[petsc-users] left and right preconditioning with a constant null space

[Matthew Knepley]

On Wed, Mar 22, 2017 at 2:58 PM, Klaij, Christiaan <C.Klaij at marin.nl> wrote:

> I'm solving the Navier-Stokes equations using PCFieldSplit type
> Schur and Selfp. This particular case has only Neumann conditions
> for the pressure field.
>
> With left preconditioning and no nullspace, I see that the KSP
> solver for S does not converge (attachment "left_nonullsp") in
> either norm.
>
> When I attach the constant null space to A11, it gets passed on
> to S and the KSP solver for S does converge in the preconditioned
> norm only (attachment "left").
>
> However, right preconditioning uses the unpreconditioned norm and
> therefore doesn't converge (attachment "right"), regardless of
> whether the nullspace is attached or not. Should I conclude that
> right preconditioning cannot be used in combination with a null
> space?
>

No, neither of your solves is working. The left preconditioned version is
just hiding that fact.

You should start by checking that the exact solve works. Namely full Schur
factorization
with exact solves for A and S. Since you do not have a matrix for S (unless
you tell it do
use "full"), just use a very low (1e-10) tolerance. My guess is that
something is off with
your null space specification.

  Thanks,

     Matt


> Chris
>
>
> dr. ir. Christiaan Klaij  | Senior Researcher | Research & Development
> MARIN | T +31 317 49 33 44 | mailto:C.Klaij at marin.nl | http://www.marin.nl
>
> MARIN news: http://www.marin.nl/web/News/News-items/H2020-marinergi-
> project-launched.htm
>
>


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