[petsc-users] Ordering of DoFs in submatrices with PCFieldsplit

[Blauth, Sebastian]

Dear PETSc developers and users,



I have a question regarding the Fieldsplit preconditioner in PETSc. In particular, I want to know how the submatrices there are created from the parent matrix. The "obvious" way would be to take the DoF indices of the corresponding split and "renumber" them so that the DoFs in the submatrix have the same order as the ones of the parent matrix. I did not find any documentation on this and as it is at least possible that the DoFs are re-ordered, I wanted to ask this question. Obviously, in case the DoFs are re-ordered, how can I get the mapping between the DoFs of the parent and the submatrix?



The thing I am wanting to work on is implementing a pressure convection diffusion preconditioner with FEniCS for the incompressible Navier-Stokes equations. The parent matrix is assembled via a mixed FEM and then I use PETSc to solve the system. I want to assemble the corresponding operators on the pressure space from a collapsed (i.e. sub-space of the mixed FEM) function space. However, FEniCS re-orders the DoFs there, but I can get a mapping between the DoFs so this should not be problematic. However, I am not sure if PETSc also does a re-ordering.



Thanks a lot in advance and best regards,

Sebastian



--

Dr. Sebastian Blauth

Fraunhofer-Institut für

Techno- und Wirtschaftsmathematik ITWM

Abteilung  Transportvorgänge

Fraunhofer-Platz 1, 67663 Kaiserslautern

Telefon: +49 631 31600-4968

sebastian.blauth at itwm.fraunhofer.de<mailto:sebastian.blauth at itwm.fraunhofer.de>

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