[petsc-users] DIVERGED_INDEFINITE_PC in algebraic multigrid

[Michele Rosso]

Jed,

thank you very much!
I will try with ///-mg_levels_ksp_type chebyshev -mg_levels_pc_type 
sor/   and report back.
Yes, I removed the nullspace from both the system matrix and the rhs.
Is there a way to have something similar to Dendy's multigrid or the 
deflated conjugate gradient method with PETSc?
Thank you,

Michele



//
On 05/19/2014 03:30 PM, Jed Brown wrote:
> Michele Rosso <mrosso at uci.edu> writes:
>
>> Jed,
>>
>> thanks for your reply.
>> By using the options you suggested, namely /-mg_levels_ksp_type
>> richardson -mg_levels_pc_type sor/, I was able to
>> solve without bumping into the DIVERGED_INDEFINITE_PC message.
>> Nevertheless, the number of iterations increases drastically as the
>> simulation progresses.
> What about SOR with Chebyshev?  (A little weird, but sometimes it's a
> good choice.)  If the solve is expensive, you can add a few more
> iterations for eigenvalue estimation.
>
>> The Poisson's equation I am solving arises from a variable-density
>> projection method for incompressible multi-phase flows.
>> At each time step the system matrix coefficients change as a consequence
>> of the change in location of the heavier phase; the rhs changes
>> in time because of the change in the velocity field. Usually the
>> black-box multigrid or the deflated conjugate gradient method are used
>> to solve efficiently this type of problem: it is my understanding -
>> please correct me if I am wrong - that AMG is a generalization of the
>> former.
> Dendy's "black-box MG" is a semi-geometric method for cell-centered
> discretizations.  AMG is not a superset or subset of those methods.
>
>> The only source term acting is gravity; the hydrostatic pressure is
>> removed from the governing equation in order to accommodate periodic
>> boundary conditions: this is more a hack than a clean solution. Could it
>> be the reason behind the poor performances/ DIVERGED_INDEFINITE_PC
>> problem I am experiencing?
> If you have periodic boundary conditions, then you also have a pressure
> null space.  Have you removed the null space from the RHS and supplied
> the null space to the solver?

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