On Apr 22, 2013, at 12:26 PM, David Scott <d.scott at ed.ac.uk> wrote:
> 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. > I have had good success with the robustness of hypre. So you might want to try that. It sounds like these pressure solves are changing over time. Using -pc_gamg_reuse_interpolation false with GAMG will make it more robust to that. > 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. >
