[petsc-users] Help for MatNullSpaceCreateRigidBody

[Jordi Manyer Fuertes]

Thanks for the prompt response. Both answers look like what I'm doing.

After playing a bit more with solver, I managed to make it run in 
parallel with different boundary conditions (full dirichlet bcs, vs 
mixed newmann + dirichlet). This raises two questions:

- How relevant are boundary conditions (eliminating dirichlet rows/cols 
vs weak newmann bcs) to the solver? Should I modify something when 
changing boundary conditions?

- Also, the solver did well with the old bcs when run in a single 
processor (but not in parallel). This seems odd, since parallel and 
serial behavior should be consistent (or not?). Could it be fault of the 
PCGAMG? I believe the default local solver is ILU, shoud I be changing 
it to LU or something else for these kind of problems?

Thank you both again,

Jordi


On 5/12/23 04:46, Matthew Knepley wrote:
> On Mon, Dec 4, 2023 at 12:01 PM Jordi Manyer Fuertes via petsc-users 
> <petsc-users at mcs.anl.gov> wrote:
>
>     Dear PETSc users/developpers,
>
>     I am currently trying to use the method `MatNullSpaceCreateRigidBody`
>     together with `PCGAMG` to efficiently precondition an elasticity
>     solver
>     in 2D/3D.
>
>     I have managed to make it work in serial (or with 1 MPI rank) with
>     h-independent number of iterations (which is great), but the solver
>     diverges in parallel.
>
>     I assume it has to do with the coordinate vector I am building the
>     null-space with not being correctly setup. The documentation is
>     not that
>     clear on which nodes exactly have to be set in each partition.
>     Does it
>     require nodes corresponding to owned dofs, or all dofs in each
>     partition
>     (owned+ghost)? What ghost layout should the `Vec` have?
>
>     Any other tips about what I might be doing wrong?
>
>
> What we assume is that you have some elastic problem formulated in 
> primal unknowns (displacements) so that the solution vector looks like 
> this:
>
>   [ d^0_x d^0_y d^0_z d^1_x ..... ]
>
> or whatever spatial dimension you have. We expect to get a global 
> vector that looks like that, but instead
> of displacements, we get the coordinates that each displacement 
> corresponds to. We make the generators of translations:
>
>   [ 1 0 0 1 0 0 1 0 0 1 0 0... ]
>   [ 0 1 0 0 1 0 0 1 0 0 1 0... ]
>   [ 0 0 1 0 0 1 0 0 1 0 0 1... ]
>
> for which we do not need the coordinates, and then the generators of 
> rotations about each axis, for which
> we _do_ need the coordinates, since we need to know how much each 
> point moves if you rotate about some center.
>
>   Does that make sense?
>
>    Thanks,
>
>       Matt
>
>     Thanks,
>
>     Jordi
>
>
>
> -- 
> 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.cse.buffalo.edu/~knepley/>
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