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Rigid-body Μodes

Posted 14.02.2014, 08:56 GMT-5 Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers, Structural Mechanics Version 4.1, Version 4.2, Version 4.2a, Version 4.3, Version 4.3a, Version 4.3b, Version 4.4 5 Replies

Nicolas Tsagarakis
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Hello guys.

I am using Comsol more and more these days, so you will be hearing from me regularly from now on. I hope we'll have a nice chat. To the point now:

In Eigenfrequency Analysis of a system, is there somewhere an option not to calculate the Rigid-body modes, or at least not to present them as results?

Thanks a lot,
Nicholas
5 Replies Last Post 06.03.2014, 05:09 GMT-5
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago 14.02.2014, 11:17 GMT-5
Hi,

If you

1. Set the 'Search for eigenfrequencies around' parameter in the Study settings for Eigenfrequency to a non-zero value which is reasonable when compared to the lowest non-zero eigenfrequency you expect

and

2. Set 'Eigenfrequency search method around shift' to 'Larger real part'

you will get rid of the rigid body modes.

A side note: If you do compute also the rigid body modes, it is also important to set 'Search for eigenfrequencies around' to get good accuracy for the non-rigid modes.

Regards,
Henrik
Hi, If you 1. Set the 'Search for eigenfrequencies around' parameter in the Study settings for Eigenfrequency to a non-zero value which is reasonable when compared to the lowest non-zero eigenfrequency you expect and 2. Set 'Eigenfrequency search method around shift' to 'Larger real part' you will get rid of the rigid body modes. A side note: If you do compute also the rigid body modes, it is also important to set 'Search for eigenfrequencies around' to get good accuracy for the non-rigid modes. Regards, Henrik
Nicolas Tsagarakis
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Posted: 1 decade ago 14.02.2014, 17:32 GMT-5
Hello Henrik,

You've been perfectly clear, that was exactly what I was looking for. Thanks a lot!
Now that we are on topic, there's something else I was curious and never asked: Do the shapes of the rigid-body modes correspond to something real, or it's something that derive mathematically?

Regards,
Nicolas T.
Hello Henrik, You've been perfectly clear, that was exactly what I was looking for. Thanks a lot! Now that we are on topic, there's something else I was curious and never asked: Do the shapes of the rigid-body modes correspond to something real, or it's something that derive mathematically? Regards, Nicolas T.
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago 16.02.2014, 14:54 GMT-5
Yes, they are physical. They represent the six (in 3D) rigid body modes; three translations and three rotations.

You may however not be able to immediately identify them as such, since the computed modes are arbitrary superpositions of these six fundamental rigid body modes.

Regards,
Henrik
Yes, they are physical. They represent the six (in 3D) rigid body modes; three translations and three rotations. You may however not be able to immediately identify them as such, since the computed modes are arbitrary superpositions of these six fundamental rigid body modes. Regards, Henrik
Nicolas Tsagarakis
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Posted: 1 decade ago 28.02.2014, 13:56 GMT-5
Hello,

I'm bringing the thread back again for a little while, I just calculated the first eigenfrequencies in a Solid 3D model as I instructed to, I did get rid of the rigid-body modes, but now some frequencies come up like this: 112.292046+2.950222e-8i
The mode shapes are as expected though, no rigid-body modes.

Thanks again,
Nicolas T.
Hello, I'm bringing the thread back again for a little while, I just calculated the first eigenfrequencies in a Solid 3D model as I instructed to, I did get rid of the rigid-body modes, but now some frequencies come up like this: 112.292046+2.950222e-8i The mode shapes are as expected though, no rigid-body modes. Thanks again, Nicolas T.
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago 06.03.2014, 05:09 GMT-5
Hi,

As long as the eigenfrequencies look like that, there is no problem. The imaginary part is 10 orders of magnitude smaller than the real part, so this is just numerical noise.

Regards,
Henrik
Hi, As long as the eigenfrequencies look like that, there is no problem. The imaginary part is 10 orders of magnitude smaller than the real part, so this is just numerical noise. Regards, Henrik

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