Frequency Domain Study
If all loads are time harmonic when seen from a) the corotating frame for a Solid Rotor and b) the stationary frame for a Beam Rotor or Solid Rotor, Fixed Frame, then a frequency domain study is an efficient method for computing the response of the rotor. In this study type, all quantities are represented by complex amplitudes, where the relation between the real and imaginary parts determines the phase.
The Frequency Domain study provides an important analysis to determine the vibration amplification factor at the resonance, which in turn helps in determining the separation margin between the operating speed and critical speed of the rotor.
In a rotordynamic analysis, the only meaningful analysis frequency in a frequency domain study is the one that corresponds to the angular speed of the rotor.
In the COMSOL Multiphysics Reference Manual:
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Frequency Domain
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Studies and Solvers
In the Structural Mechanics Module User’s Guide:
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Frequency-Domain Analysis
Prestressed Analysis
The shift in the natural frequency of the prestressed structure can also have significant effect on its frequency response. This is particularly important when the loading frequency is close to one of the natural frequencies of a structure.
Like the prestressed eigenfrequency analysis, prestressed frequency domain analysis is useful to get the response of a rotor-bearing system with a nonlinear bearing behavior for a harmonic loading.
Mode Following
When analyzing the evolution of eigenfrequencies as function of a swept parameter, it is convenient if the eigenfrequencies of the distinct modes are tracked continuously. You can achieve this by making use of mode following when performing an eigenfrequency study.
This can for example make it easier to identify critical speeds in a Campbell Diagram. In particular, if the eigenfrequencies are obtained in rotating frame of reference, and subsequently converted to corresponding eigenfrequencies in the space-fixed reference frame.
For more information about mode following, see Mode Following in COMSOL Multiphysics Reference Manual.