[CIG-SHORT] Question about accuracy in Pylith

[Demian Gomez]

Hi Brad,

Thanks. Here's the output for the mesh quality:

Tet quality, 3297540 elements:

Condition No. ranges from 1.000e+00 to 4.650e+00 (3297540 entities)

Red ranges from 4.128e+00 to 4.650e+00 (3 entities)

Magenta ranges from 3.607e+00 to 4.128e+00 (69 entities)

DkYellow ranges from 3.086e+00 to 3.607e+00 (196 entities)

Yellow ranges from 2.564e+00 to 3.086e+00 (862 entities)

Green ranges from 2.043e+00 to 2.564e+00 (4427 entities)

Cyan ranges from 1.521e+00 to 2.043e+00 (75597 entities)

Blue ranges from 1.000e+00 to 1.521e+00 (3216386 entities)


The highest Co. number is 4.65 and there are only 3 elements, mostly on the
edges. Can this distort all the solution? Also, although we want all the
elements to be as close to 1 as possible, is there an acceptable range
limit?


Thank you,

Demián


--
*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


On Fri, Oct 14, 2016 at 2:28 PM, Brad Aagaard <baagaard at usgs.gov> wrote:

> 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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>>
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