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

[David Knezevic]

On Sun, Mar 8, 2015 at 10:31 AM, Mark Adams <mfadams at lbl.gov> wrote:

>
>
>
>> 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?
>


No, just high Young's modulus in subregions. Engineers sometimes implement
a rotation boundary condition about a node by including rigid (or, in
practice, very high Young's modulus) parts in a model. For example, imagine
including a rigid pyramid in a model, in which the tip of the pyramid is is
clamped so that the base of the pyramid (which is connected to a surface in
the model) can rotate about the tip.

This works fine with a direct solver, so I was curious about getting a good
iterative solver for this case too.



> Nearly incompressible is hard.  bigger smoothers, like ASM, can help.
>

Agreed. But the Poisson ratio is still just 0.3 or so in this context, so
locking and near incompressibility aren't an issue here.

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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