Discussion Forum

I've attached my model, if anyone is willing to have a look and tell me where the imaginary eigenfrequencies are coming from I'd greatly appreciate it.

Henry,

your model is not fully defined. You need to add constraints to avoid translations and rotations of the cylinder.

To see the free eigenmodes without constraints, increase the 'Desired number of eigenfrequencies' above 6. The first 6 are the trivial ones.

Cheers
Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com

Thank you Edgar for the prompt reply!

Hi,

Just to complement Edgar's reply: If you know the approximate eigenfrequencies, you can change the value in the "Search for eigenvalues around" field to a value other than 0.

From the "Eigenfrequency search method" list, you can also select "Region" to search for eigenvalues in a specified eigenfrequency range only.

As Edgar pointed out, for a 3D object, the first 6 eigenfrequencies are 0, corresponding to three translations and three rotations (rigid body modes), so what you see as small and possibly imaginary numbers are just "0 with some numerical noise".

Best regards,
Magnus Ringh, COMSOL

This is great information Magnus, thank you.

A follow-up question: if the first 6 eigenfrequencies correspond to translational and rotational motion, which eigenfrequencies will correspond to longitudinal, transverse, and torsional modes of vibration? I have been trying to figure out how to automate comsol to extract a list of those three frequencies only, and tell me which frequency is which. Are they listed in a specific order? Or is there a way to run a global evaluation on the Derived Values node and select those specific eigenfrequencies as values to export to a table?

Thank you,
Henry

I've adjusted my model to display 15 eigenfrequencies, and I'm still imaginary results beyond the first 6 frequencies, and am getting ones that display a real and imaginary part despite no damping. I've attached the adjusted model here.

Possibly useful information: for this project my supervisor wants me to model a completely free solid cylinder to compare to our theoretical benchmark. Basically I am trying to replicate this comsol example but for a solid cylinder instead of a hollow one: www.comsol.com/model/eigenfrequency-analysis-of-a-free-cylinder-271

Thank you,
Henry

Henry,

the imaginary components are already 3 - 4 orders of magnitude smaller than the real components. Try to use a finer mesh and the imaginary components will become even smaller or disappear.

Cheers
Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com