Finite element study of a vertical rotor with tilting pads supported by an elastic structure

Abstract

Although the dynamics of vertical rotor bearing systems have been studied, the interaction between vertical rotors, bearings, and supporting structures - such as casings, bearing brackets, and foundations, remains less explored. This study presents a combined experimental and numerical investigation of a coupled vertical rotor system, incorporating a nonlinear, speed- and eccentricity-dependent bearing. The novelty lies in the description of a complex, vertical, rotor-bearing-support system incorporating a nonlinear journal bearing model, to capture the effects of the rotor’s vertical orientation, as typical of hydropower applications. The system features an elastic mid-span rotor supported by a flexible tower structure. The four-shoe tilting pad bearings impose significant stiffness variations and nonlinearities, connecting the stationary and rotating components. Modal analysis identifies the critical speeds of the flexible supporting structure, and simulations in the time domain are conducted for various run-up conditions, focusing on the bearing response across the structure’s first two natural frequencies. The results show qualitative and quantitative agreement between the experimental and simulated responses, highlighting the distinct dynamic behaviors of the upper and lower bearings. The bearing response at the structure’s first critical speed is studied and demonstrates improved accuracy during critical conditions. This model builds on established methods to accurately represent vertical rotor dynamics with nonlinear, eccentricity- and speed-dependent bearing models, while extending its applicability to more complex systems by incorporating bearing support flexibility, effectively providing a framework for simulating systems such as complete hydropower units.