Lubrication theory

About: Lubrication theory is a research topic. Over the lifetime, 1713 publications have been published within this topic receiving 50261 citations. The topic is also known as: Fluid bearing.

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.

On pyroclastic flow emplacement

Abstract: In this note I investigate some theoretical characteristics of pyroclastic flow deposits, assuming that these flows are Bingham fluids, probably the simplest non-Newtonian fluids. Pyroclastic flows are modeled as laminar debris flows moving on an inclined plane, and their physics is discussed within the classical framework of lubrication theory. Using general hydrodynamics methods, I show that the arrestment and emplacement of pyroclastic flows may be seen as the time-asymptotic limit of their equations of motion. This limit is found to be a nonlinear ordinary differential equation, whose solution gives the shape of pyroclastic flow deposits. The model suggests that these flows stop when the supply of material from the source is depleted; deposit thickness is controlled principally by the flow yield stress τz, a parameter characteristic of Bingham fluids, while deposit length, a measure of flow mobility, depends on τz, on the source flux q0, and on the slope θ of the solid substrate. Even in this simple model, theoretical analysis shows a complex correlation between flow parameters and deposit profiles.

Flow of red blood cells in narrow capillaries: role of membrane tension.

Abstract: A theoretical model is developed to describe blood flow in narrow capillaries, with inside diameters 3 microns to 6 microns. Each red blood cell is assumed to have axisymmetric geometry, and fixed surface area and volume. Cell velocities in the range 1 mm s-1 or higher are assumed, and the stress in the cell membrane is approximated by an isotropic tension. This tension is assumed to fall to zero at the concave trailing end of the cell, except in vessels whose diameter is near the minimum for passage of the cell. In the latter case, a separate analysis is used, in which the cell is effectively rigid and fully distended at each end. Lubrication theory is used to describe the plasma flow in the narrow gap between the cell and the vessel wall. Good agreement is obtained between predicted values of the tube hematocrit and apparent viscosity and published experimental values for these parameters.

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.

Flow and deformation of the capillary glycocalyx in the wake of a leukocyte

Abstract: An analysis is presented of the axisymmetric axial and radial flow and deformation fields throughout the endothelial-cell glycocalyx surface layer in the wake region behind a leukocyte moving steadily through a capillary. The glycocalyx, modeled as a thin poroelastic surface layer lining the capillary wall, is assumed to consist of a binary mixture of a linearly viscous fluid constituent and an isotropic, highly compressible, linearly elastic solid constituent having a vanishingly small solid-volume fraction. Invoking the asymptotic approximations of lubrication theory in a frame of reference translating with the leukocyte, closed-form solutions are obtained to the leading-order boundary-value problems governing the axial and radial flow and deformation fields throughout the glycocalyx as well as the axial and radial flow fields throughout the free capillary lumen within the wake. A simple asymptotic expression is obtained for the length l(char) of the wake region in terms of the translational speed U-0 of the leukocyte, and the equilibrium thickness h(0), permeability k(0), and aggregate elastic modulus H-A of the glycocalyx. The predicted wake length, as seen from an observer moving in a reference frame attached to the leukocyte, is consistent with the recovery time predicted from a one-dimensional analysis of glycocalyx deformation through a quiescent inviscid fluid. The two-dimensional fluid dynamical analysis presented here thus provides the appropriate relationships for extracting estimates of the mechanoelectrochemical properties of the glycocalyx from physiologically realistic constitutive models developed under simplified one-dimensional flow regimes. The directly measurable quantities l(char), U-0, and h(0), which are obtainable from in vivo observations of the wake region behind a leukocyte moving steadily through a capillary, can therefore be connected, through the results of this analysis, to estimates of the mechanoelectrochemical properties of the glycocalyx on vascular endothelial cells. (C) 2005 American Institute of Physics.