Olivier Bonneau

Non-linear behavior of a flexible shaft partly supported by a squeeze film damper

High speed rotors pose stability problems, especially when the speed of rotation increases above a critical speed. In this case, the dynamic behavior of the fluid bearings has an important effect. This study presents several models of the dynamic behavior of bearings, taking into account the flexibility of the shaft. The behavior of the squeeze film damper is also described. This element provides damping but its behavior is totally non-linear. An approach for an active squeeze film damper is presented: a variable clearance squeeze film damper or a variable viscosity squeeze film damper (feed with electrorheological fluid) is used to control a flexible shaft. These technologies could be very promising in the future.

Investigation about Electrorheological Squeeze FilmDamper Applied to Active Control of Rotor Dynamic

Electrorheological (ER) fluids, discovered in 1947 by W. WINSLOW, are concentrated suspensions of solid particles in an oily base liquid. Exposed to a strong electric field, their resistance to flow increases very greatly and this change is progressive, reversible and occurs very rapidly. Nowadays, ER fluids, made of lithium salt and fluorosilicon got rid of their old abrasive characteristics and are able to provide a good interface between electronics and mechanical components. A bibliographical study on ER fluids and ER technology has been carried out. The aim of this study is adapting ER technology to Squeeze Film Damper. In order to provide an active control on a flexible rotating shaft so as to command the whole shaft/bearings device in case of high rotating speed or heavy load trouble. Results of numerical computation of a shaft bearing assembly with a Squeeze Film Damper using negative ER fluid are showed in order to see the possibility of avoiding critical speeds by natural frequency shifting. A technical study of ER Squeeze Film Damper design is also presented, taking into account ER fluid properties and ER technology requirements.

Experimental Analyses of a First Generation Foil Bearing: Startup Torque and Dynamic Coefficients

This paper presents the results of the experimental analysis of static and dynamic characteristics of a generation 1 foil bearing of 38.1 mm diameter and L/D=1. The test rig is of floating bearing type, the rigid shaft being mounted on ceramic ball bearings and driven up to 40 krpm. Two different casings are used for startup and for measurement of dynamic coefficients. In its first configuration, the test rig is designed to measure the startup torque. The foil bearing casing is made of two rings separated by a needle bearing to enable an almost torque free rotation between the foil bearing and the static load. The basic results are the startup torque and the lift-off speed. In its second configuration, a different casing is used to measure the impedances of the foil bearing. Misalignment is a problem that is minimized by using three flexible stingers connecting the foil bearing casing to the base plate of the test rig. The test rig enables the application of a static load and of the dynamic excitation on the journal bearing casing and can measure displacements, forces, and accelerations. Working conditions consisted of static loads comprised between 10 N and 50 N and rotation frequencies ranging from 260 Hz to 590 Hz. Excitation frequencies comprised between 100 Hz and 600 Hz are applied by two orthogonally mounted shakers for each working condition. Stiffness and damping coefficients are identified from the complex impedances and enable the calculation of natural frequencies. The experimental results show that the dynamic characteristics of the tested bearing have a weak dependence on the rotation speed but vary with the excitation frequency.

Study of a Novel Squeeze Film Damper and Vibration Generator

A novel squeeze film damper and vibration generator (SFD&VG) is proposed as an option in the vibration control field. The SFD& VG can be used as an active squeeze film damper (ASFD) or as a vibration generator (vibrator), for unidimensional vibration damping or generation. The SFD&VG concept is connected with current research to improve a common industrial process-drilling of deep holes. The SFD&VG is based on the variable area of the lubricant film, which allows the development of a variable force, and a change in fluid film stiffness and damping. The analysis is initiated for an elementary configuration of the SFD&VG-the infinite width Rayleigh step case-and then it is developed for an advanced elliptical SFD&VG. The Reynolds equation is solved for both pure squeeze film effect which provides vibration damping, and pure hydrodynamic wedge effect which provides vibration generation. The the oretical part is continued with the SFD&VG dynamic simulation. The SFD&VG experimental device and vibration measurements, performed for the two defined regimes, ASFD and vibration generator, are presented. Finally, the experimental and theoretical results are briefly compared.

Complete Squeeze-Film Damper Analysis Based on the Bulk Flow Equations

The article presents the numerical adaptation of the “bulk flow” model for the analysis of squeeze-film dampers of industrial design. These components are characterized by high squeeze Reynolds numbers and by complex feeding and sealing systems. The “bulk flow” system of the equation is an efficient model for dealing with high Reynolds number regimes but its numerical treatment needs to be adapted for taking into account the feeding orifices and the circumferential groove as well as the openings of the piston ring seals. These adaptations are performed in the frame of the SIMPLE numerical algorithm used for dealing with the pressure-velocity coupling. The conservative character of the finite volume discretization is preserved without any penalty for the overall efficiency of the numerical procedure. The results depict how film discontinuities and local sources or sinks are embedded into the pressure field. Finally the complete numerical algorithm is validated against experimental data and its advantages are underlined by comparisons with a Reynolds based approach of squeeze-film dampers.

Evaluation of Rayleigh–Plesset Equation Based Cavitation Models for Squeeze Film Dampers

This work is intended to evaluate a cavitation model based on the complete Rayleigh–Plesset (RP) equation for use in squeeze film damper calculations. The RP equation governs the variation in the radius of the cavitation bubbles at rest, surrounded by an infinite incompressible fluid and subjected to an external pressure. This equation is obtained from the momentum equation and it takes into account the ensemble of the phenomena related to the dynamics of the bubbles (surface tension, damping, and inertia). All the terms in the RP equation will be taken into account in the present work plus a dilatation viscosity introduced by Someya in 2003. Numerical results will be compared with experimental data obtained by Adiletta and Pietra in 2006. The results underline the influence of the effects contained in the RP equation on the pressure field.

Experimental Study of the Radial and Tangential Forces in a Whirling Squeeze Film Damper

The present work introduces the main results of a detailed experimental program aimed at investigating the influence of the design parameters and of the working conditions on the global characteristics of squeeze film dampers (SFD) for serial aircraft engines. The SFD is sealed at both ends and is executing circular centered orbits. The results are presented for the tangential and the radial global force in the SFD and the leakage flow rate. The pressure field inside the SFD could also be measured and shows the influence of the inertia force, of the cavitation, and probably of the turbulence regime, as well as the interaction of these effects with the feeding groove. The limits of the existing theoretical models are also demonstrated. Review led by Bill Marscher

Influence of Axial Thrust Bearing on the Dynamic Behavior of an Elastic Shaft: Coupling Between the Axial Dynamic Behavior and the Bending Vibrations of a Flexible Shaft

This paper presents the nonlinear dynamic behavior of a flexible shaft. The shaft is mounted in two journal bearings and the axial load is supported by a hydrodynamic thrust bearing. The coupling between the axial thrust bearing behavior and the bending vibrations of the shaft is studied in particular. The shaft is modeled with typical beam finite elements. The dynamic behaviors of the fluid supports are considered as nonlinear. The dynamic behavior is analyzed using an unsteady time integration procedure. The paper shows the coupling between the axial dynamic behavior and the bending vibrations of the shaft.

Determination of the Discharge Coefficient of a Thin-Walled Orifice Used in Hydrostatic Bearings

The characteristics of hydrostatic bearings can be influenced by the compensating device they use, for example, a thin-walled orifice (diaphragm). The flow through the orifice is given by a law where an ad hoc discharge coefficient appears, and, in order to guarantee the characteristics of the hydrostatic bearing, this coefficient must be calibrated. The aim of this work is to provide an accurate estimation of the discharge coefficient under specific conditions. Therefore an experimental bench was designed and a numerical model was carried out. The results obtained then by the experimental and theoretical approach were compared with the values given by the literature. Finally, the influence of the discharge coefficient on the behavior of a thrust bearing is examined.

Experimental Analysis of Cylindrical Air-Bearing Dynamic Coefficients

The present work contains a detailed presentation of a test rig for measuring the static and dynamic characteristics of air bearings. The test rig consists of a 60-krpm spindle entraining a shaft guided by a Lomakin hybrid bearing. The test bearing is floating, overhung at one end of the shaft, and mounted on a squirrel cage. The bearing is statically loaded by a spring, and the dynamic excitation is applied by two shakers. Frequency-dependent dynamic coefficients are identified by sweeping the allowable excitation frequencies and by using a transfer function representation of the complex impedances. Test results are presented and compared with theoretical predictions for a cylindrical aerodynamic bearing of 30-mm diameter, L/D = 1, and 22-μm clearance.