[CIG-SHORT] Convergence problem with Neumann BCs

[Matthew Knepley]

On Thu, Oct 31, 2013 at 7:21 PM, Eric Lindsey <elindsey at ucsd.edu> wrote:

> Thanks very much for the help everyone.
>
> Brad and Charles, thanks for pointing out the ill-posedness, I hadn't
> thought this through properly. After adding back some Dirichlet BC the
> solver does converge, in ~400 iterations. Thanks!
>
> The initial stress field also works nicely. In this case I just want
> uniform compression and shear stress, so I can impose it with a simple
> spatialDB with just the 3 components; I believe this is compatible with the
> Airy stress function. I had some confusion getting the other boundary
> conditions to be compatible with this setup as you mentioned, but think
> I've got it worked out -- the two sides with no Dirichlet BC need a Neumann
> condition to maintain the initial normal stress.
>
> Matt, I'm just using the linux binary package (1.9.0) -- but I grabbed a
> clone from git, and tried out these solvers. They both converged in only
> 3-5 iterations, but run much slower (~2min instead of ~1sec). I'm not sure
> what the metric is for a "better" solver I guess?
>

For any performance question, you need to send

  a) The output of --petsc.log_summary

  b) The output of --petsc.ksp_view

  c) The output of --petsc.ksp_monitor

So there are slides that go with the files that explain  the options. I
find 2min and 1s hard to believe. You can always
try them on the same benchmark problems that we used in the paper.

  Thanks,

     Matt


> Thanks,
> Eric
>
>
> On Thu, Oct 31, 2013 at 1:12 PM, Charles Williams <willic3 at gmail.com>wrote:
>
>> Hi Eric,
>>
>> Sorry to be slow in responding.  I just had a look at your problem, and
>> it appears that you have changed all the BC to Neumann, which means that
>> your displacements are completely unconstrained.  This is an ill-posed
>> problem, and I wouldn't expect it to work.  I'm not quite sure what you're
>> trying to represent, but I'm wondering whether initial stresses might be
>> what you want.  Alternatively, you need to pin at least 3 DOF (to prevent
>> translation/rotation).  Usually, with traction BC there is some sort of
>> symmetry, so you could probably fix one of your boundaries.  I'm assuming
>> your traction variations aren't that complicated, however, which means you
>> could use an Airy stress function to describe the stresses within the
>> domain (see, for example, the elasticity book by Timoshenko).  Your
>> displacement BC would have to be compatible, though.
>>
>> Cheers,
>> Charles
>>
>>
>> On 31/10/2013, at 11:43 AM, Eric Lindsey wrote:
>>
>> Hi,
>>
>> I'd like to apply background stresses to a simple elastic 2D plane strain
>> model in pylith, then add fault slip or other quasistatic deformation;
>> later I'll modify the material properties etc. Of course the stresses also
>> cause some deformation in the volume that I'd like to ignore. So I'm
>> imposing the slip at the second time step, with the hopes of subtracting
>> out the deformation from the first time step. (Is this the right way to do
>> this?)
>>
>> However, I noticed that the effects from the boundary conditions continue
>> to increase, and then oscillate, and don't stabilize until I let it run to
>> the 8th step or so (ideally I'd only use 2 time steps, I think). I think
>> the problem is with the solver's convergence; I see the message
>>
>> ...
>> 499 KSP Residual norm 9.952574507238e-04
>> 500 KSP Residual norm 9.952555275971e-04
>> Linear solve did not converge due to DIVERGED_ITS iterations 500
>>
>> I've taken most of the configuration straight from
>> examples/2d/greensfns/strikeslip, and just modified the boundary conditions
>> from Dirichlet to Neumann. Config file is attached; any advice would be
>> great.
>>
>> Thanks,
>> Eric
>>
>> <neumann_test.cfg>_______________________________________________
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>>
>>  Charles A. Williams
>> Scientist
>> GNS Science
>> 1 Fairway Drive, Avalon
>> PO Box 30368
>> Lower Hutt  5040
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>> fax (office): 0064-4570-4600
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>>
>>
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>
>
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