Why does FEM meshing doesn't generate volume elements in some cases

About the development of the FEM module/workbench.

Moderator: bernd

vejmarie
Posts: 551
Joined: Mon Jan 04, 2016 4:52 pm
Location: Somewhere between France, USA and Taiwan
Contact:

Why does FEM meshing doesn't generate volume elements in some cases

Postby vejmarie » Sat Apr 16, 2016 5:42 pm

I am struggled since a couple of hours to try to assemble a stair with two steps.

Attached is the model I have made. The step are complex shape, they are assembly of sheet metal parts, and extruded pipe (square shape). The assembly is supposed to be welded. Anyway, this is not the issue I face currently. When I run a FEM analysis on a single step, the mesher generate a volume mesh. If I merge the steps into a single element to have 2 steps, then I do not have volume elements anymore. I can use merge or join from the Part workbench nothing works :(.

Is this a known issue, is there some workaround ? Any idea on how to assemble this 2 solids ? I checked after the assembly and they are declared as solid element :(

I tested with the pattern function from the Draft workbench, same issue (instead of copying the element)

vejmarie
stair.png
stair.png (81.69 KiB) Viewed 3429 times
Attachments
step.FCStd
(186.91 KiB) Downloaded 53 times
User avatar
sgrogan
Posts: 5331
Joined: Wed Oct 22, 2014 5:02 pm

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby sgrogan » Sat Apr 16, 2016 6:42 pm

I fused the solids and tried to run a frequency result on the resulting fusion (Fusion passes geomentry check with BOP checks = True)
Step.PNG
Step.PNG (110.76 KiB) Viewed 3423 times
OS: Windows 7
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.6700 (Git)
Build type: Release
Branch: releases/FreeCAD-0-16
Hash: 7b925d11aa69ac405b423635adb1e2833f18a817
Python version: 2.7.8
Qt version: 4.8.6
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17
vejmarie
Posts: 551
Joined: Mon Jan 04, 2016 4:52 pm
Location: Somewhere between France, USA and Taiwan
Contact:

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby vejmarie » Sat Apr 16, 2016 6:54 pm

Hi sgrogan

You have the same issue than i have. Your mesh is a shell not a volume one which explain your calculix error
Can you click on the mesh and look at view/data and validate that volume element is set to 0 ? There should be plenty Of them

Vejmarie
User avatar
nemesis
Posts: 344
Joined: Tue Mar 25, 2014 11:24 pm
Location: France, Lyon

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby nemesis » Sat Apr 16, 2016 9:34 pm

Hi,
I reworked the file, and was able to have a close to working file.
"only" jacobian error....
Attachments
step.FCStd
(268.44 KiB) Downloaded 59 times
Last edited by nemesis on Sun Apr 17, 2016 12:22 pm, edited 1 time in total.
vejmarie
Posts: 551
Joined: Mon Jan 04, 2016 4:52 pm
Location: Somewhere between France, USA and Taiwan
Contact:

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby vejmarie » Sat Apr 16, 2016 10:43 pm

Hi Nemesis,

Thanks to have a look. The issue is raised only when you have 2 steps. With a single one everything works :(
I have worked a little bit more on the model, and still face the same issue even if I generate the mesh from a SOLID which contains both steps. I make a fusion of the 2 steps and then generate a solid out of the fusion without any issue in the part workbench. But whatever happen I got a mesh which doesn't contains volume.

vejmarie

ps: as to get ride of the negative Jacobian you need a fix that I still need to implement regarding minh which is passed to Netgen with a value of 0 while it should be at least 1 or 2, to avoid too small cells. I have posted on another post regarding that.
stair2.png
stair2.png (784.83 KiB) Viewed 3399 times
User avatar
nemesis
Posts: 344
Joined: Tue Mar 25, 2014 11:24 pm
Location: France, Lyon

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby nemesis » Sun Apr 17, 2016 12:19 pm

vejmarie wrote:Thanks to have a look. The issue is raised only when you have 2 steps. With a single one everything works
sorry going to fast ;)

I tried again with 2 steps, and also rework the model, to have only one cylinder for the 2 step (no tangent face)
so as you said, it didn't work.
so, I tried with Gmsh macro and it work.
default max size was a 5, so the mesh is quite huge.
step2.png
Gish Result
step2.png (453.01 KiB) Viewed 3374 times
I put 1000 (as in netgen by default)
and surprise, it didn't work. So I decrease the max size to find, when the volume starts to be ok, and it's 25 (test on 500, 100, 50, 25).

put 25 in netgen max size element "et voila"
step3.png
step3.png (319.09 KiB) Viewed 3374 times
it doesn't explain why, but at least, it's maybe a clue. there maybe a link between the max size element, and Volumes. (I'm completely noob in FEM so...)

I try a frequency analysis, ad add only 2 jacobin errors

Code: Select all

*ERROR in e_c3d: nonpositive jacobian         determinant in element     1018936
*ERROR in e_c3d: nonpositive jacobian         determinant in element     1296839
User avatar
nemesis
Posts: 344
Joined: Tue Mar 25, 2014 11:24 pm
Location: France, Lyon

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby nemesis » Sun Apr 17, 2016 12:54 pm

and the file (without FEM of course)
Attachments
step.FCStd
(395.95 KiB) Downloaded 51 times
vejmarie
Posts: 551
Joined: Mon Jan 04, 2016 4:52 pm
Location: Somewhere between France, USA and Taiwan
Contact:

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby vejmarie » Sun Apr 17, 2016 1:29 pm

Hey,

Thanks for the hint, I will have a look tonight (I am on west coast of the US), I don't really know why maxh parameter is disabling potentially the generation of volume elements. I will try to understand that.

vejmarie

ps: I am currently fully redrawing the model with new FreeCAD version, which has a brand new Part Design interface preparing the Assembly tools.

OS: Mac OS X
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.17.7414 (Git)
Build type: Unknown
Branch: master
Hash: f48e57c99f4300baefa873d9f79b300e0f6ba965
Python version: 2.7.11
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.18-dev
User avatar
bernd
Posts: 8332
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby bernd » Sun Apr 17, 2016 2:42 pm

nemesis wrote:and the file (without FEM of course)
I gave the step.FCStd a try too in GMSH. I used min size = 2 and max size = 25, but I used hight order tools to generate the 2nd order elements and optimized it. The result is a minimum jacobian of 0.129

See attached screens.
elements with jacobian < 0.25
screen1.jpg
screen1.jpg (88.88 KiB) Viewed 3357 times
the first 4 eigenshapes:
stairs.jpg
stairs.jpg (150.68 KiB) Viewed 3357 times
User avatar
bernd
Posts: 8332
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

Re: Why does FEM meshing doesn't generate volume elements in some cases

Postby bernd » Sun Apr 17, 2016 3:00 pm