November 25, 2020 at 9:01 pm

peteroznewman

Subscriber

nA free-free modal analysis results in the first six modes being the zero frequency rigid body modes. You only care about modes 7 and above which are the bending modes of the structure.nYou need the structure to be grounded to get a static solution, but you don't want a Fixed Support to be included in the modal analysis.nI suggest you try a Kinematic connection to ground. This requires you to have three non-colinear vertices on your geometry. For an object with rotational symmetry, I would suggest the following.nFirst create a Cylindrical Coordinate System at the center of the face, with Z normal to the flat face along the rotor axis.nThen create a Displacement support using the Cylindrical Coordinates that has X = Free, Y = 0 and Z = 0.nIn the Cylindrical system, X = R, Y = Theta and Z = Z.nIf you have sliced your geometry, select 3 vertices, A,B,C. If you have not sliced your geometry, select 3 nodes which could be roughly 90 or 120 degrees apart.nWhen you run the Modal analysis, there will be no rigid body modes, but I expect the frequency of mode 1 of the model with the Kinematic Displacement BC will be the same or similar to mode 7 in a Free-Free. Try it without the prestress and see what you get and let me know the results in your reply.n