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modal analysis and mass participation factors

Posted May 24, 2011, 2:02 a.m. EDT Acoustics & Vibrations, Results & Visualization 4 Replies

Mikael Evander
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Hi.

I currently modeling the resonant behavior of some different structures in the acoustics module. I have previously just looked at the eigenfrequencies to try to find the most interesting resonances. One major problem have been how to judge how strong the different resonances will be.

After spending some time yesterday I discovered that I might be able to use modal analysis and extract the mass participation factors to get a measure of the energy contained within each resonance mode. I can't however find any useful documentation on how to do this in COMSOL 4.1. If I understood things right, it should be possible to do this kind of analysis in 4.1 but since I have never worked with it before it would be very useful with some kind of example/tutorial or documentation on it.

Does anyone know of anything useful or where to look?

best regards
Mikael
4 Replies Last Post May 25, 2011, 7:10 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago May 24, 2011, 10:28 a.m. EDT
Hi

indeed the mass participation factors (for linear displacements along u,v,w only, no rotation yet) are awailable in 4.1,

You must turn them "on" check the "Output" normalisation field in the "Eigen-value/frequency solver" node (the one deepest down there) and change from RMS to Mass matrix mod1_u. Then in the Derived Values use a "global evaluation" solver Particpation factor MPF_mod1_u, v, w to dump the values in a table

I can remind you that the SUM over all modes of MPF^2 = Mass of mobile object (for u,v and w), so if you directly ask for the square of the particpation factor, divided by the mobile mass then you have the particpation factor in %

I have noticed that in V4.2 if you have a RBE (Rigid Body Element) you have further normalisation choices, but I havent had to check these yet

--
Good luck
Ivar
Hi indeed the mass participation factors (for linear displacements along u,v,w only, no rotation yet) are awailable in 4.1, You must turn them "on" check the "Output" normalisation field in the "Eigen-value/frequency solver" node (the one deepest down there) and change from RMS to Mass matrix mod1_u. Then in the Derived Values use a "global evaluation" solver Particpation factor MPF_mod1_u, v, w to dump the values in a table I can remind you that the SUM over all modes of MPF^2 = Mass of mobile object (for u,v and w), so if you directly ask for the square of the particpation factor, divided by the mobile mass then you have the particpation factor in % I have noticed that in V4.2 if you have a RBE (Rigid Body Element) you have further normalisation choices, but I havent had to check these yet -- Good luck Ivar
Mikael Evander
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Posted: 1 decade ago May 24, 2011, 12:09 p.m. EDT
Thanks Ivar. You are the true hero of these forums :)

I can't wait to try it tomorrow morning at work.
Thanks Ivar. You are the true hero of these forums :) I can't wait to try it tomorrow morning at work.
Mikael Evander
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Posted: 1 decade ago May 25, 2011, 2:40 a.m. EDT
Yup, worked like a charm :)

For the record, and I assume it's because I'm in the pressure acoustic mode, the variable is called MPF_mod1_p.

Oh and btw, since I have a damped model, I get complex eigenfrequencies which seem to translate into complex participation factors. Would you happen to have any idea on how to interpret that or where to read more about it? I feel a bit unsure if I should be looking at either the real or imaginary part, calculate the magnitude or what would be the most correct way of comparing the modes.

thanks again!
Yup, worked like a charm :) For the record, and I assume it's because I'm in the pressure acoustic mode, the variable is called MPF_mod1_p. Oh and btw, since I have a damped model, I get complex eigenfrequencies which seem to translate into complex participation factors. Would you happen to have any idea on how to interpret that or where to read more about it? I feel a bit unsure if I should be looking at either the real or imaginary part, calculate the magnitude or what would be the most correct way of comparing the modes. thanks again!
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago May 25, 2011, 7:10 a.m. EDT
Hi

I assume then the name is because you solve for "p" in acoustics and not necessarily for "u" as "usually" in Solids

Indeed I haven't though about the complex mass participation factors: so should one take the abs() or the realdot() ? probably not only the real().

That would need some tests

--
Good luck
Ivar
Hi I assume then the name is because you solve for "p" in acoustics and not necessarily for "u" as "usually" in Solids Indeed I haven't though about the complex mass participation factors: so should one take the abs() or the realdot() ? probably not only the real(). That would need some tests -- Good luck Ivar

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