Squeeze Film Dampers (SFDs) are widely used in aero-engines, gas turbines and rotating machines in order to reduce the shaft vibration and eliminate instabilities. Especially in aero-engines, use of SFDs is mostly preferred due to the undesired unbalance loadings on the rotor system and short length SFDs are desired in order to satisfy low weight and space requirements. Hydrodynamic squeeze film pressure forces are generated by SFDs due to dynamic forces on the rotor-bearing system. Therefore, advanced linear rotor dynamic models for rotor-bearing system and nonlinear models for squeeze film dampers are required in order to investigate the nonlinear vibration characteristic of rotor-bearing systems supported by SFDs.In this study, nonlinear forced response of a 1D finite element based multi-mass flexible rotor-bearing system supported by short length SFDs is studied. The effects of rotor parameters, geometrical parameters of the SFD (Length, diameter and clearance), oil parameters (viscosity, density) and cavitation type of the oil are studied. Rotordynamic model of the system is developed by using 1D Timoshenko beam elements for flexible shafts, lumped inertias for disks, linear springs and viscous dampers for bearings and structural damping for the system. Stiffness, mass, damping and gyroscopic matrices are obtained for the rotor-bearing system supported by SFDs. The SFD model is based on the analytical solution of Reynolds equation for open ends short length SFDs assuming circular-centered orbit motion. The nonlinear damping coefficient matrix and mass coefficient matrix generated due to the effect of fluid inertia are obtained. In this model, rotor system matrices and nonlinear SFD coefficient matrices are incorporated to obtain the full system mass, gyroscopic, damping and stiffness matrices. The resulting nonlinear differential equations of motion are converted into a set of nonlinear algebraic equations by using Harmonic Balance Method (HBM), which is solved by using Newton’s method with arc-length continuation.For the numerical case study, rotor of a small turbojet engine is used. The model without SFD is analyzed and the critical speeds and unbalance responses are obtained and compared with the results available in the literature. The nonlinear responses of the rotordynamic system are investigated for different SFD parameters, different bearing locations, different amount of unbalance forces and their locations. It is observed that the jump phenomenon occurs due to the nonlinear forces generated by SFDs, especially, for half film cavitation due to the presence of cross-coupled terms in the damping and mass matrices. It is observed that behavior of the jump region is highly dependent on the parameters of the SFD and the amount of unbalance.

Nonlinear Vibrations of Rotor-Bearing Systems Supported by Squeeze Film Dampers Due to Unbalance Excitation

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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