Extracting Modes Of Vibration Extracting Modes Of Vibration

If one wishes to investigate the vibration modes of a body, he should issue the command:


     MODES,  BODY_NAME, -OPTIONS

and the options are:


     -NUM_EVAL, NEV

     -NO_FIX

Here, BODY_NAME defines the name of the body for which the modes will be extracted, and the option -NUM_EVAL defines the number of modes, NEV, which will be extracted. If -NUM_EVAL is omitted, 20 modes will be computed. The -NO_FIX option controls the manner in which connectors and restraints are treated. MOSES uses the subspace iteration method outlined by Bathe in the book Finite Element Procedures in Engineering Analysis.

The way in which MOSES can use the modes as generalized degrees of freedom make the extraction of the modes themselves an interesting question. Normally, one takes the body in the specified configuration with the defined connections and computes the modes. For use as generalized degrees of freedom, however, we really want the unconstrained modes. Also, the body will move and as a result, its mass matrix will also change. What is the correct mass to use, and what should be done about the connections? These questions are left to the user. The mass due to ballast, weight, and any Morison's Equation added mass will be used when the modes are extracted. No added mass due to diffraction will be used since it is frequency dependent. If the -NO_FIX option is used, then all active connectors and restraints will be used. Without the option, the last node in the stiffness matrix will be fixed and no restraints or connectors will be applied.

Neither the concern about mass nor about connectors is very important if you intend to use the modes as generalized degrees of freedom. Here, one is simply looking for a reasonable subspace of the N degrees of freedom which "adequately" describe the deflection of the system. The correct connections and the correct mass will be added when any generalized degree of freedom analysis is performed.

As with the other commands in the STRUCTURAL menu, MODES produces no reports directly. One can look at the modes and the eigenvalues in the Structural Post-Processing Menu.