[CIG-SHORT] Convergence

[Matthew Knepley]

You should first try with a direct solver (hopefully the problem is small
enough),
--petsc.pc_type=lu --petsc.ksp_type=preonly

   Matt

On Wed, Dec 10, 2008 at 10:49 AM, Tabrez Ali <stali at purdue.edu> wrote:

> Brad/Charles
>
> Thanks for your reply. Yes the residuals do increase after each time
> step. I have a low dipping fault which curves from very shallow dip to
> somewhat steeper dip (as in shallow subduction).
>
> If I specify slip on the steeper part then things work fine. Things also
> work fine when I dont specify slip on this particular fault but have
> other faults/BC's in place (on the same mesh).
>
> Its slip on the shallow part which seems to mess it up.
>
> Also my mesh quality isnt that bad. I do have a few elements with aspect
> ratios of 50+ (in cubit) but nothing greater than 100. And my material
> property for the fault is the same as used in the examples/benchmarks.
>
> Regards
> Tabrez
>
> Brad Aagaard wrote:
> > 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
> >> 1 Fairway Drive, Avalon
> >> PO Box 30368
> >> Lower Hutt  5040
> >> New Zealand
> >> ph (office): 0064-4570-4566
> >> fax (office): 0064-4570-4600
> >> C.Williams at gns.cri.nz
> >> NOTE NEW E-MAIL ADDRESS
> >>
> >
> >
> >
>
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-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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