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Frequency domain - mechanical

Posted Apr 15, 2012, 5:21 a.m. EDT Structural Mechanics Version 4.4 10 Replies

Mir
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Hi,
I am an electronics student and am trying to simulate the mechanics of a square plate.
My plate has the 4 side faces fixed and a uniform distributed load on one of the larger faces.
The stress and displacement are close to calculated values - hence I can justify that my understanding is verified by the simulation.
I tried to find the eigen frequencies. My query - is the solution given by comsol for eigen frequency = omega (= 2.pi.f) or is it = "f"?
I tried the frequency domain simulation. Here, the frequency that I enter are omega (= 2.pi.f) or is it "f"?

Please recommend me books where, from formulae, I can calculate the resonant frequency and compare with the simulated results. I tried Timoshenko - strength of materials -2, but did not get the frequency.

Thanks in advance.
10 Replies Last Post Apr 17, 2014, 6:27 p.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 16, 2012, 2:11 a.m. EDT
Hi

eifgenfrequency is in Hz or "f" eigenvalue in omega or rad/s. A frequency domain solving you should give the frequency in Hz. Check the model library and do some of the exercices therein (do not forget to update your model library first)

PS: try to get a newer version, 4.0 was a "first out" of an important change in the software, it was not very stable

--
Good luck
Ivar
Hi eifgenfrequency is in Hz or "f" eigenvalue in omega or rad/s. A frequency domain solving you should give the frequency in Hz. Check the model library and do some of the exercices therein (do not forget to update your model library first) PS: try to get a newer version, 4.0 was a "first out" of an important change in the software, it was not very stable -- Good luck Ivar
Mesut
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Posted: 1 decade ago Mar 10, 2014, 1:47 p.m. EDT
Hi
I am trying to get a frequency response using COMSOL 4.4 from a clamp-free shaft cantilever. The problem is that COMSOL solve it for a limited frequency ranges but not for my desired range. Can any one tell me what's wrong? I attached my simulation with the freq range that is working but I want to run it in range of 1-1.5 KHz and it doesn't work.
Regards, Mesut

Hi I am trying to get a frequency response using COMSOL 4.4 from a clamp-free shaft cantilever. The problem is that COMSOL solve it for a limited frequency ranges but not for my desired range. Can any one tell me what's wrong? I attached my simulation with the freq range that is working but I want to run it in range of 1-1.5 KHz and it doesn't work. Regards, Mesut

Attachments:

Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago Mar 12, 2014, 12:36 p.m. EDT
Hi,

Please see

www.comsol.com/community/forums/general/thread/42949/

Regards,
Henrik
Hi, Please see http://www.comsol.com/community/forums/general/thread/42949/ Regards, Henrik
Mir
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Posted: 1 decade ago Mar 16, 2014, 12:46 a.m. EDT
Resonance is a point of singularity, so the simulation (numerical number crunching) goes awry around that point (like facing a 1 divided by 0). The best you can do is
1. Find the resonance frequency from Eigen frequency (that option is available)
2. Run frequency response at all frequencies except close to the Eigen frequency
3. The peaking in response near the resonant frequency should suffice to show the overall trend in freq resp.
4. Remember, theory and simulation are different from practical. Fabrication process variations can really throw up different results. Use the simulation only as a confidence building measure.10% error or more is acceptable as long as your answers are within one order of variation.
Best wishes
Resonance is a point of singularity, so the simulation (numerical number crunching) goes awry around that point (like facing a 1 divided by 0). The best you can do is 1. Find the resonance frequency from Eigen frequency (that option is available) 2. Run frequency response at all frequencies except close to the Eigen frequency 3. The peaking in response near the resonant frequency should suffice to show the overall trend in freq resp. 4. Remember, theory and simulation are different from practical. Fabrication process variations can really throw up different results. Use the simulation only as a confidence building measure.10% error or more is acceptable as long as your answers are within one order of variation. Best wishes
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago Mar 17, 2014, 3:08 a.m. EDT
Hi,

As long as there is damping in your model (and there always is in real life) there is no problem in solving also at the resonance frequency. Determining an appropriate damping for a certain system can be difficult, though.

Regards,
Henrik
Hi, As long as there is damping in your model (and there always is in real life) there is no problem in solving also at the resonance frequency. Determining an appropriate damping for a certain system can be difficult, though. Regards, Henrik
Mesut
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Posted: 1 decade ago Mar 19, 2014, 4:18 p.m. EDT
Thanks Mir and Henrik for your response. I tried many numbers (even those that are irrelevant to this problem) and I could get some results for the 2nd to 10th mode, but the simulation doesn't converge for the first mode.
Thanks Mir and Henrik for your response. I tried many numbers (even those that are irrelevant to this problem) and I could get some results for the 2nd to 10th mode, but the simulation doesn't converge for the first mode.
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago Mar 20, 2014, 6:46 a.m. EDT
Hi Mesut,

If you get a message about non-convergence in the frequency response analysis even when having appropriate damping, please submit the model to support.

Regards,
Henrik
Hi Mesut, If you get a message about non-convergence in the frequency response analysis even when having appropriate damping, please submit the model to support. Regards, Henrik
Mesut
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Posted: 1 decade ago Mar 21, 2014, 10:22 a.m. EDT
Thank you Henrik. I attached the file. This simulation works for 5000-340000 Hz but it can't converge for 1000-2000 Hz.
Regards, Mesut
Thank you Henrik. I attached the file. This simulation works for 5000-340000 Hz but it can't converge for 1000-2000 Hz. Regards, Mesut

Attachments:

Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago Mar 25, 2014, 3:08 a.m. EDT
Hi,

The problem here is that the 'dynamical stiffness matrix' becomes extremely ill-conditioned. This kind of geometry should preferably be treated using beam elements. Using solids, you would need even more elements to arrive at an accurate solution.

But if you want to force this problem to be solved, you much switch of the check of the conditioning in the linear equation solver: In the 'Direct' node in the solver sequence, locate the 'Error' section. Set 'Check error estimate' to 'No'.

Regards,
Henrik
Hi, The problem here is that the 'dynamical stiffness matrix' becomes extremely ill-conditioned. This kind of geometry should preferably be treated using beam elements. Using solids, you would need even more elements to arrive at an accurate solution. But if you want to force this problem to be solved, you much switch of the check of the conditioning in the linear equation solver: In the 'Direct' node in the solver sequence, locate the 'Error' section. Set 'Check error estimate' to 'No'. Regards, Henrik
Mesut
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Posted: 1 decade ago Apr 17, 2014, 6:27 p.m. EDT
Hi everyone,
Does anyone know How can I get a displacement PSD (Power Spectrum Density) from a frequency response simulation in COMSOL? Does it matter if I use base motion or force excitation?
Regards, Masoud
Hi everyone, Does anyone know How can I get a displacement PSD (Power Spectrum Density) from a frequency response simulation in COMSOL? Does it matter if I use base motion or force excitation? Regards, Masoud

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