[petsc-users] Advice Being Sought

[Mark F. Adams]

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.
>