[petsc-users] 2 Dirichlet conditions for one Element in PetscFE

Matthew Knepley knepley at gmail.com
Wed Nov 22 11:53:47 CST 2017

On Wed, Nov 22, 2017 at 12:39 PM, Yann Jobic <yann.jobic at univ-amu.fr> wrote:

> Hello,
> I've found a strange behavior when looking into a bug for the pressure
> convergence of a simple Navier-Stokes problem using PetscFE.
> I followed many examples for labeling boundary faces. I first use
> DMPlexMarkBoundaryFaces, (label=1 to the faces).
> I find those faces using DMGetStratumIS and searching 1 as it is the value
> of the marked boundary faces.
> Finally i use DMPlexLabelComplete over the new label.
> I then use :
>   ierr = PetscDSAddBoundary(prob, DM_BC_ESSENTIAL, "in", "Faces", 0,
> Ncomp, components, (void (*)(void)) uIn, NWest, west, NULL);CHKERRQ(ierr);
> in order to impose a dirichlet condition for the faces labeled by the
> correct value (west=1, south=3,...).
> However, the function "uIn()" is called in all the Elements containing the
> boundary faces, and thus impose the values at nodes that are not in the
> labeled faces.
> Is it a normal behavior ? I then have to test the position of the node
> calling uIn, in order to impose the good value.
> I have this problem for a Poiseuille flow, where at 2 corner Elements i
> have a zero velocity dirichlet condition (wall) and a In flow velocity one.

I believe I have fixed this in knepley/fix-plex-bc-multiple which should be
merged soon. Do you know how to merge that branch and try?



> The pressure is then very high at the corner nodes of those 2 Elements.
> Do you think my pressure problem comes from there ? (The velocity field is
> correct)
> Many thanks,
> Regards,
> Yann
> PS : i'm using those runtime options :
> -vel_petscspace_order 2 -pres_petscspace_order 1 \
> -ksp_type fgmres -pc_type fieldsplit -pc_fieldsplit_type schur
> -pc_fieldsplit_schur_fact_type full  \
> -fieldsplit_velocity_pc_type lu -fieldsplit_pressure_ksp_rtol 1.0e-10
> -fieldsplit_pressure_pc_type jacobi
> ---
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> logiciel antivirus Avast.
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What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
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