This contribution is concerned with the computational analysis of a rigid rotor supported by means of two self-acting foil air journal bearings. Even though the overall equation system is thereby typically written in a nondimensional form, prior knowledge about realistic value ranges of occurring dimensionless numbers is required in order to parameterize and interpret such simulations correctly. Unlike all other quantities, the nominal lubrication gap clearance between the rotating journal and the undeformed foil structure is reported to be only poorly known. Thus, even in the light of an advanced understanding of the bearing rotor system's fundamental behavior, the quantitative reproduction and prediction of experimental results by means of computational analysis need to be viewed critically. In this study, the sensitivity of numerical results towards the assumed nominal lubrication gap clearance will be investigated. To this end, the stability of the system is considered and the characteristics of occasionally observed equilibrium points and limit cycles are addressed.
Motivation and ModelingOffering reduced wear and power loss compared to rolling-element bearings, self-acting foil air journal bearings are an upcoming technology in high-speed rotating machinery. The load-carrying capacity of such bearings is achieved via a thin film of ambient air forming an aerodynamic lubrication wedge. Some undesirable side-effects, e.g., the occurrence of self-excited vibrations, may be reduced by concepts featuring a compliant foil structure inside the lubrication gap [1]. In current research on this topic, reliable numerical tools are of major interest with regard to the complexity and costliness of experiments. When operating within the full fluid film lubrication regime, the pressure distribution p = p(ϕ, z, t) is governed by the REYNOLDS equation for compressible ideal gases. Depending on the rotor's angular velocity ω 0 , the PDE can be stated as [2] Figure 1 illustrates the bearing model with the fluid film thickness h = h(ϕ, z, t) = C − e(t) cos [ϕ − Γ(t)] − q(ϕ, z, t) involving the nominal lubrication gap clearance C = R − r, the journal eccentricity e(t), the attitude angle Γ(t), and the foil structure deformation field q(ϕ, z, t). With increasing accuracy, the foils are modeled using either a rigid shell, an elastic foundation [3], or elastically supported beams [4]. The bearing forces acting on the considered rigid rotor are obtained by pressure integration.