[petsc-users] Advice Being Sought

[David Scott]

Thanks for the suggestion.

I had tried '-pc_type gamg -pc_gamg_agg_nsmooths 1' with an earlier 
version of the code without success. I have tried it again but I get 
NaN's after only 90 time steps whereas with block Jacobi it runs quite 
happily for 36,000 time steps and produces physically sensible results.

David

On 22/04/2013 13:12, Matthew Knepley wrote:
> On Mon, Apr 22, 2013 at 7:56 AM, David Scott <d.scott at ed.ac.uk
> <mailto: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


-- 
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.