[CIG-SHORT] Question about accuracy in Pylith

Demian Gomez demiang at gmail.com
Fri Oct 14 11:17:07 PDT 2016

Dear Brad, Matt and Charles,

I have a question regarding the accuracy of the solution using tets. I have
a model with a biased tet mesh (4 km at the fault and 160 km at the edges,
~2200 km away) from which I am trying to get the strain and stress on some
depth profiles at ~ 400 km from the fault. I am running Pylith with the
refiner on (only one level) to refine my mesh and improve the resolution.

The problem I'm having is that when I plot the strains and stresses, the
plots are very "noisy" (see profiles_70.png). The displacement looks ok,
maybe a few bumps and kinks here and there, but acceptable. I think these
small displacement kinks are translating into the "noise" and larger kinks
in strain and stress. I did tests in 2D (on a cross section of my 3D model)
to figure out the best discretization size, and if I use a mesh with
constant element size (say, 1 km), then everything is smooth and nice (see
profiles_70_2D.png). However, a 3D model of the size that I need meshed
with 1 km elements is huge and very impractical. Moreover, there shouldn't
be any problems with using a biased mesh since there are examples within
Pylith were you guys use this type of mesh.

I know that I can improve the accuracy by using hexes, but unfortunately
I've been trying to mesh my model with hexes (in Trelis) without any
success. The model has the shape of a spherical cap and apparently there is
something that Trelis doesn't like about this geometry. No matter how I
divide and subdivide the model to help the mesher, there is always one
volume that I cannot mesh. With tets, however, it works fine.

Do you have any suggestions on what can I try to improve these results,
without increasing the number of elements? I am at the limit of resources
in terms of the model size (right now I'm at 125 GB of required memory to
run my model). I could start using the HPC but it seems that there should
be another way to solve this problem other than "brute force", i.e. making
the model larger and using a bigger computer. You may also have suggestions
regarding the meshing process. I would appreciate any advise that can help
me to solve my problem. Let me know if there is any additional information
you may need that I did not include.


PS: I've attached the cfg files, just in case you want to see how I'm
running the problem.

*Dr. Demián D. Gómez*
Postdoctoral Researcher
The Ohio State University - School of Earth Sciences
275 Mendenhall Laboratory
125 South Oval Mall
Columbus, Ohio 43210
Cell: +1 (901) 900-7324
email: gomez.124 at osu.edu
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