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The discrepancy of results of eigenfrequency and frequency-domain modal studies

Posted 21.08.2015, 12:24 GMT-4 Structural Mechanics Version 5.0 5 Replies

Koray Akcengiz
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Hi,

I first performed an eigenfrequency analysis of a simple hollow cylinder, in which I obtained 6 different eigenfrequenies, however, I then performed a frequency-domain modal study (defined harmonic boundary load acting on the outer shell) and obtained the maximum displacements occuring at differing frequencies as a table. When I plot the table, I see that three of the eigenfrequencies show no resonant like behaviors at all.

I tried differing meshes (also the model is extremely simple) and also very narrow step sizes (range(0,0.1,3000)) so I do not think that it is due to mesh size or step size.

Any ideas why this might happen?

Thanks,
Koray
5 Replies Last Post 03.07.2017, 12:36 GMT-4
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Hello Koray Akcengiz

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Thanh Le
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Posted: 7 years ago 23.05.2017, 23:25 GMT-4
Hi,
I have the same problem when simulating a hollow cylinder. Have you figure out how to fix it?
Thanks,
Thanh.


Hi,

I first performed an eigenfrequency analysis of a simple hollow cylinder, in which I obtained 6 different eigenfrequenies, however, I then performed a frequency-domain modal study (defined harmonic boundary load acting on the outer shell) and obtained the maximum displacements occuring at differing frequencies as a table. When I plot the table, I see that three of the eigenfrequencies show no resonant like behaviors at all.

I tried differing meshes (also the model is extremely simple) and also very narrow step sizes (range(0,0.1,3000)) so I do not think that it is due to mesh size or step size.

Any ideas why this might happen?

Thanks,
Koray


Hi, I have the same problem when simulating a hollow cylinder. Have you figure out how to fix it? Thanks, Thanh. [QUOTE] Hi, I first performed an eigenfrequency analysis of a simple hollow cylinder, in which I obtained 6 different eigenfrequenies, however, I then performed a frequency-domain modal study (defined harmonic boundary load acting on the outer shell) and obtained the maximum displacements occuring at differing frequencies as a table. When I plot the table, I see that three of the eigenfrequencies show no resonant like behaviors at all. I tried differing meshes (also the model is extremely simple) and also very narrow step sizes (range(0,0.1,3000)) so I do not think that it is due to mesh size or step size. Any ideas why this might happen? Thanks, Koray [/QUOTE]
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 7 years ago 24.05.2017, 08:28 GMT-4
Hi

An Eigenfrequency study (in 3D) gives you all possible modes in all directions, a frequency domain study implies a load and a direction and will excite well only some particular modes. Have you considered this too ?

In 2D, 2D axi ... or in 3D with symmetry or anti-symmetri conditions, you might not see all modes in an eigenfrequency model, as some are cancelled out by the symmetry conditions or the specific 2D, 2D-axi ... implicit symmetries

--
Good luck
Ivar
Hi An Eigenfrequency study (in 3D) gives you all possible modes in all directions, a frequency domain study implies a load and a direction and will excite well only some particular modes. Have you considered this too ? In 2D, 2D axi ... or in 3D with symmetry or anti-symmetri conditions, you might not see all modes in an eigenfrequency model, as some are cancelled out by the symmetry conditions or the specific 2D, 2D-axi ... implicit symmetries -- Good luck Ivar
Thanh Le
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Posted: 7 years ago 02.07.2017, 21:22 GMT-4

Hi

An Eigenfrequency study (in 3D) gives you all possible modes in all directions, a frequency domain study implies a load and a direction and will excite well only some particular modes. Have you considered this too ?

In 2D, 2D axi ... or in 3D with symmetry or anti-symmetri conditions, you might not see all modes in an eigenfrequency model, as some are cancelled out by the symmetry conditions or the specific 2D, 2D-axi ... implicit symmetries

--
Good luck
Ivar


Hi Ivar,

Thank you for your response. I haven't consider the excitements in my simulation yet. My project is to figure out at which frequencies a cavity with known dimension can be resonated to compare with the measurement. The measurement is done with a piezoelectric transducer plate to excite the cavity and an acoustic sensor to get the signal. In the simulation, I use a point source to excite and obtain the total displacement at some positions on the outer surface of the cylinder. However, there some points that I'm really not sure about:

1. Is the excitement I used correct in the frequencies domain study?
2. I used solid mechanics model which I'm not sure suitable for a cavity resonants simulation
3. I used only Fine/Finer/ExtraFine the physics-controlled meshing in Comsol to simulation. Does the meshing quality affect much to the eigenfrequencies?

It would be great if you can help me to figure them out.

Thanks,
Thanh Le.
[QUOTE] Hi An Eigenfrequency study (in 3D) gives you all possible modes in all directions, a frequency domain study implies a load and a direction and will excite well only some particular modes. Have you considered this too ? In 2D, 2D axi ... or in 3D with symmetry or anti-symmetri conditions, you might not see all modes in an eigenfrequency model, as some are cancelled out by the symmetry conditions or the specific 2D, 2D-axi ... implicit symmetries -- Good luck Ivar [/QUOTE] Hi Ivar, Thank you for your response. I haven't consider the excitements in my simulation yet. My project is to figure out at which frequencies a cavity with known dimension can be resonated to compare with the measurement. The measurement is done with a piezoelectric transducer plate to excite the cavity and an acoustic sensor to get the signal. In the simulation, I use a point source to excite and obtain the total displacement at some positions on the outer surface of the cylinder. However, there some points that I'm really not sure about: 1. Is the excitement I used correct in the frequencies domain study? 2. I used solid mechanics model which I'm not sure suitable for a cavity resonants simulation 3. I used only Fine/Finer/ExtraFine the physics-controlled meshing in Comsol to simulation. Does the meshing quality affect much to the eigenfrequencies? It would be great if you can help me to figure them out. Thanks, Thanh Le.
Shubham Shubham
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Posted: 7 years ago 03.07.2017, 12:29 GMT-4
Hi,

My case is also similar as I can see only my first and fifth resonance modes (of a 3D square diaphragm model) in the graph after plotting displacement vs frequency.

Is there any other way around to calculate the exact displacements (not relative because of normalization) at the eigen frequencies?

Any help will be appreciated.

Best.
Shubham
Hi, My case is also similar as I can see only my first and fifth resonance modes (of a 3D square diaphragm model) in the graph after plotting displacement vs frequency. Is there any other way around to calculate the exact displacements (not relative because of normalization) at the eigen frequencies? Any help will be appreciated. Best. Shubham
Shubham Shubham
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Posted: 7 years ago 03.07.2017, 12:36 GMT-4
Hi,

I have posted my detailed thread here:
www.comsol.com/community/forums/general/thread/149152/


My other doubt is how to remove this normalization effect to get accurate displacements at eigen frequency modes.

One of the examples for Bracket analysis says "Note that the displacement values are normalized and have no physical significance. The normalization method can be changed in the Eigenvalue Solver node, available under the Solver Configuration node"

But it doesn`t mention what setting to change? Any idea anyone?

Best.
Shubham
Hi, I have posted my detailed thread here: https://www.comsol.com/community/forums/general/thread/149152/ My other doubt is how to remove this normalization effect to get accurate displacements at eigen frequency modes. One of the examples for Bracket analysis says "Note that the displacement values are normalized and have no physical significance. The normalization method can be changed in the Eigenvalue Solver node, available under the Solver Configuration node" But it doesn`t mention what setting to change? Any idea anyone? Best. Shubham

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