Dynamics of a rotor-structure-soil system: transient response by iterative coupling

Abstract

In this paper, the transient dynamics of a rotor-foundation-structure-soil system is studied, aiming to obtain unbalance response of the rotor considering the effect of the structure and the influence of the unbounded soil. The transient responses are obtained through iterative coupling between the rotor subsystem and the frame-soil subsystem. The non-linear rotor subsystem uses the model of a Laval rotor supported by rigid bearings and presenting external and internal damping mechanisms. These equations of motion are integrated using the fourth-order Runge-Kutta method. In the frame-soil subsystem, the frame is modeled using the Finite Element Method (FEM), and the homogeneous half-space is modeled using 3D version of the Direct Boundary Element Method (DBEM) in the frequency domain. To obtain time domain equations of motion of this second system a methodology is developed, which is based on the extraction of modal quantities from the Frequency Response Functions of the coupled soil-foundation system. The modal parameters are obtained using the Rational Fraction Polynomial Method (RFPM). The equivalent time domain equations of motion for the soil-structure subsystem may be integrated by standard techniques. The methodology renders transient response for the rotor and structure with very small time steps, allowing an accurate simulation of the rotor runup phase and the analysis of the dynamics of the system going through resonance frequencies.