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Edge rotation for shell with prestressed eigenfrequency analysis always gives 0

Posted 22 mai 2018, 06:01 UTC−4 Results & Visualization, Studies & Solvers, Structural Mechanics Version 5.3a 2 Replies

Carlo Resta
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Hello! I'm having troubles trying to evaluate the rotation of one edge of a shell element around the axis parallel to it (in my case, the z axis). I'm running a prestressed eigenfrequency study, i.e., a two-step study with stationary and eigenfrequency steps. Trying to plot shell.thZ (or any other shell.th) gives a clean 0.0 over the whole shell surface.

Funny thing is, if I use a different study with only an eigenfrequency analysis (no stationary one), I get reasonable values for the same variable .

Am I forgetting or overlooking something obvious? Thank you in advance to anyone who will shed a light on this!

2 Replies Last Post 22 mai 2018, 11:35 UTC−4
Henrik Sönnerlind COMSOL Employee

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Posted: 7 years ago 22 mai 2018, 11:15 UTC−4

Hi Carlo,

Looking at numerical values of the rotations is not useful for an eigenfrequency analysis in any case. Since an eigenmode has an arbitrary scaling, the computed rotation angles will be nonsensical.

The only thing that can be useful to some extent is to look at the values of the rotational degress of freedom, since they scale in the same way as the displacements.

The reason you get different results without and with prestress:

In the first case, the angles are computed using the geometrically linear assumption (using a cross product). That expression does give values, but they have no meaning unless your eigenmode happens so be scaled to that the rotations are small.

In the second case, the expressions for geometric nonliearity are used. But those expressions give the value zero, because of the perturbation nature of the solution.

If you are interested, please refer to the user's guide for the exact expressions used to compute the rotations in the two cases.

Regards,
Henrik

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Henrik Sönnerlind
COMSOL
Hi Carlo, Looking at numerical values of the rotations is not useful for an eigenfrequency analysis in any case. Since an eigenmode has an arbitrary scaling, the computed rotation angles will be nonsensical. The only thing that can be useful to some extent is to look at the values of the rotational degress of freedom, since they scale in the same way as the displacements. The reason you get different results without and with prestress: In the first case, the angles are computed using the geometrically linear assumption (using a cross product). That expression does give values, but they have no meaning unless your eigenmode happens so be scaled to that the rotations are small. In the second case, the expressions for geometric nonliearity are used. But those expressions give the value zero, because of the perturbation nature of the solution. If you are interested, please refer to the user's guide for the exact expressions used to compute the rotations in the two cases. Regards, Henrik
Carlo Resta
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Posted: 7 years ago 22 mai 2018, 11:35 UTC−4

Thank you very much for your prompt answer!

Thank you very much for your prompt answer!

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