On the Multiphysics Modelling of Rotor-Bearing-Foundation Dynamics & the Model-Based Control Design of Active Fluid Film Bearings

Tilting-pad journal bearings (TPJB) are commonly used by the industry in turbomachinery due to their ex-cellent stability properties. The main drawback is the lack of damping at high rotational speeds, which can be overcome by using a squeeze-film damper attached to their housing (passive approach) or active radial oil injection (mechatronic approach).

TPJB with active oil injection (ATPJB) uses high-pressure oil to adjust the rotor posi-tion and increase damping. Modelling these controllable bearings is not a trivial task from the viewpoint of fluid dynamics and strong fluid film forces couple the lateral movements of the rotor to the bearing housing and founda-tion.

In this framework the entire rotor-bearing-foundation system must be considered and the coupled dynamics modelled. In this paper the complex coupling between rotor, bearing, and foundation dynamics is described by a multi-physics model instead of approaching the foundation dynamics via equivalent impedance.

Individual components of a test-rig composed of a flexible rotor, ATPJB, pedestals, and flexible foundation are discretised using FE methods in MatLab and ANSYS, based on the geometrical and material properties of each component. By implementing solid 3D models in ANSYS and using Craig-Bampton reduction, the accuracy and efficiency of the model is evaluated.

Experimental modal analysis (EMA) is used to measure the natural frequen-cies, damping ratios and mode shapes of the coupled rotor-bearing-foundation system. The results are used to validate the mathematical multi-physics model. The theoretical and experimental results fit with a high degree of accuracy, the first 7 theoretical and experi-mental natural frequencies and mode shapes deviate less than 8%.

More tuning and optimisation of the model is recommended, particularly the coupling between ATPJB and other components. Nevertheless the current model provides useful insights to how these rotor-bearing-foundation systems can be described and is a step closer to-wards regulating the ATPJB using model-based control design.