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Eigenfrequency analysis problem

Posted 26 nov 2015, 03:52 GMT-5 Acoustics & Vibrations, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers, Structural Mechanics 3 Replies

Hung-Wei Lee
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Hi,

I'm dealing with a nano rod vibration.
I want to know the displacement of some point on the eigen mode. (0.5 L, 0.125 L, .....)
However, I found that I can't find the same value that is I will get different answer after evaluation (on all the same condition).

ex.
sol.1 : 700Hz 20um
sol.2 : 700Hz 30um

The following is the processes I did,

Study: eigenfrequency
desired number: 30

after compute

use cut point 3D set some point in the center of rod but at different L

use derived value, Point Evaluation , then change the Data set to cut point 3D

then find u, v, w

the result is that i can't find the totally consistent answer in two time~~

(I thing is not the mesh problem)



L=length of the rod.


Thank you very much.


HW

3 Replies Last Post 9 dic 2015, 01:57 GMT-5
Jeff Hiller COMSOL Employee

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Posted: 9 years ago 30 nov 2015, 16:01 GMT-5
Hello Hung-Wei,
The magnitude of eigenmodes is arbitrary, so the pointwise values of the displacements of an eigenmode depend on the normalization used as much as anything else: they have no inherent meaning.
Best,
Jeff
Hello Hung-Wei, The magnitude of eigenmodes is arbitrary, so the pointwise values of the displacements of an eigenmode depend on the normalization used as much as anything else: they have no inherent meaning. Best, Jeff
Hung-Wei Lee
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Posted: 9 years ago 7 dic 2015, 05:05 GMT-5
Hello Jeff,

Thank you so much.

But I'm still wondering that if I choose stored energy for the rod, is it arbitrary?

Thus, how to I calculate the energy transmission in a rod?

Thank you
Hello Jeff, Thank you so much. But I'm still wondering that if I choose stored energy for the rod, is it arbitrary? Thus, how to I calculate the energy transmission in a rod? Thank you
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 9 years ago 9 dic 2015, 01:57 GMT-5
Hi

From my understanding, and as stated above, as Eigen mode analysis are normalized to arbitrary displacements, hence the strain levels are also arbitrary. As the Young modulus relates linear stress and strain, the Stress levels are also arbitrary. The energy stored is the integration over the domain of the stress and the displacement, so yes: for Eigen mode analysis the modal energies are also arbitrary, but are qualitatively comparable (between modes).

To get an absolute information of stored energy of a vibrating rod (or any object) you need to inject a given finite energy, have true damping included and then you can get your absolute amplitudes and energy out.
This is best done with the frequency sweep solvers, by entering a representative frequency span and a known PSD (Power Spectral Density) distribution. Here COMSOL is still a bit week as you need to tweak it a bit to get such models quickly done. Classical vibration tests and shock tests are not prepared with standard scripts, and rotational modes are more complex to identify than for older classical engineering tools, but everything (and, in fact, far more physics than in the classical tools) are built into COMSOL so its a question of time to solve and writing and doing carefully our VV&C activities.

--
Good luck
Ivar
Hi From my understanding, and as stated above, as Eigen mode analysis are normalized to arbitrary displacements, hence the strain levels are also arbitrary. As the Young modulus relates linear stress and strain, the Stress levels are also arbitrary. The energy stored is the integration over the domain of the stress and the displacement, so yes: for Eigen mode analysis the modal energies are also arbitrary, but are qualitatively comparable (between modes). To get an absolute information of stored energy of a vibrating rod (or any object) you need to inject a given finite energy, have true damping included and then you can get your absolute amplitudes and energy out. This is best done with the frequency sweep solvers, by entering a representative frequency span and a known PSD (Power Spectral Density) distribution. Here COMSOL is still a bit week as you need to tweak it a bit to get such models quickly done. Classical vibration tests and shock tests are not prepared with standard scripts, and rotational modes are more complex to identify than for older classical engineering tools, but everything (and, in fact, far more physics than in the classical tools) are built into COMSOL so its a question of time to solve and writing and doing carefully our VV&C activities. -- Good luck Ivar

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