[CIG-SHORT] Question about accuracy in Pylith

[Brad Aagaard]

Demian,

Have you looked at mesh quality (aspect and condition numbers close to 
1.0)? Distorted cells (slivers, squashed tets, etc) will be stiffer and 
may cause local stress/strain concentrations. My guess is that your 2-D 
mesh has better quality. Look to see if there is a correlation between 
condition number or aspect ratio and the kinks, etc in your stress field 
(you can do this in ParaView). If so, then spend some time playing with 
the bias value, cell size, and smoothing to improve the mesh quality.

Regards,
Brad


On 10/14/2016 11:17 AM, Demian Gomez wrote:
> 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.
>
> Cheers,
> Demián
>
> 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 <mailto:gomez.124 at osu.edu>
>
>
>
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