Research on lateral–torsional coupled vibration induced by substantial imbalance in an overhung flexible rotor

Abstract

Substantial imbalance is a real possibility in aero–engines and is usually caused by blade off, bird strikes, etc. A substantially unbalanced rotor system will exhibit extremely complicated nonlinear lateral–torsional behavior, and the related dynamics are critical to the safe design of a rotor system. In this paper, a comprehensive nonlinear model of an imbalanced flexible rotor system is proposed, in which translational–torsional coupling and angular–torsional coupling are introduced simultaneously. Based on the model, the lateral–torsional coupling features are discussed, and the lateral as well as the torsional vibration behaviors are further analyzed in detail. The results show that the lateral–torsional coupling factors only exist under the nonsynchronous whirl condition. Thus, if the imbalance is small, the lateral vibration exhibits the linear feature of a traditional unbalanced response, and the torsional vibration does not occur in steady state. However, under substantially unbalanced conditions, the vibration responses are significantly changed. In lateral vibration, many combined frequency components between rotation frequency (f_u) and torsional natural frequency (f_t1 and f_t2) such as f_u-f_t1, f_u-2f_t1 and f_u-f_t2 can be induced, which can lead to combined resonance phenomena and many additional resonance peaks. The lateral–torsional coupling effect is strong at that time. Both the lateral translational–torsional coupling excitation and lateral angular–torsional coupling excitation exhibits the multiple harmonics whose frequencies are closely related to f_t1 and f_t2. Consequently, the torsional vibration in those combined resonance regions can be very high. The corresponding vibration exhibits the steady harmonic feature with vibration frequency being f_t1 or f_t2.