For the Shell Solver, as discussed before, it gave out incorrect eigen results for box-type structures. The case you metnioned below regarding eigen analysises dones't have right corners so it might be correct.
As you may understanding, right corner cases are quite common for shell structures. If the shell solver can not deal with this kind of cases, it is very difficult to use for many analysises.
mika wrote: ↑09 Sep 2020, 17:17I just updated the source codes of both shell solvers in order to enable mass-proportional damping. If the material section has the command
Rayleigh Damping Alpha = Real ...
the solver should now create a damping matrix D = alpha * M, with M being the mass matrix. I didn't have time to run any shell case with damping, but in principle this should work after recompilation.
I nevertheless hesitate over how much time I'm willing to spend developing the facet shell solver, since the analysis presented in the article
Bernadou, M, Trouve, P. Approximation of general shell problems by flat plate elements. Part 2: Addition of a drilling degree of freedom. Comput. Mech., 6 , 359–378 (1990)
gives a good reason to think that for general shells the facet finite element approach gives curvature-dependent errors which cannot be reduced by refining the mesh. The article proposes a perturbation term and a way to estimate the shell curvatures to get rid of this error, but the facet shell solver of Elmer doesn't try to add such a modification. Moreover, the plate formulation used in Elmer is different from that used in the article, so one should perhaps do some maths to find an appropriate perturbation term for the Reissner-Mindlin model. I believe there are better approaches to general shell problems.
What makes you think that switching to eigenanalysis makes the solution obtained with the shell solver incorrect? There are some verification cases where the results of shell eigenanalysis has been compared with results given in literature, obtained with commercial software or given by other models of the same problem
https://github.com/ElmerCSC/elmerfem/tr ... s_Cylinder
https://github.com/ElmerCSC/elmerfem/tr ... _Spherical
https://github.com/ElmerCSC/elmerfem/tr ... enanalysis
If wrong results are obtained, I doubt the root reason may not be that the solution type is selected to be eigenanalysis, but something else.
A non-traditional feature of the (newer) shell solver of Elmer is that it models the straining in the normal direction. Unfortunately the normal strain is a difficult concept when the director data makes large jumps. It has now become known that the discontinuities in the director data would need some special treatment to get the equations right. Hopefully we can improve this somehow in future, so that this wouldn't be a cause for opting for the otherwise doubtful facet shell model.
On the other hand, the ability of the shell solver to model normal strain makes the coupling with solid elements more straightforward and we have recently developed some functionality related to such coupling. In addition to the test Shell_with_Solid_Eigenanalysis, other tests related to this option are
https://github.com/ElmerCSC/elmerfem/tr ... hmarkCase1
https://github.com/ElmerCSC/elmerfem/tr ... hmarkCase2
https://github.com/ElmerCSC/elmerfem/tr ... Solid_Beam
https://github.com/ElmerCSC/elmerfem/tr ... Beam_Eigen
I'm afraid trying to repeat these coupling tests with the facet shell solver would require more than a quick modification of the code. Adding beam sections would be an easier task by making some additional modularization. Nevertheless, with very limited resources it would be ideal to put efforts to developing one shell solver rather than sharing the work between two solvers. Perhaps this is an ideal thought as in some cases it might be nice to have an option to use solid shell elements (very special 3-D finite elements) and this would be a third approach.
-- Mika