Abstract: The modal-perturbation method, which is the combination of the modal analysis method and matrix perturbation method，is applied in nonlinear rotor system. The natural frequencies and the corresponding modes at the initial point are solved by subspace iterative method, the modes at other step are solved by matrix perturbation method. In the modal―perturbation method, the local modes need not solve again by the subspace iterative method, and the nonlinear DOF need not iterative calculate when the excitation is not nonlinear. A nonlinear rotor system simulation shows that the modal-perturbation method is effective for analyzing the long term behavior of dynamical systems with nonlinearity.

Key words: Matrix perturbation, Mode analysis, Mode truncation, Nonlinear dynamics, Rotor system