extraction of negative eigenvalues

About the development of the FEM module/workbench.

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HarryvL
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Re: extraction of negative eigenvalues

Postby HarryvL » Sat Jul 06, 2019 12:19 pm

bernd wrote:
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



Box_Mesh.dat.txt



Screenshot_20190706_125522.png
EDIT: see https://forum.freecadweb.org/viewtopic. ... 30#p319635
Last edited by HarryvL on Tue Jul 09, 2019 12:25 am, edited 2 times in total.
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bernd
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Re: extraction of negative eigenvalues

Postby bernd » Sat Jul 06, 2019 12:31 pm

HarryvL wrote:
Sat Jul 06, 2019 12:19 pm
I don’t understand. You have 5 values per line so the last one should be imaginary part in Hz as per my post.
no the number and four values ... see my screen from dat file !
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bernd
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Re: extraction of negative eigenvalues

Postby bernd » Sat Jul 06, 2019 2:01 pm

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HarryvL
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Re: extraction of negative eigenvalues

Postby HarryvL » Sat Jul 06, 2019 2:04 pm

EDIT: see https://forum.freecadweb.org/viewtopic. ... 30#p319635
Last edited by HarryvL on Tue Jul 09, 2019 12:26 am, edited 2 times in total.
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HarryvL
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Re: extraction of negative eigenvalues

Postby HarryvL » Sat Jul 06, 2019 2:06 pm

Last edited by HarryvL on Tue Jul 09, 2019 12:26 am, edited 2 times in total.
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HarryvL
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Re: extraction of negative eigenvalues

Postby HarryvL » Sat Jul 06, 2019 2:18 pm

bernd wrote:
Sat Jul 06, 2019 12:31 pm
HarryvL wrote:
Sat Jul 06, 2019 12:19 pm
I don’t understand. You have 5 values per line so the last one should be imaginary part in Hz as per my post.
no the number and four values ... see my screen from dat file !
EDIT: see https://forum.freecadweb.org/viewtopic. ... 30#p319635
Last edited by HarryvL on Tue Jul 09, 2019 12:27 am, edited 1 time in total.
Jee-Bee
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Re: extraction of negative eigenvalues

Postby Jee-Bee » Sat Jul 06, 2019 3:27 pm

Why not using numpy arrays against math with a loop as you use now?
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bernd
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Re: extraction of negative eigenvalues

Postby bernd » Mon Jul 08, 2019 7:44 am

I am unsure, but we may do something totally wrong or missunderstand ...

Attached a screen. The green value is presentet to the user as Eigenmode Frequency for mode no 8. This is what FreeCAD does ATM. As far as I read the table ther is no imaginary part in cycles / time. Attached the screen again.

What makes me unsure is the Eigenvalue. AFAIK the eigenvalue is the natural frequency, but we have two columns one for Eigenvalue and one for the frequency ...

Mhh, I should do more earthqake analyses ... I would better know than ...
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HarryvL
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Re: extraction of negative eigenvalues

Postby HarryvL » Mon Jul 08, 2019 9:50 am

This is surprising and different from what I remember. I will look into it and compare to what I did for the bridge.

EDIT: the natural frequency in rad/s is the square root of the eigenvalue of the free vibration problem.

EDIT: see https://forum.freecadweb.org/viewtopic. ... 30#p319635
Last edited by HarryvL on Tue Jul 09, 2019 12:30 am, edited 1 time in total.
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bernd
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Re: extraction of negative eigenvalues

Postby bernd » Mon Jul 08, 2019 10:03 am

HarryvL wrote:
Mon Jul 08, 2019 9:50 am
This is surprising and different from what I remember. I will look into it and compare to what I did for the bridge.
great
HarryvL wrote:
Mon Jul 08, 2019 9:50 am
PS: yes the natural frequency in rad/s is the eigenvalue of the free vibration problem.
than what is the column eigenvalue for? Do you know some paper which explains all this without tons of integrals and matrix operations ... :oops: wikipedia does not a good job here IMHO.