Frequency Spectrum

Limitations

Currently frequency responses are allowed only for sizing optimization.

Options in Context of Frequency Response

The SOL108 (direct frequency response) and SOL111 (modal frequency response) are supported for MSC Nastran®. Gravity loading and temperature loadings are not allowed in finite element modeling.

Solution excitation frequencies defined using FREQ, FREQ1, and FREQ2 are allowed in the finite element input file. The solution excitation frequencies defined using FREQ3, FREQ4, and FREQ5 should be used with care because the eigenvalues usually change a lot during optimization.

In frequency response, structural damping PARAM, G, and GE on the MATi are allowed for all design elements. The structural damping should be the same for all design elements, and the structural damping for the design elements should also be defined using OPT_PARAM yielding,

OPT_PARAM
  ...
  DAMP_STRUCTURAL_STIFF = G or GE
  ...
END_

where G or GE is the structural damping of the design elements.

Important: All kinds of discrete damping elements and other kind of damping are allowed outside the design domain if they are not a function of the eigenfrequencies or eigenmodes. That is, modal damping (for example, defined using SDAMPING) is prohibited.