Showing papers on "Lubrication theory published in 1984"

Volume-cycled oscillatory flow in a tapered channel

Abstract: Oscillatory viscous flow in a tapered channel is analysed under conditions of fixed stroke volume. A lubrication theory is developed for small taper and the general result is steady bidirectional drift and an induced steady pressure gradient. The characteristics of the drift profiles change significantly as the Womersley parameter is increased. For large values difficulties arise in the matched asymptotics method which are resolved by introducing a steady drift layer that is much thicker than the Stokes layer. This double boundary layer does not arise in pressure-cycled oscillations. Both Eulerian and Lagrangian drift are examined. The results are compared qualitatively to experimental observations which primarily focus on the application to ventilation in the lung.

The effect of surface contamination on thermocapillary flow in a two-dimensional slot

Abstract: In an analysis of the effect of insoluble surfactants on the steady capillary flow in a shallow slot, the problem is formulated as a two-dimensional channel heated differentially from the side, with a free surface on which an insoluble surface species is adsorbed. The vertical boundaries are considered to be isothermal and impenetrable and the lower horizontal boundary to be insulated and impenetrable. Lubrication theory and matched asymptotic expansions are used to reduce the problem to a single nonlinear integral-algebraic equation. For small elasticity E, the surface velocity is nearly constant, with the result that, for small Pe, the surface concentration is nearly uniform while, for large Pe, it develops a boundary layer near the stagnation point at the cold wall, resulting in high local surface concentrations. Condensed surfactant layers will form only with E much less than one and Pe much greater than one.

Performance Characteristics of Elliptical Journal Bearings in Turbulent Flow Regime

Abstract: In this paper, the performance characteristics of elliptical journal bearings in turbulent flow regime are studied theoretically and experimentally. Solving the turbulent lubrication equation by the semi-analytical finite element method, the static and dynamic characteristics such as the Sommerfeld number, the locus of shaft center, the friction coefficient and the whirl onset speed are obtained for various mean Reynolds' numbers and ellipticity ratios, and the results are compared with the experimental ones. It is found that the turbulence affects significantly the static and dynamic characteristics of such bearings and the stability limits of the bearings become larger with an increase of ellipticity ratio.

A Simulation Method for Hydrodynamic Lubrication of Surfaces with Two-Dimensional Isotropic or Anisotropic Roughness Using Mixed Average Film Thickness

Abstract: A new method is presented for simulating hydrodynamic lubrication with two-dimensional roughness. As a result of utilizing the concept of flow conductance in lubricating film with a unit two-dimensional roughness or a regular series of the unit roughness, it is found that a smooth surface equivalent to the rough one is given by mixed average film thickness of arithmetic and harmonic average film thickness. A mixed average Reynolds equation for the rough surface is derived, where mixing ratios for averaging film thickness can be determined by using the unit roughness with symmetrical film thickness. The equation is justified by comparison with results computed from real rough film thickness. This method is applied to some selected isotropic and anisotropic roughnesses. The most important conclusion is that micro topography of a unit roughness governs the total surface roughness effects.

Lubrication of Elastic-Isoviscous Line Contacts Subject to Cyclic Time-Varying Loads and Entrainment Velocities

Abstract: An analytical approach was developed for the difficult problem of predicting the dynamic variation in fluid-film thickness of elastic-isoviscous line contacts under isothermal conditions. A numerical solution procedure was constructed and applied to the experiments of Hirano and Murakami who were investigating the lubrication of O-ring seals. Reasonable agreement was obtained. The present approach was extended to the lubrication of compliant layered surfaces as part of a study of human ankle joint lubrication. Although dependent on many assumptions, the present analysis provided a simple method for predicting the film thickness in both Hertzian and non-Hertzian contacts subject to cyclic time-varying loads and entrainment velocities. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Hartford, Connecticut, October 16–20, 1983

Power-law creep of a material being compressed between parallel plates: a singular perturbation problem

Abstract: The flow of a power-law creeping or viscous material which is being compressed in a narrow gap between parallel plates is studied. A perturbation scheme based on the small gap size is developed and the approximations which lead to classical lubrication theory are formally identified. The solution obtained from lubrication theory is shown to correspond to an outer solution which is not uniformly valid because it predicts infinite longitudinal stresses along both the centerline of the gap and across the entire gap on the line connecting the plate midpoints. The failure of lubrication theory to describe the flow in these regions is not due to an inherent failure of the power-law constitutive equation to model material behavior, but is due to a breakdown in the approximations made in lubrication theory. The solution is corrected by constructing inner solutions in the regions where lubrication theory fails and a uniformly valid solution for the stress field and velocity field is obtained.

Measurements of Squeeze Film Bearing Forces to Demonstrate the Effect of Fluid Inertia

Abstract: Squeeze film dampers are commonly applied to high speed rotating machinery, such as aircraft engines, to reduce vibration problems. The theory of hydrodynamic lubrication has been used for the design and modeling of dampers in rotor dynamic systems despite typical modified Reynolds numbers in applications between ten and fifty. Lubrication theory is strictly valid for Reynolds numbers much less than one, which means that fluid viscous forces are much greater than inertia forces. Theoretical papers which account for fluid inertia in squeeze films have predicted large discrepancies from lubrication theory, but these results have not found wide acceptance by workers in the gas turbine industry. Recently, experimental results on the behavior of rotor dynamic systems have been reported which strongly support the existence of large fluid inertia forces. In the present paper direct measurements of damper forces are presented for the first time. Reynolds numbers up to ten are obtained at eccentricity ratios 0.2 and 0.5. Lubrication theory underpredicts the measured forces by up to a factor of two (100% error). Qualitative agreement is found with predictions of earlier improved theories which include fluid inertia forces.Copyright © 1984 by ASME

Removal of a Thin Liquid Film From a Flat Surface Using an Axisymmetric Impinging Jet

Abstract: On presente une theorie simple permettant de prevoire la vitesse d'amincissement d'une couche liquide visqueuse recouvrant initialement une surface plane rigide soumise a l'action d'un jet. Etude experimentale confirmant cette theorie

Dynamic Properties of Externally-Pressurized Gas Journal Bearings with Circular Slot Restrictors

Abstract: The externally-pressurized gas journal bearings with circular slot restrictors are investigated theoretically and experimentally to determine their dynamic properties. The theoretical analyses are presented for a single-row admission bearing and a double-row admission one. respectively, by using Reynolds equations in which inertial forces of a gas film are considered. The accuracy of the theoretical method is consequently shown that the present theoretical method can accurately predict the dynamic properties of a gas journal bearing with a circular slot restrictor.

The Effect of Fluid Inertia and Viscoelasticity in Squeeze-Film Damper Bearings.

Abstract: : In the modeling and analysis of rotor dynamic systems, the behavior of squeeze film damper bearings is normally predicted by the Reynolds equation of hydrodynamic lubrication. Large bearing gaps and high speeds can combine to create conditions in practical applications where fluid inertia and viscoelastic effects may become significant, violating the assumptions under which Reynolds equation can be applied. The analysis shows that the results of lubrication theory can be greatly in error with regard to phase effects between bearing forces and displacements, which may have profound implications regarding critical speed and forced response behavior. Very large deviations from lubrication theory have been predicted in a series of analytical papers under this contract. In the present study direct measurements of damper forces are presented for the first time. Reynolds numbers up to ten are obtained at eccentricity ratios 0.2 and 0.5. Lubrication theory underpredicts the measured forces by up to a factor of two (100% error). Qualitative agreement is found with predictions of the improved theory which includes fluid inertia forces.