[petsc-users] Advice on setting BCs in a PLEX

[Aldo Bonfiglioli]

Dear users,

I have been using petsc's KSP for over 20 years and I am considering 
using DMPLEX to replace my own data structure in a mixed FEM/FVM CFD code.

To do so, I am trying to understand DMPLEX by writing a 1D code that 
solves u_t = u_xx using PSEUDOTS.

In order to prescribe Dirichlet BCs at the endpoints of the 1D box, I 
use PetscSectionSetConstraintDof when building the PetscSection.

> A global vector is missing both the shared dofs which are not owned by 
> this process, as well as /constrained/ dofs. These constraints 
> represent essential (Dirichlet) boundary conditions. They are dofs 
> that have a given fixed value, so they are present in local vectors 
> for assembly purposes, but absent from global vectors since they are 
> never solved for during algebraic solves.

My global Vec has indeed two entries less than the local one.

When initializing the solution or evaluating the rhs function I transfer 
data from the global to local representation, do the calculation, then 
transfer back.

I am doing something wrong, though, which shows up in the attached log file.

My simple code is also attached.

Thank you for your advice.

Aldo

-- 
Dr. Aldo Bonfiglioli
Associate professor of Fluid Machines
Scuola di Ingegneria
Universita' della Basilicata
V.le dell'Ateneo lucano, 10 85100 Potenza ITALY
tel:+39.0971.205203 fax:+39.0971.205215
web:https://urldefense.us/v3/__http://docenti.unibas.it/site/home/docente.html?m=002423__;!!G_uCfscf7eWS!cWgDpzKW9KGEkqxu775gDU488CBQ5itLl2l_HUsDKcPstbcuSm0_bZPHkMciXajTi3539fuMPe8TD1YZuirDfmSBMSXXxrQZWns$ 
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DM Object: box 1 MPI process
  type: plex
box in 1 dimension:
  Number of 0-cells per rank: 11
  Number of 1-cells per rank: 10
Labels:
  marker: 1 strata with value/size (1 (2))
  Face Sets: 2 strata with value/size (1 (1), 2 (1))
  depth: 2 strata with value/size (0 (11), 1 (10))
  celltype: 2 strata with value/size (0 (11), 1 (10))
[0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
[0]PETSC ERROR: Invalid argument
[0]PETSC ERROR: Point 10: Global dof 0 != 1 size - number of constraints
[0]PETSC ERROR: WARNING! There are unused option(s) set! Could be the program crashed before usage or a spelling mistake, etc!
[0]PETSC ERROR:   Option left: name:-ts_max_steps value: 10 source: command line
[0]PETSC ERROR:   Option left: name:-ts_monitor (no value) source: command line
[0]PETSC ERROR: See https://petsc.org/release/faq/ for trouble shooting.
[0]PETSC ERROR: Petsc Release Version 3.22.0, Sep 28, 2024 
[0]PETSC ERROR: ./ex15f90 with 1 MPI process(es) and PETSC_ARCH linux_gnu on DESKTOP-04AR6V6.lan by abonfi Mon Nov  4 19:28:00 2024
[0]PETSC ERROR: Configure options: --with-mpi=1 --with-mpi-dir=/usr/lib64/mpi/gcc/mpich --with-shared-libraries=1 --with-debugging=1 --with-blas-lib=/usr/lib64/libblas.a --with-lapack-lib=/usr/lib64/liblapack.a --with-triangle=1 --download-triangle=yes
[0]PETSC ERROR: #1 PetscSFSetGraphSection() at /home/abonfi/src/petsc-3.22.0/src/vec/is/sf/utils/sfutils.c:179
[0]PETSC ERROR: #2 DMCreateSectionSF() at /home/abonfi/src/petsc-3.22.0/src/dm/interface/dm.c:4783
[0]PETSC ERROR: #3 DMGetSectionSF() at /home/abonfi/src/petsc-3.22.0/src/dm/interface/dm.c:4722
[0]PETSC ERROR: #4 DMGlobalToLocalBegin() at /home/abonfi/src/petsc-3.22.0/src/dm/interface/dm.c:2851
[0]PETSC ERROR: #5 DMGlobalToLocal() at /home/abonfi/src/petsc-3.22.0/src/dm/interface/dm.c:2812
[0]PETSC ERROR: #6 ex15f90.F90:280
Abort(62) on node 0 (rank 0 in comm 16): application called MPI_Abort(MPI_COMM_SELF, 62) - process 0
 there are           11  entries in depth:            0
 there are           10  entries in depth:            1
 Setting up a section on           11  meshpoints with            1  dofs
 The DM is marked
 Point           10 is a boundary point
 Point           20 is a boundary point
 Array u has type : seq                                                                             
 (Global) Array u has size :            9
 coordVec has size           11
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