[petsc-users] Removing rigid body motion using Null Space
Barry Smith
bsmith at mcs.anl.gov
Tue Jul 28 16:44:31 CDT 2015
> On Jul 28, 2015, at 4:33 PM, Jed Brown <jed at jedbrown.org> wrote:
>
> Hom Nath Gharti <hng.email at gmail.com> writes:
>
>> Thanks for your response. My example is also a elasticity problem
>> where displacment is a vector-valued.
>
> You called it "electrostatic".
To be fair he said
> a simple
> elastostatic problem. I have an unconstrained sphere and I need to
> compute displacement field due to a point load on the surface.
which made it clear to me that it involved mechanics as well.
> Anyway, use the actual null space for
> your operator and run the tests I mentioned.
>
>>
>> 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.
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