[CIG-SHORT] Question about accuracy in Pylith

Tabrez Ali stali at geology.wisc.edu
Fri Oct 14 17:16:29 PDT 2016

The displacements should not be noisy. Are you sure the solver converged 
for the 3D case?


On 10/14/2016 02:17 PM, 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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