[CIG-SHORT] Convergence problem with Neumann BCs

[Charles Williams]

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
> 
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Charles A. Williams
Scientist
GNS Science
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C.Williams at gns.cri.nz

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