[petsc-users] Nullspaces

[Mark Adams]

Can you please give more details on what 'does not work' is.
More detail on how you judge what works would also be useful.
Mark

On Tue, Dec 7, 2021 at 11:19 AM Marco Cisternino <
marco.cisternino at optimad.it> wrote:

> Good morning,
>
> I’m still struggling with the Poisson equation with Neumann BCs.
>
> I discretize the equation by finite volume method and I divide every line
> of the linear system by the volume of the cell. I could avoid this
> division, but I’m trying to understand.
>
> My mesh is not uniform, i.e. cells have different volumes (it is an octree
> mesh).
>
> Moreover, in my computational domain there are 2 separated sub-domains.
>
> I build the null space and then I use MatNullSpaceTest to check it.
>
>
>
> If I do this:
>
> MatNullSpaceCreate(getCommunicator(), PETSC_TRUE, 0, nullptr, &nullspace);
>
> It works
>
>
>
> If I do this:
>
> Vec nsp;
>
> VecDuplicate(m_rhs, &nsp);
>
> VecSet(nsp,1.0);
>
> VecNormalize(nsp, nullptr);
>
> MatNullSpaceCreate(getCommunicator(), PETSC_FALSE, 1, &nsp, &nullspace);
>
> It does not work
>
>
>
> Probably, I have wrong expectations, but should not it be the same?
>
>
>
> Thanks
>
>
>
> Marco Cisternino, PhD
> marco.cisternino at optimad.it
>
> ______________________
>
> Optimad Engineering Srl
>
> Via Bligny 5, Torino, Italia.
> +3901119719782
> www.optimad.it
>
>
>
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