schrieb Jed Brown <j...@jedbrown.org>: > >> Mark Adams <mfad...@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. >>