[CIG-SHORT] Convergence

[Brad Aagaard]

Tabrez-

Does the solution blow up (residual increases) or just fail to converge 
(residual approaches some value)? If the solution blows up, something is 
wrong. Either the problem is not setup correctly or there is a bug.

There are a number of things that can cause the solution to converge very 
slowly. In addition to the mesh quality issue that Charles pointed out, some 
other things you might look into include:

(1) Do you get the same behavior with a coarser mesh?

(2) Does the solution converge when you omit the fault in the parameters 
(i.e., use the same mesh but don't have any fault interface conditions)? If 
you have zero displacement BC, try simple compression without a fault.

(3) What is Poisson's ratio? Do any cells have a Poisson ratio greater than 
0.45 (only values greater than about 0.48 should cause problems)?

(4) What physical properties are you using for the fault (this affects the 
conditioning of the system and can affect convergence if they are not 
reasonable)? In a test problem, the solution converged slightly faster for 
uniform physical properties for the fault (only used to condition the system) 
compared with the actual 3-D variation.

Brad


On Wednesday 10 December 2008 12:05:31 am Charles Williams wrote:
> I would look at the element quality in the mesh.  You can do this in
> ParaView.  The low angles may be giving you poorly-formed elements,
> and this could cause problems.  I doubt that incorrect fault
> parameters is causing the problem, unless you're doing something
> that's giving you very large strains.
>
> Charles
>
> On Dec 10, 2008, at 6:43 PM, Tabrez Ali wrote:
> > Brad/PyLith Users
> >
> > It seems that whenever I use a low angle thrust fault (very low dip)
> > the
> > solution (to the quasi-static problem) after a certain number of time
> > steps fails to converge and blows up. Can wrong fault parameters
> > such as
> > up_dir or normal_dir also cause this?
> >
> > Regards
> > Tabrez
> > _______________________________________________
> > CIG-SHORT mailing list
> > CIG-SHORT at geodynamics.org
> > http://geodynamics.org/cgi-bin/mailman/listinfo/cig-short
>
> Charles A. Williams
> Scientist
> GNS Science
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