The effects of air film pressure constraints and top foil detachment on the static equilibrium, stability and modal characteristics of a foil-air bearing rotor model

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Abstract

The linear force coefficients method (LFCM) has been a mainstay in the computation of the onset of instability speed (OIS) and Campbell diagrams of foil-air bearing (FAB) rotor systems, despite reported inconsistencies with low amplitude dynamic analysis of the nonlinear system. The author recently established a linearisation method that eliminates such errors since it is based on the Jacobian of the nonlinear system. Unlike LFCM, this static equilibrium, stability and modal analysis (SESMA) procedure is not restricted to a spring-damper foil model. This paper investigates for the first time the effects of air film constraints and top foil detachment using SESMA, backed by transient nonlinear dynamic analysis (TNDA). The novel FAB model comprises an air film acting on a modal beam top foil that can detach from discretely distributed bumps according to a smoothed bilinear model. Application to a symmetric rigid rotor on single-pad FABs reveals abrupt shifts in modal frequencies and damping as the contact state of a bump with the top foil changes with speed. The applicability of the Gümbel correction for simpler models is illustrated. The SESMA procedure operates efficiently and accurately. The TNDA results present the first independent verification of previously presented top foil detachment simulations.
Original languageEnglish
Pages (from-to)115590
JournalJournal of Sound and Vibration
Early online date17 Jul 2020
DOIs
Publication statusPublished - 2020

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