Saeid Dousti

Temporal and Convective Inertia Effects in Plain Journal Bearings With Eccentricity, Velocity and Acceleration

This paper extends the theory originally developed by Tichy (Tichy and Bou-Said, 1991, Hydrodynamic Lubrication and Bearing Behavior With Impulsive Loads,” STLE Tribol. Trans. 34 , pp. 505–512) for impulsive loads to high reduced Reynolds number lubrication. The incompressible continuity equation and Navier-Stokes equations, including inertia terms, are simplified using an averaged velocity approach to obtain an extended form of short bearing Reynolds equation which applies to both laminar and turbulent flows. A full kinematic analysis of the short journal bearing is developed. Pressure profiles and linearized stiffness, damping and mass coefficients are calculated for different operating conditions. A time transient solution is developed. The change in the rotor displacements when subjected to unbalance forces is explored. Several comparisons between conventional Reynolds equation solutions and the extended Reynolds number form with temporal inertia effects are presented and discussed. In the specific cases considered in this paper, the primary conclusion is that the turbulence effects are significantly more important than inertia effects.

An Extended Reynolds Equation Including the Lubricant Inertia Effects: Application to Finite Length Water Lubricated Bearings

Water lubricated bearings used in nuclear coolant pumps and sub-sea applications exhibit large lubricant inertia forces in the magnitude order of viscous forces. To model these bearings the traditional Reynolds equation is not adequate. An extended Reynolds equation is developed in this study which takes into account the turbulence and inertia effects: both convective and temporal. The most complete form of temporal inertia which applies to the turbulent regime as well, is developed that consists of primary and secondary temporal inertia terms. The convective inertia model follows Constantinescu’s approach [1,2]. The turbulence model is also Constantinescu’s which is tuned with a CFD work. The dynamic coefficients including the lubricant added mass coefficients of a full cylindrical fixed geometry water bearing are obtained. It is observed that the convective inertia increases the load capacity and stability of the bearing. Significant lubricant added mass coefficients comparable to the shaft mass are calculated, which exhibit destabilizing effects in general.Copyright

Time Transient Analysis of Horizontal Rigid Rotor Supported With O-Ring Sealed Squeeze Film Damper

This study addresses the nonlinear dynamic behavior of O-ring seals as the retaining spring in squeeze film dampers (SFDs). An analytical model is developed to predict the restoring and hysteresis forces of elastomer O-rings based on experimental and numerical data. This model takes into account the temperature softening and excitation frequency hardening effects in O-rings as well as the installation conditions in the form of radial and vertical preloads, σ and γ, respectively. Long bearing assumption is adopted for the solution of Reynolds equation. The equations of motion of horizontal unbalanced rigid rotor are derived, and a dimensional analysis is conducted on them. The numerical results substantiates the synchronizing effects of bearing parameter, B and vertical preload, γ, and the asynchronizing effects of O-ring parameter, O and radial preload, σ. It is shown that the variation of temperature and rotational speed as operating conditions influence the rotor response significantly.Copyright

Elastomer O-Rings as Centering Spring in Squeeze Film Dampers: Application to Turbochargers

This study addresses the nonlinear dynamic behavior of O-ring seals as the centering spring in squeeze film dampers. An analytical model is developed to predict the restoring and hysteresis characteristics of elastomer O-rings and extends the existing models to non-axisymmetric load conditions. Also, the inclusion of axial seals increases the damping capacity of squeeze film dampers. O-rings forces are investigated for different sizes, installation preloads, and operating conditions and contrasted with squeeze film damper’s hydrodynamic forces. This model is incorporated in linear and time transient rotordynamic analyses of a vertical turbocharger. It is shown that the O-rings contribution in rotor response for low rotational speeds and low amplitude vibrations is dominant and can not be ignored.© 2013 ASME

Sliding Mode Control in Ziegler’s Pendulum With Tracking Force: Novel Modeling Considerations

Zieglers pendulum is an appropriate model of a nonconservative dynamic system. By considering gravity effect, new equations of motion are extracted from Newtons motion laws. The instability of equilibriums is determined by linear stability analysis. Chaotic behavior of the model is shown by numerical simulations. Sliding mode controller is used for eliminating chaos and for stabilizing the equilibriums. Copyright

ROTOR DYNAMIC MODELING OF GEARS AND GEARED SYSTEMS

The ability to accurately predict rotating machine resonant frequencies and to assess their stability and response to external forces is crucial from a reliability and preventive maintenance perspective. Resonant frequencies and forced response become more difficult to predict when additional complicated components such as gearboxes are present in the rotor system. Gearbox dynamics contain many complex interactions and many of the simplifying assumptions provided in the literature do not apply to most geared systems. A finite element formulation of the gearbox, which couples the axial, lateral, and torsional degrees-of-freedom of the low and high-speed shafts, is developed. It has the capability to apply to a wide variety of both spur and helical geared systems and is sufficiently robust to account for arbitrary orientation angles between the parallel shafts. This study presents a rotor dynamic analysis of an industrial drive-train consisting of a steam turbine, herringbone gearbox, and a generator using 1-D Timoshenko beam elements. The rotor dynamic analysis consists of the calculation of the damped natural frequencies, mode shapes, and provides insight into the stability of the industrial drive-train.Copyright

Forced Response of a Flexible Rotor With Squeeze Film Damper Under Parametric Change

Squeeze film dampers play an important role in the dynamics of modern turbomachinery by improving vibrational response and stability. The present paper develops an effective tool for evaluating the forced response of these systems under parametric changes. A flexible rotor with multiple masses supported on a squeeze film damper at one end is investigated. The forced response of this asymmetrically supported system is obtained using the harmonic balance method with a predictor-corrector procedure. This response is examined with various parameters including unbalance forces with and without fluid inertia effects, unidirectional loads, stiffness of centering spring of the damper and the gyroscopic effects of the disks. The developed tool predicts the nonlinear jump phenomenon of the damper with large unbalance forces, indicates the present of multiple harmonics within the response with high damper eccentricity and shows the insensitivity of the damper to surrounding gyroscopic variation.Copyright