Effects of Forward/Backward Whirl Mechanism on Nonlinear Normal Modes of a Rotor/Stator Rubbing System

Abstract

In this paper, the effects of forward and backward whirl mechanism on the existence and the stability of multiple nonlinear normal modes (NNMs) in a four degree-of-freedom (DOF) rotor/stator rubbing system with cross-coupling stiffness and dry friction are investigated analytically. The NNMs may possess either positive or negative modal frequencies, corresponding, respectively, to the inherent motions of forward or backward whirl, and can be either stable or unstable. The relationship between the NNMs, regarding to their stability, and the forced system responses of the system is of great interest. It is found that a stable NNM corresponds to a forced harmonic response with the same whirl direction and frequency as the NNM, and an unstable NNM may still influence some forced system responses by contributing a frequency component equal to the modal frequency to the response spectrum.