extraction of negative eigenvalues

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UR_
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Re: extraction of negative eigenvalues

Post by UR_ »

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

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PrzemoF
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Re: extraction of negative eigenvalues

Post by PrzemoF »

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
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Re: extraction of negative eigenvalues

Post by UR_ »

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

FemCalculixCantilever3D.FCStd
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Box_Mesh.dat.zip
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bernd
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Re: extraction of negative eigenvalues

Post by bernd »

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 ...


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Re: extraction of negative eigenvalues

Post by bernd »

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.
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Re: extraction of negative eigenvalues

Post by bernd »

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
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HarryvL
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Re: extraction of negative eigenvalues

Post by HarryvL »

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

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<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.
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Re: extraction of negative eigenvalues

Post by HarryvL »

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.
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bernd
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Re: extraction of negative eigenvalues

Post by bernd »

- 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
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Box_Mesh.dat.txt
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bernd
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Re: extraction of negative eigenvalues

Post by bernd »

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