[petsc-users] Advice Being Sought

[Matthew Knepley]

On Mon, Apr 22, 2013 at 7:56 AM, David Scott <d.scott at ed.ac.uk> wrote:

> Hello,
>
> I am working on a fluid-mechanical code to solve the two-phase
> Navier–Stokes equations with levelset interface capturing. I have been
> asked to replace the pressure calculation which uses the SOR and Jacobi
> iterative schemes with a Krylov subspace method. I have done this and the
> code is working but as I have never used PETSc before I would like to know
> if improvements to my code, or the run time parameters that I am using,
> could be made.
>
> I am using GMRES with a Block Jacobi pre-conditioner. I have tried
> Conjugate Gradient with a Block Jacobi pre-conditioner but it diverges. If
> I use GMRES for the first few thousand time steps and then swap to CG it
> does converge but the speed of execution is somewhat reduced.
>

Krylov methods do not work with preconditioners. You have a Poisson
problem, so as abundantly documented in the literature, you should use
multigrid. The easiest thing to try is

  -pc_type gamg -pc_gamg_agg_nsmooths 1

   Thanks,

     Matt


> I have attached relevant excerpts from the code.
>
> Yours sincerely,
>
> David Scott
> --
> Dr. D. M. Scott
> Applications Consultant
> Edinburgh Parallel Computing Centre
> Tel. 0131 650 5921
>
> The University of Edinburgh is a charitable body, registered in
> Scotland, with registration number SC005336.
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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