[petsc-users] Removing rigid body motion using Null Space

[Jed Brown]

Hom Nath Gharti <hng.email at gmail.com> writes:

> Dear all,
>
> After searching old mailing lists, it appears that the rigid body
> motion

"rigid body motion" refers to rotations and translations, which appear
in vector-valued problems like elasticity or fluids.

> can be removed using Null Space features in PETSc.  Unfortunately, I
> could not successfully implement this to a simple elastostatic
> problem. 

What is vector-valued in your electrostatic problem?

> I have an unconstrained sphere and I need to compute displacement
> field due to a point load on the surface.
>
> I tried:
> call MatNullSpaceCreate(PETSC_COMM_WORLD,PETSC_TRUE,0,0,nullspace,ierr)
> call MatSetNullSpace(Kmat,nullspace,ierr)
> call MatNullSpaceRemove(nullspace,bvec,PETSC_NULL_OBJECT,ierr)
> call MatNullSpaceDestroy(nullspace,ierr)
>
> But the solution did not converge! Any help/hint would be greatly appreciated.

Check that this really is the null space of the operator.  You can use
-ksp_test_null_space, for example.

Check that the preconditioner is stable.  Use a simple preconditioner on
a tiny problem to start with.
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