Influence of fluid film bearing nonlinearities on monitoring the fluid-induced instability threshold

In rotor dynamics, one of the most common ways for modelling hydrodynamic forces is through the linear coefficients of stiffness and damping since this approach is much less time-consuming. However, in several situations, the rotating system has highly nonlinear behaviour. In these, methods to calculate the hydrodynamic forces in their true nature, without any further simplifications, are required, such as in the fluid-induced instability regime. Therefore, the present work compares numerical vibration data in the time and frequency domains for a real turbine rotor, using linear and nonlinear models to represent the dynamics of hydrodynamic bearings with different geometries. The main analyses are focused on evaluating the effects of nonlinearities on the system's instability threshold and proposing a way to monitor this process. As a result of using the proposed method, it was observed at the instability threshold, determined in the nonlinear run-up simulations, the excitation of several frequencies and not only the natural frequency as would be expected. This fact evidences the nonlinear character of the phenomenon and the importance of considering nonlinear models to assess the instability threshold of rotating systems. It is concluded that the proposed analysis can monitor the onset of fluid-induced instability in real-time. (AU)