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Frequency domain modal analysis of a cantilevered beam

Posted 11 mars 2012, 12:41 UTC−4 MEMS & Nanotechnology, MEMS & Piezoelectric Devices, Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 4.1, Version 4.2, Version 4.2a, Version 4.3, Version 4.3a, Version 4.3b, Version 4.4 9 Replies

Sankha Mukherjee
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Hello all,

I am trying to run frequency domain modal analysis of a cantilevered beam. I have built the model in Comsol 4.1 and have assigned desired material properties. Have applied fixed constraint criteria for one end of the beam. Now, the resonance frequency of the beam is 610 Hz. I am interested in the analyzing the beam between 400-900 Hz. The problem is that when I run the model I do not get any modal displacements. For all the frequencies I get '0' displacement throughout the model. But this can not be true because the beam has resonance at 610 Hz. I have tried using boundary load at the fixed boundary and body load also. But the problem persists. I am attaching the model with this mail. Can anyone suggest where am I going wrong?


Thanks

Sankha
9 Replies Last Post 27 févr. 2014, 13:43 UTC−5
Sankha Mukherjee
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Posted: 1 decade ago 12 mars 2012, 01:05 UTC−4
Hello,

It seems that the frequency sweep is working. However, if the step is less than 10, the software shows errors and stops. The error is shown just before the resonance frequency. For example if I run the seep from 300 Hz to 700 Hz at a step of 10 Hz, the model works but it does not for the same range and a step less than 10 Hz. Does anyone have an idea as to why this is happening?

I have one more question though. What is the difference between frequency domain analysis and frequency domain modal analysis?

-Sankha
Hello, It seems that the frequency sweep is working. However, if the step is less than 10, the software shows errors and stops. The error is shown just before the resonance frequency. For example if I run the seep from 300 Hz to 700 Hz at a step of 10 Hz, the model works but it does not for the same range and a step less than 10 Hz. Does anyone have an idea as to why this is happening? I have one more question though. What is the difference between frequency domain analysis and frequency domain modal analysis? -Sankha
Sankha Mukherjee
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Posted: 1 decade ago 12 mars 2012, 01:42 UTC−4
Hello,

There is something else which I do not understand either. If I use a finer mesh this error message pops up "Failed to find a solution for all parameters, even when using the minimum parameter step. The relative error (X) is greater than the relative tolerance Returned solution has not converged.". Only If I use a normal or coarse mesh (which are there by default) the module runs successfully. I wonder why this is happening?

-Sankha
Hello, There is something else which I do not understand either. If I use a finer mesh this error message pops up "Failed to find a solution for all parameters, even when using the minimum parameter step. The relative error (X) is greater than the relative tolerance Returned solution has not converged.". Only If I use a normal or coarse mesh (which are there by default) the module runs successfully. I wonder why this is happening? -Sankha
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 12 mars 2012, 11:36 UTC−4
Hi

when you do frequency sweeps (particularly without any damping) then often the peaks are very narrow, and you need to manually tweek the frequency steps you ask for to carefully resolve the peaks/resonances and if possible try to avoid just THE resonance frequency

--
Good luck
Ivar
Hi when you do frequency sweeps (particularly without any damping) then often the peaks are very narrow, and you need to manually tweek the frequency steps you ask for to carefully resolve the peaks/resonances and if possible try to avoid just THE resonance frequency -- Good luck Ivar
Victor Soares
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Posted: 1 decade ago 17 juil. 2013, 09:28 UTC−4
Try add a Boundary Load, then right click on it and select Harmonic Load.
Try add a Boundary Load, then right click on it and select Harmonic Load.
Nicolas Tsagarakis
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Posted: 1 decade ago 14 févr. 2014, 06:54 UTC−5

Try add a Boundary Load, then right click on it and select Harmonic Load.



It seems like the Frequency Domain-Modal Study needs a load with "Harmonic Perturbation" in order to work and unlikewise the Frequency Domain Study works only with a simple load.

The reason is not quite obvious to me from the references, shall anyone know and explain this part?
[QUOTE] Try add a Boundary Load, then right click on it and select Harmonic Load. [/QUOTE] It seems like the Frequency Domain-Modal Study needs a load with "Harmonic Perturbation" in order to work and unlikewise the Frequency Domain Study works only with a simple load. The reason is not quite obvious to me from the references, shall anyone know and explain this part?
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago 14 févr. 2014, 10:38 UTC−5
Hi,

You can find a discussion about this in the posting from May 23, 2013 in

www.comsol.com/community/forums/general/thread/35454

Regards,
Henrik
Hi, You can find a discussion about this in the posting from May 23, 2013 in http://www.comsol.com/community/forums/general/thread/35454 Regards, Henrik
Nicolas Tsagarakis
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Posted: 1 decade ago 14 févr. 2014, 16:39 UTC−5
Thanks Henrik,
I will check it out and continue there then.

Regards,
Nicolas
Thanks Henrik, I will check it out and continue there then. Regards, Nicolas
Jyani Vaddi
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Posted: 1 decade ago 24 févr. 2014, 19:12 UTC−5
Is it possible to apply a non-zero prescribed displacement boundary condition with harmonic perturbation for a frequency domain modal study? I am trying to apply a frequency dependent displacement (using the global variable "freq") but the values does not seem to be updating with each frequency step.

The documentation says that in order for a modal analysis to work, there should be no non-zero displacement BCs. Can this be the reason why my displacement variable does not update?
Is it possible to apply a non-zero prescribed displacement boundary condition with harmonic perturbation for a frequency domain modal study? I am trying to apply a frequency dependent displacement (using the global variable "freq") but the values does not seem to be updating with each frequency step. The documentation says that in order for a modal analysis to work, there should be no non-zero displacement BCs. Can this be the reason why my displacement variable does not update?
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago 27 févr. 2014, 13:43 UTC−5
Hi,

Since all eigenmodes have zero displacement where there is a constraint, any superposition of them will also be zero there. This is why it is not possible to have nonzero prescribed displacements in any modal superposition.

If it is really necessary, there is an old and ugly engineer's trick:

1. Replace the constraint with a *very* large mass. This will not change the eigenvalues much.

2. In the reponse analysis, replace the prescribed displacement with a force which moves the mass as much as you need. Since the mass is so large, you can use F=m*a=-m*u*omega^2.

But beware; selecting a proper value of the mass is not trivial. If you want to try it, start with some simple models.

Regards,
Henrik
Hi, Since all eigenmodes have zero displacement where there is a constraint, any superposition of them will also be zero there. This is why it is not possible to have nonzero prescribed displacements in any modal superposition. If it is really necessary, there is an old and ugly engineer's trick: 1. Replace the constraint with a *very* large mass. This will not change the eigenvalues much. 2. In the reponse analysis, replace the prescribed displacement with a force which moves the mass as much as you need. Since the mass is so large, you can use F=m*a=-m*u*omega^2. But beware; selecting a proper value of the mass is not trivial. If you want to try it, start with some simple models. Regards, Henrik

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