[Gmsh] R: Stress on Gauss Points

Gaetano Camarda gaetano_18 at live.it
Sat Feb 16 12:30:45 CET 2019

Hi guys,

Yes, the goal for me was to plot the data on Gmsh, but i would like to ask if there’s a way to show sigx, sigy and so on as view in Gmsh Post-processing, it’s look like all the tensor is plotted all together, am I wrong?

I will do a little brief on my work, maybe it could be better understanding. I’m sorry in advance if my english is not so perfect.
I’m working on a Multiscale RVE, not the one I’,ve send but another one more complex, but I’m working in parallel with this smaller one for having more control on the code and on the operations.
First thing first, I’ve generated the mesh on Gmsh and I processed the operation on Matlab, part of the code was made by me, and for the solver I’ve used CalFem function (freeware script for matlab and python for studying FEM). The .txt file with stress matrix in gauss point and nodal forces for each element are the result of the Calfem function’s I’ve used. The Gauss stress matrix I’ve sent as .txt has 8 point for each element, this is why it has 512 row, cause the element are 64 with 8 gauss point = 512.

Now the post-processing part, my goal is to plot the data on Gmsh postprocessing, thanks to Max now I’ve understood a little bit more about the input data Gmsh needed, but I would like to plot stress field separately (as sigx view, sigy view and so on); I also have another matrix for strain, but I didn’t sent because I think the process to do is the same for the gauss-points stresses.

About this question from Max:
“For the displacement, does it seem like it aligns? I don't want to make a mistake in interpreting the data. “
I haven’t understand well your question, my english is a little bit limited, what to you mean with “it seem like it alligns?” If I’ve understood the question you mean the fact that displacement are not the same in faces dx and sx? if that’s your meaning, i’ve noticed it too, and I think it depends on the boundary conditions I gave to the element.


Inviato da Posta<https://go.microsoft.com/fwlink/?LinkId=550986> per Windows 10

Da: Jeremy Theler <jeremy at seamplex.com>
Inviato: Saturday, February 16, 2019 11:29:03 AM
A: Max Orok
Cc: Gaetano Camarda; gmsh at onelab.info
Oggetto: Re: [Gmsh] Stress on Gauss Points

Hey Max

On Fri, 2019-02-15 at 11:01 -0500, Max Orok wrote:

My impression going in was that the Gauss point steps were part of the simulation and the goal was just to plot the data.

Well, there is a fine line here but it can be considered that the modeling [1] is finished when the stresses (or the heat fluxes, etc) are computed at the gauss points.

Actually, I think the original question was: "given that I have data at the Gauss points (and not at the nodes nor at the cell centers), how can I plot them on Gmsh?"

Indeed, I think Gaetano wanted to have Gmsh to do something like to re-mesh the base grid and have new cells around the gauss points and plot the data as cell-centered values. I do not know if I am clear, but for a 8-node hexa with 2 Gauss points per dimensions that would be to replace the original element with 8 little hexas, one in each octant and then assign a cell value from the dataset to each of them. For other elements it would be more complex.

This is definetely in the post-processing step of the worflow.

I think for this data set, the shared node data does repeat, but a single element will vary as a function of its nodes.

As far as I can see, the data corresponding to the shared nodes does not repeat. Again, this is what is expected from secondary fields computed as derivatives of a primary field at the Gauss points.

For any averaging etc., perhaps this could be done during the simulation?

Yes, it could. This is why I am interested in this issue, and raised the question on how to average if the solver has to output nodal values of stresses. Weighted average? With the element's volume? With the element's quality?

[1] https://www.seamplex.com/blog/say-modeling-not-simulation.html<https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.seamplex.com%2Fblog%2Fsay-modeling-not-simulation.html&data=02%7C01%7C%7C3651368f07bb492242d008d693f99612%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636859097498313953&sdata=cPWVCtVwxuo9fb8E2II9hE10fpEeh7LSXQcXkvKQaDg%3D&reserved=0>

Jeremy Theler
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