[petsc-users] Preconditioner for a elasticity problem with Robin boundary conditions

[Matthew Knepley]

On Mon, May 14, 2018 at 5:45 PM, Salazar De Troya, Miguel <
salazardetro1 at llnl.gov> wrote:

> Hello,
>
>
>
> Up until now, I have been solving my elasticity problem using the gamg
> preconditioner with the following options:
>
>
>
> -ksp_monitor_true_residual
>
> -ksp_converged_reason
>
> -ksp_type cg
>
> -log_view
>
> -mg_levels_esteig_ksp_type cg
>
> -mg_levels_ksp_chebyshev_esteig_steps 10
>
> -mg_levels_ksp_type chebyshev
>
> -mg_levels_pc_type sor
>
> -pc_type gamg
>
> -pc_gamg_verbose 7
>
> -pc_gamg_type agg
>
> -pc_gamg_agg_nsmooths 1
>
> -pc_gamg_threshold 0.001
>
> -snes_linesearch_type basic
>
> -snes_atol 1e-6
>
> -ksp_atol 1e-7
>
> -ksp_rtol 1e-9
>
> -ksp_norm_type unpreconditioned
>
>
>
> It has worked great so far, even for my problem where I have two materials
> with more than 1e3 ratio.
>
>
>
> Now, when I add robin boundary conditions, the solver fails with
> DIVERGED_INDEFINITE_PC. Using -ksp_type bcgs makes the solver converge. I
> imagine it is because it can handle the indefinite PC. Why is the PC
> indefinite for the Robin problem?
>

It matters how you are enforcing the conditions. Just write out the problem
completely for 1 vertex constrained. It should be clear whether its
symmetric or not.


> Is there any way to make it positive definite?
>

It should be possible if the BC are imposed in the right way.


> I am also passing the rigid body modes of my mesh to the solver. I am not
> sure if these modes change with the Robin boundary conditions, they should
> not, should they?
>

No, these are precisely the null modes of the symbol, and not affected by
BC.

  Thanks,

    Matt


> Thanks
>
> Miguel
>
>
>
> --
>
>
>



-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
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