extraction of negative eigenvalues

About the development of the FEM module/workbench.

Moderator: bernd

UR_
Posts: 977
Joined: Tue Jan 03, 2017 8:42 pm

Re: extraction of negative eigenvalues

Postby UR_ » Mon Jul 01, 2019 11:55 am

bernd wrote:
Mon Jul 01, 2019 11:05 am
the reader code never changed over the years

phpBB [video]
User avatar
PrzemoF
Posts: 2649
Joined: Fri Jul 25, 2014 4:52 pm
Contact:

Re: extraction of negative eigenvalues

Postby PrzemoF » Mon Jul 01, 2019 12:07 pm

UR_ wrote:
Mon Jul 01, 2019 9:16 am
Just tried a frequency analysis by modifying cantilever3D example from start page.

Got mode 1 to 15.

If I select result object e.g. "CalculiX_frequency_mode_1_results", I see in properties:


Screenshot 001.png


But same values for all modes :?

Where is the beaf? :confused:
Check if you see the results in the file produced by CalculiX. If it's there then the reader is broken, but I doubt it - it worked since it was added to the FreeCAD code. Another option is change of formatting by CalculiX, but that's unlikely. Post the model, so someone can check it.

P.S. I never dreamt that the reader code might survive that long without modifications :D
UR_
Posts: 977
Joined: Tue Jan 03, 2017 8:42 pm

Re: extraction of negative eigenvalues

Postby UR_ » Mon Jul 01, 2019 12:17 pm

PrzemoF wrote:
Mon Jul 01, 2019 12:07 pm
Post the model, so someone can check it.

FemCalculixCantilever3D.FCStd
(620.53 KiB) Downloaded 4 times
Box_Mesh.dat.zip
(2.13 KiB) Downloaded 6 times
User avatar
bernd
Posts: 8329
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

Re: extraction of negative eigenvalues

Postby bernd » Mon Jul 01, 2019 12:51 pm

I can confirm the problem for 0.19 Just open FEM 3D example change from static to frequancy analysis.

Ahh it works in 0.18.2 ... :) which means I may am the culprit in one of the lots of code refactoring commits ... I will have a look ...


screen.jpg
screen.jpg (176.32 KiB) Viewed 198 times
User avatar
bernd
Posts: 8329
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

Re: extraction of negative eigenvalues

Postby bernd » Mon Jul 01, 2019 4:04 pm

my vault ... :oops:

I deleted this line https://github.com/FreeCAD/FreeCAD/blob ... em.py#L284 I will have a look how to fix this.
User avatar
bernd
Posts: 8329
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

Re: extraction of negative eigenvalues

Postby bernd » Mon Jul 01, 2019 7:45 pm

git commit 20b78fc fixes the eigenmode reading. Back to the origin question ...

Is the eigenmode we gone read from dat file the right one ??? https://github.com/FreeCAD/FreeCAD/blob ... py#L94-L95
User avatar
HarryvL
Posts: 1043
Joined: Sat Jan 06, 2018 7:38 pm

Re: extraction of negative eigenvalues

Postby HarryvL » Mon Jul 01, 2019 11:56 pm

Judging from the table in the first post the entries in the frequency line are as follows:

Code: Select all

<mode number> <real part of frequency in rad/s> <real part of frequency in Hz> <imaginary part of frequency in rad/s> <imaginary part of frequency in Hz>
EDIT: this is not correct. see: https://forum.freecadweb.org/viewtopic. ... 30#p319635
Last edited by HarryvL on Tue Jul 09, 2019 12:10 am, edited 1 time in total.
User avatar
HarryvL
Posts: 1043
Joined: Sat Jan 06, 2018 7:38 pm

Re: extraction of negative eigenvalues

Postby HarryvL » Tue Jul 02, 2019 12:08 am

A practical example of a natural frequency analysis where the imaginary part of the response needs to be considered:

https://forum.freecadweb.org/viewtopic. ... 50#p232717

EDIT: this relates to imaginary response modes and not to imaginary frequencies
Last edited by HarryvL on Tue Jul 09, 2019 12:23 am, edited 1 time in total.
User avatar
bernd
Posts: 8329
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

Re: extraction of negative eigenvalues

Postby bernd » Sat Jul 06, 2019 10:50 am

- start FreeCAD
- load FEM 3D example
- change to analysis type frequency
- change from first 15 to first 8 frequencies
- delete constraint fixed and constraint force
- run analyisis
- the imaginary part in Hz is missing
- screen and file attached

test.FCStd
(330.26 KiB) Downloaded 3 times

Box_Mesh.dat.txt
(3.07 KiB) Downloaded 3 times

Screenshot_20190706_125522.png
Screenshot_20190706_125522.png (58.25 KiB) Viewed 85 times
User avatar
bernd
Posts: 8329
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

Re: extraction of negative eigenvalues

Postby bernd » Sat Jul 06, 2019 11:03 am

but I can the rad/Time use as well ... https://en.wikipedia.org/wiki/Radian_per_second just use /2pi ...