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Solution of an eigenfrequency problem

Posted 14 mar 2010, 14:18 GMT-4 5 Replies

Amit Finkler
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Hi,

As a test model, I am trying to find the eigenfrequencies of a 440 Hz tuning fork I drew on Inventor and imported into Comsol. I do get the proper eigenfrequencies, but the displacement amplitude is in meters! It is a 10 cm tuning fork, so I can't see how it gets to such high amplitudes.

How does one set the driving amplitude of the eigenfrequency calculation?

Thanks,

Amit
5 Replies Last Post 17 mar 2010, 09:02 GMT-4
Mihir Shetye
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Posted: 1 decade ago 16 mar 2010, 18:42 GMT-4
Hi Amit,

Modal analysis cannot be used to estimate displacements. It is a method to extract the natural frequencies only. The displacements shown depend on the method used for normalization. If you want displacements, you would have to do a harmonic analysis by applying the appropriate loads.

Mihir
Hi Amit, Modal analysis cannot be used to estimate displacements. It is a method to extract the natural frequencies only. The displacements shown depend on the method used for normalization. If you want displacements, you would have to do a harmonic analysis by applying the appropriate loads. Mihir
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 17 mar 2010, 03:32 GMT-4
Hi

Yes indeed amplitudes in frequency analysis of COMSOL should be seen as relative items, unitless

In anycase, for modal analysis, amplitudes depends on the quality factor Q that is directly coupled to the loss or damping values you use, therefore either you know the losses and you get correct absolute values (relative they will anyhow be OK), or you measure your damping factor via the resonance heights versus half widths values and you run a parametric sweep (or an optimisation) of your damping factor on your model to identify how to match both.

What you can find out though, with an eigenfrequency calculation, is the relative participation factors of the different modes, and how much relative energy there are in the different ones.
This is not directly derived by COMSOL, as you have it in Nastran, Ansys, COSMOS and most others FEM tools, because COMSOL uses a different eigenfrequency normalisation. But you can, if your model is not to heavy, export the matrices into matlab and normalise them ther.

Take a look at :

www.comsol.com/community/forums/general/thread/2650/

for the litterature you have good descriptions in "Actrive Control of Structure", A. Preumont and K Seto, Wiley, 2008, chapter 1.

By the way Prof. A. Preumont (Springer, and Kluever) has some very interesting books with good examples to run in COMSOL on Piezo structures and control thereof

Have a nice reading and
Good luck
Ivar
Hi Yes indeed amplitudes in frequency analysis of COMSOL should be seen as relative items, unitless In anycase, for modal analysis, amplitudes depends on the quality factor Q that is directly coupled to the loss or damping values you use, therefore either you know the losses and you get correct absolute values (relative they will anyhow be OK), or you measure your damping factor via the resonance heights versus half widths values and you run a parametric sweep (or an optimisation) of your damping factor on your model to identify how to match both. What you can find out though, with an eigenfrequency calculation, is the relative participation factors of the different modes, and how much relative energy there are in the different ones. This is not directly derived by COMSOL, as you have it in Nastran, Ansys, COSMOS and most others FEM tools, because COMSOL uses a different eigenfrequency normalisation. But you can, if your model is not to heavy, export the matrices into matlab and normalise them ther. Take a look at : http://www.comsol.com/community/forums/general/thread/2650/ for the litterature you have good descriptions in "Actrive Control of Structure", A. Preumont and K Seto, Wiley, 2008, chapter 1. By the way Prof. A. Preumont (Springer, and Kluever) has some very interesting books with good examples to run in COMSOL on Piezo structures and control thereof Have a nice reading and Good luck Ivar
Cheng Yang
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Posted: 1 decade ago 17 mar 2010, 06:00 GMT-4
Eigenfrequency analysis only gives you modes, but not displacement. It is the basic of mechanics.
Eigenfrequency analysis only gives you modes, but not displacement. It is the basic of mechanics.
Cheng Yang
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Posted: 1 decade ago 17 mar 2010, 06:07 GMT-4
Ivar, do you know how to describe complex eigenfrequency in COMSOL? And thanks for recommending the book, fortunately I have found it in the library :-)
Ivar, do you know how to describe complex eigenfrequency in COMSOL? And thanks for recommending the book, fortunately I have found it in the library :-)
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 17 mar 2010, 09:02 GMT-4
Hi

Well complex values eigenmodes/frequencies should be linked to damping in structural, and in comsol its the "phase" variable that represents the complex "i", try an indexed search on your doc with "phasor variable" and start in the guide.pdf

But I beleive that in structural, the complex mode is not "on" by default, pls check your solver settings, but it might implicitely turn on with the definition of a damping <> 0

Good luck
Ivar
Hi Well complex values eigenmodes/frequencies should be linked to damping in structural, and in comsol its the "phase" variable that represents the complex "i", try an indexed search on your doc with "phasor variable" and start in the guide.pdf But I beleive that in structural, the complex mode is not "on" by default, pls check your solver settings, but it might implicitely turn on with the definition of a damping 0 Good luck Ivar

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