Modal analysis strategy and nonlinear dynamic characteristics of complicated aero-engine dual-rotor system with rub-impact

This paper aims to gain insight into the nonlinear modal characteristics and the possible influence of the modes on the responses for the practical dual-rotor system with rub-impact in aero-engine. The finite solid element method combined with a constraint stiffness model produced by rub-impact is introduced to build the governing equation of the complicated nonlinear dual-rotor system. In order to deal with the efficiency and numerical divergence in the process of solving the nonlinear modes of this large-scale nonlinear system, an analysis strategy is proposed by integrating a two-layer reduction technique into the harmonic balance method. The effectiveness of the analysis strategy is validated by applying to a simple rotor system, which can easily obtain the theoretical result. Based on the modeling method and analysis strategy, the modal characteristics of an aero-engine dual-rotor system with rub-impact are revealed. The results show that the modal frequency of the dual-rotor system increases when rub-impact occurs and has the feature of interval, which allows us to obtain the critical speeds of the rubbing system by traditional Campbell diagram. The rotation direction is an important factor since it can not only affect the gyroscopic effect but also change the friction effect of the rub-impact. It is found that the modal frequencies of the counter-rotation dual-rotor are less than those of co-rotation condition. More importantly, the forward modes of the counter-rotation dual-rotor may be instable when rub-impact occurs at a certain rotor, while the corresponding modes under the co-rotation condition are always stable. Furthermore, by analyzing the rubbing response of the dual-rotor, it is found that the modal characteristics have an important influence on rotor’s response. The instable forward modes existing in the counter-rotation dual-rotor may lead to the divergence of the response when passing the corresponding critical speed.