[petsc-users] Help for MatNullSpaceCreateRigidBody

[Matthew Knepley]

On Tue, Dec 5, 2023 at 7:57 AM Jordi Manyer Fuertes <jordi.manyer at monash.edu>
wrote:

> 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?
>
> The rigid body kernel is independent of boundary conditions, and is only
really important for the coarse grids. However, it is really easy to ruin a
solve with inconsistent boundary conditions, or with conditions which cause
a singularity at a change point.

> - 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?
>
This is unlikely. We have many parallel tests of elasticity (SNES ex17,
ex56, ex77, etc). We do not see problems. It seems more likely that the
system might not be assembled correctly in parallel. Did you check that the
matrices match?

> I believe the default local solver is ILU, shoud I be changing it to LU or
> something else for these kind of problems?
>
Do you mean the smoother for AMG? No, the default is Chebyshev/Jacobi,
which is the same in parallel.

  Thanks,

     Matt

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/>
>
>

-- 
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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