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March 14, 2010 6:18pm UTC

Solution of an eigenfrequency problem

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

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March 17, 2010 7:32am UTC in response to Mihir Shetye

Re: Solution of an eigenfrequency problem

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

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March 17, 2010 10:07am UTC in response to Ivar Kjelberg

Re: Solution of an eigenfrequency problem

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 :-)

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