Determination of Modal Parameters in Complex Nonlinear Systems

This paper describes a methodology for evaluating the modal parameters of complex nonlinear systems. It combines four different tools: the Coleman post-processing, the partial Floquet analysis, the moving window analysis, and the signal synthesis algorithm. The approach provides a robust estimation of the linearized modal parameters and qualitative information about the nonlinear behavior of the system. It operates on one or multiple discrete time signals and is able to deal with both time-invariant and periodic systems. The method is computationally inexpensive and can be used with multiphysics computational tools, and in principle, with experimental data. The proposed approach is validated using a simple, four degree of freedom model of a wind turbine. The predictions for the linear system are validated against an exact solution of the problem. For the nonlinear system, it is demonstrated that qualitative information concerning the nonlinear behavior of the system is obtained using the proposed method. Finally, the nonlinear behavior of a realistic, three-bladed horizontal axis wind turbine model is investigated.