A Comparison Between The Traditional Frequency Response Function (Frf) And The Directional Frequency Response Function (Dfrf) In Rotordynamic Analysis

Abstract

In any individual FRF plot of a rotor, the negative frequency region of the FRF is
merely a duplicate of the positive frequency region. Therefore, it is only necessary to treat
with one region of the FRF, conventionally the positive one, because it yields some physical
meaning. Thus, the directivity of a mode, forward or backward, generally cannot be easily
distinguishable in frequency domain through the use of traditional modal analysis in rotors.
The complex modal analysis is related with the application of classical modal analysis
principles to rotating systems, where the inputs and outputs are described by complex
variables. The advantage of this methodology, in comparison with the traditional modal
analysis in rotors, is the ability of incorporating directionality. The method separates the
backward and forward modes in the dFRF (Directional Frequency Response Function), so
that effective modal parameter identification is possible. In this paper, both theories of
traditional and complex modal analysis are revised. Aspects of numerical modeling are
discussed and numerical results are presented. Special attention is paid to the identification
of forward and backward precessional modes of isotropic and anisotropic rotor finite element
models in both FRF and dFRF plots.