[CIG-SHORT] Pylith question

[Matthew Knepley]

On Wed, Jan 13, 2016 at 10:43 PM, Demian Gomez <demiang at gmail.com> wrote:

> Dear Matt,
>
> I'm Demián Gómez from The University of Memphis and I work with Eunseo
> Choi. We briefly met in 2014 during the Pylith workshop at Stanford. Back
> then, I was just starting to use Pylith with a full spherical model (not
> just a section of the sphere) that you may remember.
>
> I'm bothering you with a question because I was unable to find a solution
> to my problem, so I decided it was time to ask you. I've been trying to add
> gravity to my spherical model, which I've been using to obtain the
> coseismic deformation from the Maule earthquake. I activate gravity as
> shown in the manual (gravity_field = spatialdata.spatialdb.GravityField)
> with the coordinate system set to coordsys = spatialdata.geocoords.CSGeo in
> the pylithapp.cfg (version 2.1.0). My spatial databases are configured to
> have is-geocentric = true and ellipsoid = sphere to specify that my mesh of
> Cartesian XYZ coordinates refer to the geocenter (section C.2.1.2 of the
> manual).
>
> Everything works fine until I activate gravity. When I turn gravity on,
> the problem never converges and the residual norm shown during the
> integration process is very large (in cases without gravity it starts at
> ~10^-4 and converges at ~10^-12, with gravity it is ~ 10^2 and it never
> decreases!). I don't know if it's important, but without gravity my model
> converges in ~250 iterations.
>
> I am not sure if this is a configuration problem or something that Pylith
> cannot handle. If you could please point me in the right direction to
> correct this problem, I'd really appreciate it. Maybe there is a way to
> specify a spatialdb with the direction of the gravity field? I read the
> documentation but I could not figure it out how to do it.
>
> If you need more information (I could send you the complete model if
> needed), or if I should direct this question to somebody else, please let
> me know. I've attached the config files and one spatialdb just in case you
> want to take a look at them.
>

Hi Demian,

With something like this, I think the right thing to do is get as simple as
possible first.

So you are solving a whole sphere problem, which we never do in our
examples. Thus we should
make a simple example for this that we completely understand. How about
making a uniform sphere
with gravity directed toward the center. This is analytically tractable. If
you can set this up, we will
incorporate it into the PyLith test examples, and work out an analytic
solution.

If that works, we can start adding back the complications in your model.

  Thanks,

     Matt


> Thank you for any help you can provide me,
> Demián Gómez
>
> --
> *Demián D. Gómez*
> Graduate Research Assistant & PhD Candidate
> Center for Earthquake Research and Information
> 3890 Central Ave, Memphis TN 38152
> Tel: +1 (901) 678-4809
> Cell: +1 (901) 900-7324
>



-- 
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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