Modelling of Gas Foil Bearings Towards Controllable Operation Multi-domain Analysis

The feasibility of combining gas lubrication with compliant surfaces has been known since at least the 1960s. These principles are combined in the Gas Foil Bearing (GFB) offering a versatile, mechanically simple, oil–free and environmentally
friendly mechanism for the support of lightweight high-speed rotating machinery. The applicability of GFBs is, however, restricted by several factors; a limited number of start–stop cycles, an inherently low level of damping and rich hardto-
predict dynamics. This project is motivated by the latter and is aiming at improving the fundamental understanding of GFB dynamics and the available modelling tools. Furthermore, it is an objective to leverage the improved modelling
capacity to push towards augmentation of GFBs with injection, eventually enabling a mechatronic GFB with adaptable properties. The first two papers are devoted to the modelling of friction. A truss-based bump foil model, a beam-based top foil model and a smooth friction model are combined and it is concluded that the foil mass must be included to retain simultaneity. It is demonstrated that a dynamic friction model is not sufficient to explain the observed discrepancies between simulations and experimental observations, and it is hypothesized that a model allowing sticking would be required. The third paper presents an extended perturbation method treating the foil degrees of freedom explicitly. This is demonstrated to provide onset speeds of instability in better agreement with the non-linear time integration than the classical perturbation technique in which only the rotor states are perturbed. The fourth paper presents an all-new GFB simulation tool. This includes a finite volume-based discretization of the Reynolds Equation coupled to generic foil and rotor models along with clearly defined domain interfaces. Importantly, the paper also details the assembly of the system Jacobians. The fifth paper deals with eigenvalue analysis of the system Jacobians. The predicted stability limits are shown to exactly match those of the extended perturbation, while diverging from the classical perturbation as the compliance level increases. Furthermore, Campbell diagrams are extracted, and the multi-domain modes are visualized. In the final paper, a scheme for modelling the injection mass flow based on interpolation from Computational Fluid Dynamics results is coupled to the bearing code. The feasibility of adding injection to an existing GFB supported test rig is investigated, leading to a predicted 20% increase of the stability limit and reduced sub-synchronous vibrations.