FEM Mesh Creation - What Are "Tet Parameters"?

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johnsmith
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Joined: Sun Mar 12, 2017 5:35 pm

FEM Mesh Creation - What Are "Tet Parameters"?

Postby johnsmith » Sun Mar 12, 2017 5:51 pm

I'm new to FreeCAD, CAD in general, and know nothing at all about FEM, so I am probably doing something incorrectly.

I have 7 solids that I have positioned with the assembly2 workbench. I combined them into one object using the Part Workbench's Make Compound tool. When I start to process of making a mechanical analysis, I get the mesh creation dialog ("Tet Parameter"), and when I press OK, it seems like it will never end. It just sits there using a full CPU core, and 5-10GB for RAM. I have 16GB of RAM, so that's fine, but I have a feeling it's trying to create a mesh more complex than practical.

What exactly do all the "Tet parameters" mean, and how can I modify them to speed up the mesh creation process?

OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.6706 (Git)
Build type: Release
Branch: releases/FreeCAD-0-16
Hash: f86a4e411ff7848dea98d7242f43b7774bee8fa0
Python version: 2.7.8
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17
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r-frank
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Re: FEM Mesh Creation - What Are "Tet Parameters"?

Postby r-frank » Mon Mar 13, 2017 9:05 am

Hello John.

Welcome to FreeCAD and the forum.

Mastering 3D CAD can be a challenge and doing FEA Analysis is another level.
When doing an analysis on a solid we need to mesh the solid, apply "values of behaviour" (that means material
definitions like young's modulus, poisson ration and yield strength, ...) apply loads and constraints, and
then run the solver ...

For meshing we have different types of elements and different options for meshing.
For starters you may read here more ...

What is a good mesh and what not is one of the hardest questions to answer but without knowing more about your model
and your situation it's only guessing ...

Roland
Deutsche FreeCAD Tutorials auf Youtube
My GrabCAD FreeCAD-Projects
FreeCAD lessons for beginners in english

Native german speaker - so apologies for my english, no offense intended :)
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bernd
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Re: FEM Mesh Creation - What Are "Tet Parameters"?

Postby bernd » Mon Mar 13, 2017 10:22 am

johnsmith wrote:Version: 0.16.6706 (Git)
Hi and welcome to the world of FEM.If you would like to work with FreeCAD FEM it is highly recommended to use the latest development version 0.17.x You can download it here: https://github.com/FreeCAD/FreeCAD/releases It is what all users and developers of FreeCAD FEM do because there have been tons of fixes and changes in 0.17 in FEM
johnsmith wrote:I have 7 solids that I have positioned with the assembly2 workbench. I combined them into one object using the Part Workbench's Make Compound tool. When I start to process of making a mechanical analysis, I get the mesh creation dialog ("Tet Parameter"), and when I press OK, it seems like it will never end. It just sits there using a full CPU core, and 5-10GB for RAM. I have 16GB of RAM, so that's fine, but I have a feeling it's trying to create a mesh more complex than practical.

What exactly do all the "Tet parameters" mean, and how can I modify them to speed up the mesh creation process?
It is difficult to say what it the right way to go for your model because we do not know your mode. Best results you will get if you make a boolean union of all solids and extract the solid out of this compound union. This is the best for most models but might not be suitable at all for your model because you model might has its own requirements. Best is to post the FreeCAD file of your assembly model.
r-frank wrote:For meshing we have different types of elements and different options for meshing. https://www.comsol.com/blogs/meshing-yo ... ent-types/ ...
cool link :) But as johnsmith is a beginner I would highly recommend to use second order tetra10 (10-node tet-elements) elements only

bernd
johnsmith
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Re: FEM Mesh Creation - What Are "Tet Parameters"?

Postby johnsmith » Tue Mar 14, 2017 6:28 am

Hmm. Read the article, and I think the problem is my solids consist of long thin sections. If the mesher is trying to create tets with a low aspect ratio, then it will need to create many tets to fill the lengths. What would be the best way to remedy this in FreeCAD? Can I tell the mesher to create blocks instead of tets? Is there a better way to approach this problem?

I'm not afraid of using Python or the terminal to resolve this issue. I'm a programmer by trade, and even have some experience using Numpy for machine learning tasks.

Here is the FreeCAD document: https://drive.google.com/open?id=0B4R2V ... 3NMQ1Yya0k
table-frame.png
table-frame.png (119.63 KiB) Viewed 947 times
table-frame-close.PNG
table-frame-close.PNG (53.52 KiB) Viewed 947 times
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bernd
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Re: FEM Mesh Creation - What Are "Tet Parameters"?

Postby bernd » Wed Mar 15, 2017 7:04 am

For such a structure I would use line mesh analysis with cross section data. The problem is FreeCAD and CalculiX support for line mesh is limited an only the cross sections of some standard profiles are supported. Your one for sure not.

If you would mesh it with solid you would need to make some solid solid at the connection. But the real problem is somewhere else. Your profile has so many details, if it is meshed with volume elements it needs billions of elements. Try to mesh a 5 cm long part of your profile. Just this one has thousands of nodes and elements. Imagine how many the whole table frame would have. Thus I would use 1D elements or simplify your profile.

Mhh may be Z88 is a bet for 1D in this case ... It is included in FreeCAD FEM too.
Last edited by bernd on Wed Mar 15, 2017 7:20 am, edited 1 time in total.
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bernd
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Re: FEM Mesh Creation - What Are "Tet Parameters"?

Postby bernd » Wed Mar 15, 2017 7:18 am

460562 elements ...
screen.jpg
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johnsmith
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Re: FEM Mesh Creation - What Are "Tet Parameters"?

Postby johnsmith » Wed Mar 15, 2017 5:06 pm

Ok. Thanks for pointing me in the right direction, I'll look into it. You guys are awesome.