Open veiguf opened 4 years ago
@KratosMultiphysics/structural-mechanics
The cantilever beam has an analytical solution for eigen modes: https://www.comsol.fr/multiphysics/eigenfrequency-analysis
may be related with the torsional modes?
Torsional modes might be a good tip! We saw it on another model and saw the same "problem" with the example from the repository.
What we are seeing in this case is that the 8th and 22nd modes have values of the eigenvector a magnitude higher than the other modes. Something with the angles? Scaling with the mass matrix? Rad deg? Or do we have a mistake in our thinking?
The problem came up when using the eigenvectors for calculating the analytical sensitivities of the eigenfrequencies and assumed that the eigenvectors were scaled to the mass matrix ("normalize_eigenvectors": true in JSON). Has anyone use analytical gradients when optimizing beams considering eigenfrequencies?
I had similar experiences
@armingeiser maybe you have an idea?
Hm the material/cross section parameters for this example do not seem to be "physical". I suggest to run the analysis using values for moment of inertia and area that are calculated from a "real" cross section e.g. rectangular or circular.
I had the experience that if these values do not have proper relations to each other, the behaviour of beams can be quite unexpected.
@veiguf: The third degree of freedom of each node or globally?
I checked again and it is not only the third but also the forth and the fifth degrees of freedom of each node. As an example, I attach a screenshot of the EIGENVECTOR_MATRIX of the first node.
this might be related to #6434
We saw the problem described in this issue with beams and shells, though not with volume elements. Therefore, we are thinking that it has to do with the rotational degrees of freedom. We had a couple ideas where we want to look, but have not had time yet. Next couple weeks...
I have not been following issue 6434 in detail. Is that problem with all element types?
I think it depends on the eigensolver and hence I would expect it to be more or less independent of the element type, but not sure
The eigenvalues appear to be good, some degrees of freedom of the eigenvectors seem to be strange, but only beams and shells.
What I would like to look at the coming weeks:
Hi, It seems to me that there are some strange results on the eigenvector-matrix. I get them with the following code:
Using the example of the beam_eigenvalue_analysis, the values for the 1st, the 8th and the 22th frequency (index 0, 7 and 21 in Python) and the 3rd (index 2 in Python) degree of freedom have a completely different order of magnitude compared to the other values. Can this be correct? How are the eigenvectors normalized?