[petsc-users] Near nullspace for preconditioners other than GAMG?

[Mark Adams]

On Sat, Mar 7, 2015 at 6:27 PM, David Knezevic <david.knezevic at akselos.com>
wrote:

> OK, thanks for letting me know.
>
> I've tried GAMG and ML with MatNullSpaceCreateRigidBody and those both
> work well for me. (I tried ML after Jed pointed out that ML also uses the
> near nullspace.)
>
> I have also tried hypre but the convergence hasn't been as good for the
> elasticity models I've been using, though I may not have been setting the
> hypre options in an optimal way for elasticity.
>

As Jed said, SA should be better for elasticity but it is worth testing.


> Another situation I've been meaning to ask about: If the elasticity model
> includes some "near rigid" regions (very high Young's modulus)
>

Do you mean Poisson ratio?  Nearly incompressible is hard.  bigger
smoothers, like ASM, can help.


> all iterative solvers I've tried fare poorly. I guess it's because the
> highly contrasting stiffnesses give a large condition number. Is there a
> standard way to get the preconditioner compensate for this?
>



>
> Thanks,
> David
>
>
>
> On Sat, Mar 7, 2015 at 5:57 PM, Mark Adams <mfadams at lbl.gov> wrote:
>
>> FYI, stüben used classical AMG for elasticity but he has articulated his
>> code for elasticity more than Hypre as I understand it.  Hypre can work OK
>> for elasticity in my experience.  Its worth a try.
>>
>> Mark
>>
>> On Thu, Mar 5, 2015 at 5:27 PM, David Knezevic <
>> david.knezevic at akselos.com> wrote:
>>
>>> OK, got it, thanks!
>>>
>>> David
>>>
>>>
>>> On Thu, Mar 5, 2015 at 5:08 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>
>>>> David Knezevic <david.knezevic at akselos.com> writes:
>>>> > I was just wondering if its possible to achieve the same sort of
>>>> thing with
>>>> > other AMG solvers (e.g. BoomerAMG)? I assume that MatSetNearNullSpace
>>>> does
>>>> > nothing for external solvers like hypre, right?
>>>>
>>>> It is used by ML (smoothed aggregation), but not BoomerAMG (classical
>>>> AMG) which uses an algorithm that doesn't have a natural place for such
>>>> information.  To my knowledge, classical AMG is not widely used for
>>>> elasticity.  It is very robust for M-matrices.
>>>>
>>>
>>>
>>
>
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