Many thanks for your interest and providing your point of views.

You are right. This model supposes to have 3 rigid-body modes. Their frequencies should be zero or numerical zero. The results from FreeCAD and Ansys both have these 3 modes. It is what I expect and thus I compare frequencies except these 3 rigid-body modes.

This very well explains the pair modes I got and the difference between them. Could I ask if you have some references about this?mac_the_bike wrote: ↑Sat Jan 22, 2022 1:01 pm One feature of a vibration analysis of a cylinder, which is completely axisymmetric, is that the non RBM modes come in pairs, i.e 2 modes will have the same frequency, see Note 3. The RBM modes will not be in pairs.

In the figures there appear to be modes that are almost in pairs. This discrepancy can be assigned to at least the following:

1. the mesh is not the same over the surface. If you look at the TDC of the mesh you will a line of elements - this pattern doesn't appear anywhere else.

2. the nodes don't lie strictly on a cylinder, see Note 4.

Um... I am not sure if it is a coincidence or they really share some sort of relation. But I modify my analysis settings and get the results which are quite close to the analytical solution. Seemac_the_bike wrote: ↑Sat Jan 22, 2022 1:01 pm Note 2: I am also questioning the FreeCAD frequency values, of the second analysis, because if you divide the FreeCAD frequencies by PI the new numbers are close to the Ansys values.