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

[Hom Nath Gharti]

Thanks for your response. My example is also a elasticity problem
where displacment is a vector-valued.



On Tue, Jul 28, 2015 at 3:38 PM, Jed Brown <jed at jedbrown.org> wrote:
> 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.