FEM meshing doesn't generate volume elements

About the development of the FEM module/workbench.

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nquantum
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Joined: Fri Aug 03, 2018 1:29 am

FEM meshing doesn't generate volume elements

Postby nquantum » Tue Sep 04, 2018 1:30 pm

It quite a while what is status of the issue "why sometime no volume?"

Me too I use Freecad 0.17 and someime no volume on create mesh. I notice that the smaller or fine the mesh size then usually no volume. Both Gmsh and Netgen. But if mesh is courses or big it can have volume.

Why? And good work around?
I will later add pictures.

EDIT: related topic https://forum.freecadweb.org/viewtopic.php?f=18&t=15328
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bernd
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Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby bernd » Tue Sep 04, 2018 3:20 pm

Usually it is a problem of your geometry, means you should not add pictures but a FreeCAD file with your geometry.
nquantum
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Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby nquantum » Tue Sep 04, 2018 4:09 pm

I see. Here is the file.
I design cnc and would like to see deformation of structure to know estimate how strong or weak point of each of design.


I hide other part only left "a001" for FEM. It is base for x-axis. I've try create mesh from whole body, all of them created no volume. So I try on x-axis only. I can create mesh with volume and can solve to show result only by Gmsh (use max value 0 or more than 50).
Netgen create no volume no matter max size is 10,100,1000,10000 coarse , fine , moderate.

So here it is. Coarse mesh can create with volume. Fine mesh no volume. On same shape. But for simple shape like couple of rectangles in tutorial no problem on both Netgen and Gmsh. What am I wrong?

PS. file is 2mb big cannot upload so I put in the google drive and here is link.
https://drive.google.com/open?id=1TFGZK ... FOGdd_Uvkq
Attachments
netgen.jpg
netgen
netgen.jpg (299.99 KiB) Viewed 613 times
gmsh-course-------2.jpg
gmsh way smaller
gmsh-course-------2.jpg (319.31 KiB) Viewed 613 times
gmsh-course-.jpg
gmsh smaller
gmsh-course-.jpg (298.59 KiB) Viewed 613 times
gmsh-course.jpg
gmsh course1
gmsh-course.jpg (301.38 KiB) Viewed 613 times
UR_
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Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby UR_ » Wed Sep 05, 2018 9:02 pm

To get reliable meshing results, you should provide model data following an improved quality standard.

What's this:
2018-09-05 22_54_18-FreeCAD 0.18.png
2018-09-05 22_54_18-FreeCAD 0.18.png (73.45 KiB) Viewed 588 times
nquantum
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Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby nquantum » Sat Sep 08, 2018 2:20 am

To get reliable meshing results, you should provide model data following an improved quality standard.
What you mean by provide model data? You mean I must set some value or data something in Freecad? Or should I send new file? Or should I provide better picture like you in example u provided?

An I think I found cue, I've created self-intersect mesh from those round-rail with those rectangle block with hole. If I meshing them seperately then no problem. Once they together like the picture many time they have self-intersect mesh which produce no volume.

Maybe this cause of it? I try to find how the way to avoid and/or how to fix.
Pls also advice
Attachments
images.jpg
that is I try to model
images.jpg (19.48 KiB) Viewed 557 times
nquantum
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Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby nquantum » Sat Sep 08, 2018 7:00 am

I want volume mesh for use in FEM module to see how strong structure
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bernd
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Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby bernd » Sat Sep 08, 2018 3:47 pm

As mostly UR_ is on the right track. Without the sleds your geometry meshes for me. Attached the file with geometry and meshes cleared. But the mesh still has some problems, see screen ...


cnc2-bernd1_cleared.FCStd
(124.24 KiB) Downloaded 6 times


Screenshot_20180908_173920.png
Screenshot_20180908_173920.png (702.92 KiB) Viewed 529 times
nquantum
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Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby nquantum » Sat Sep 08, 2018 3:59 pm

As mostly UR_ is on the right track. Without the sleds your geometry meshes for me. Attached the file with geometry and meshes cleared. But the mesh still has some problems, see screen ...
I'm sorry bernd, I don't understand what you try to tell me or want me do. Can you please tell me again? I've problem about geometry?
Attachments
a.jpg
a.jpg (227.16 KiB) Viewed 529 times
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bernd
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Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby bernd » Sat Sep 08, 2018 4:20 pm

If I use the geometry from your file but do not use the sleds which are on the rails, the geometry meshes fine for me with volumes. This is what _UR has shown on his screen. There is some problem with the sleds on the rails.

The question is, what exactly would you like to analyze? Depending on the answer you do not need the sleds and thus you can use your geometry without the sleds.

bernd
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bernd
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Re: FEM meshing doesn't generate volume elements

Postby bernd » Sat Sep 08, 2018 4:26 pm

Since this is not really related to the topic from the first post and the topic was years old, I made an own topic.