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Eigenfrequencies of a spring WITH Gravity

Posted Jul 29, 2013, 6:28 p.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers, Structural Mechanics Version 4.3a 1 Reply

Ben Levy
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In the real world, I have a flat steel beam that is acting as a spring. It starts out curved into a semicircular shape, and then it is loaded with a hanging weight that brings it down flat. I want to find the eigenfrequencies of the loaded spring using COMSOL. How can I do this?

I have been able to successfully find the natural frequencies for a beam that is "pre-flattened," but I don't know how to add the effects of a hanging weight (subject to gravity, of course).

I am very new to COMSOL, so I would greatly appreciate anyone who could take the time to explain this in a simple manner. Thanks!
1 Reply Last Post Jul 30, 2013, 9:45 a.m. EDT
Josh Thomas Certified Consultant
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Posted: 1 decade ago Jul 30, 2013, 9:45 a.m. EDT
Ben-

I believe you will need to do this in 2 steps. First, a stationary step that calculates the "pre-stressed" state which is used as an input for your eigenfrequency analysis.

I'd recommend following tutorial model called "Vibrating String" as a template for your problem. This model is found in the model library.


With the eigenfrequency study all loads and sources are taken to be zero regardless of how your physics set-up is defined (so this is why you need to use an Initial Stress and Section definition). Just FYI - the eigenvalue and eigenfrequency studies are basically the same as the because they use the same solver. The relationship between the eigenvalues, lambda, and the eigenfrequencies, f, is as follows: f=-lambda/(2*pi*i).

Also, here's how the documentation describes an eigenfrequncy study:

If all sources are removed from a frequency-domain equation, its solution becomes zero for all but a discrete set of angular frequencies, omega, where the solution has a well-defined shape but undefined magnitude. These solutions are known as eigenmodes and the corresponding frequencies are eigenfrequencies.

Hope that gets you on the right track.

Best regards,
Josh Thomas
AltaSim Technologies




Ben- I believe you will need to do this in 2 steps. First, a stationary step that calculates the "pre-stressed" state which is used as an input for your eigenfrequency analysis. I'd recommend following tutorial model called "Vibrating String" as a template for your problem. This model is found in the model library. With the eigenfrequency study all loads and sources are taken to be zero regardless of how your physics set-up is defined (so this is why you need to use an Initial Stress and Section definition). Just FYI - the eigenvalue and eigenfrequency studies are basically the same as the because they use the same solver. The relationship between the eigenvalues, lambda, and the eigenfrequencies, f, is as follows: f=-lambda/(2*pi*i). Also, here's how the documentation describes an eigenfrequncy study: If all sources are removed from a frequency-domain equation, its solution becomes zero for all but a discrete set of angular frequencies, omega, where the solution has a well-defined shape but undefined magnitude. These solutions are known as eigenmodes and the corresponding frequencies are eigenfrequencies. Hope that gets you on the right track. Best regards, Josh Thomas AltaSim Technologies

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