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The internal structure of many aero-engines is designed with a dual-rotor system. Up to now, there have been few studies on the influence of aerodynamic excitation on the dual-rotor system. The phenomenon of synchronous impact may occur when the frequency of the aerodynamic excitation force of the fan blade is close to the characteristic frequency of the inter-shaft bearing. This paper investigates the dynamic characteristics of a dual-rotor system under the condition of synchronous impact. The system's motion equations are formulated considering the complex nonlinearities of the inter-shaft bearing, such as Hertz contact force of 10/9 exponential function, clearance, and periodic varying compliance. In addition, the inter-shaft bearing with a local defect is considered. The fan blade’s aerodynamic excitation force is modeled by synthesizing multiple harmonic excitation forces, the amplitudes of which are obtained by the Fourier series expansion. Numerical simulations are employed to get the dynamic responses of the system. The results show that the dynamic characteristic of the dual-rotor system at the primary resonance caused by the high-pressure (H.P.) rotor is not changed by the aerodynamic excitation force, while the primary resonance caused by the low-pressure (L.P.) rotor increases significantly. However, three aerodynamic resonances of the amplitude-frequency response of the dual-rotor system are emerging in the low-frequency region (124, 146 and 186 rad/s). When the synchronous impact phenomenon occurs, the amplitude of the three resonance peaks will increase twice compared to the original status, leading to a doubled increase in the dynamic load of the inter-shaft bearing. The characteristics of the dual-rotor system affected by the parameters such as initial phase difference of local defect, rotor eccentricity of system, clearance of inter-shaft bearing, and the stiffness and damping of supports are discussed in detail. The results obtained provide a deep insight into the mechanism of synchronous impact.

Quasi-zero-stiffness (QZS) vibration isolator is widely used in low-frequency vibration isolation due to its high-static-low-dynamic-stiffness (HSLDS) characteristics. The complex nonlinear force of the QZS vibration isolator increases the difficulty of solving it while realizing the HSLDS characteristics. The typical analysis method is to use Taylor expansion to simplify the nonlinear force and make it approximate to polynomial form, which leads to inaccurate analysis results in the case of large excitation and small damping. Therefore, the modified incremental harmonic balance (IHB) method is used to directly analyze the dynamic characteristics of the QZS vibration isolator without simplification in this paper. The classical three-spring QZS vibration isolation model is used as the calculation example. The results are different from the previous approximate equation analysis results in three aspects: (1) There is no unbounded response of the system under displacement excitation; (2) Even harmonics and constant terms also exist in the response of the system and can lead to multiple solution intervals; (3) In the case of small damping and large excitation, both displacement excitation and force excitation have subharmonic resonance, reducing the vibration isolation performance of the system. In addition, the accuracy of the solution obtained by the IHB method is verified by the Runge-Kutta method. The accurate analysis method in this paper provides favorable theoretical support for the design and optimization of vibration isolators.

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