I´ve found something weird using Natural Vibrations module to obtain the vibrational modes of a simple square plate simply supported at the corners. Whereas most modes look fine, e.g. mode 0 or 6 (see below), some of them seem wrong, e.g. try mode 8 or 13 in the blue slider. PlateVibrations.gh (21.0 KB) These latter modes do not have any amplitude, even using high deformation values in Display Scales tab from Model View component. I also tried to reduce Eps and increase MaxIter in NVibes without success.
the weird displacement pattern comes from drilling degrees of freedom (see attached file: PlateVibrations_cp.gh (20.4 KB)
In K3D shell elements do not have a drilling stiffness. That is why in case of a flat plate one needs to support one node against rotation about the axis perpendicular to the middle plane otherwise there is a rigid body mode in the system. The relation between the drilling DOFs of a shell element in K3D is not a rigid mechanism but maintained with a certain stiffness. This is I think the reason why it shows up in the Eigenfrequency calculation. I will check that however.
Sorry, I´m a bit confused. First you say that K3D shell elements do not have drilling stiffness, but latter you say the relation between the drilling DOFs of a shell element in K3D is not a rigid mechanism but maintained with a certain stiffness. (I know these things are hard to explain here)
In case there exists such drilling stiffness in K3D shell elements, is there a way to adjust it?
I´m trying to reproduce Chladni figures (“nodal patterns”) on a flat square plate, at least virtually with K3D. A short video here if you´re curious.
the drilling dof-relation within a triangular element in K3D sets up a mechanism: when rotating one of these DOFs the other two will rotate about the same angle. That’ why blocking the drilling DOF in one node is sufficient for a flat plate.
I need to check whether the issue you face is related to the element-formulation or the eigenvalue-solver.
A quick workaround could be to place drilling-supports at all nodes.