Transient analysis of rotor-bearing systems using approximate support models

A computer model is developed to predict the transient and steady-state response of a flexible rotor on nonlinear fluid film supports. A modal approach is used for the rotor that incorporates uncoupled (planar) modes which are acted upon by physical forces transformed into modal coordinates. The modal state variables are marched through time by explicit or implicit numerical integration while the requested physical responses are extracted at each time step through inverse transformation from modal to physical coordinates.; Nonlinear hydrodynamic models are included for squeeze film dampers and tilting pad bearings. The squeeze film damper model consists of a one-dimensional integration of the isoviscous Reynolds equation to solve the circumferential pressure distribution. The tilting pad bearing model is based on tabulated polynomial approximations of pad dimensionless load and dimensionless damping from which the pad forces are computed at each integration time step. The individual pad forces are then assembled (added) vectorially to yield the net journal forces.; The linear aspects of the model are verified through comparison to closed-form solutions and to existing rotordynamic analyses. The nonlinear damper model is verified through comparison to two other analytical solutions based on circular orbit iterations. A lack of analytical or experimental data for tilting pad bearings precluded direct verification of the tilting pad bearing model. However, response predictions for a compressor on nonlinear tilting pad bearings exhibited very reasonable results.; A single-stage overhung compressor supported in tilting pad bearings is analyzed with both rigid and flexible bearing supports. The results show that the bearing nonlinear effects can couple the rotor with the foundation, creating a coupled mode that can have a much higher amplification factor than that predicted by a linear analysis. An aircraft turbofan engine is analyzed under steady-state unbalance and transient surge excitations for various squeeze film damper designs. The predicted responses indicate that a new damper design having a greater length and larger radial clearance is expected to offer improved attenuation of the high-pressure rotor response under high unbalance conditions.