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

[David Knezevic]

On Tue, Mar 10, 2015 at 5:50 PM, Mark Adams <mfadams at lbl.gov> wrote:

> Penalty methods are hell on solvers.  There is a good change that GAMG
> will do the right thing with a finite threshold. and keep this stiff
> section separate.  You might want to start wit a small number of levels and
> a large coarse grid and get it working.  Then add more levels and see if it
> breaks.
>


OK, thanks for the help, much appreciated. I'll look into this.

Best regards,
David







>
> On Sun, Mar 8, 2015 at 11:38 AM, David Knezevic <
> david.knezevic at akselos.com> wrote:
>
>> 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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