Showing papers on "Lubrication theory published in 1983"

Nonlinear adjustment of a thin annular film of viscous fluid surrounding a thread of another within a circular cylindrical pipe

Abstract: A nonlinear analysis, based on lubrication theory, is presented for the adjustment under surface tension of an initially uniform annular film of viscous fluid confined within a circular cylindrical pipe The film surrounds a thread of another viscous fluid Small axisymmetric interfacial disturbances of sufficiently long wavelength are found to grow, leading to the break-up of the initially continuous outer film into a number of isolated rings of fixed length on the pipe wall The implications for the rupture of fluid threads surrounded by moderately thin films in confined geometries are discussed

An Analysis of Valve Train Friction in Terms of Lubrication Principles

Abstract: Friction losses in a motored 1.6L valve train can be reduced by roller tappets, by needle bearing inserts placed in the rocker arm/fulcrum contact and in the cam journals and by reducing spring tension. Friction reducing engine oil additives reduce valve train friction substantially, but oil viscosity has only a limited effect. These results can be quantitatively accounted for by a simple friction model based on lubrication theory. Both the model and the experimental results are consistent with the idea that the friction losses in the valve train are mainly due to boundary and mixed lubrication.

Spreading kinetics of a drop on a rough solid surface

Abstract: The effect of surface roughness on spreading rates has been analyzed using a model in which a liquid drop spreads over the surface of a porous medium filled with the same liquid. The equations of motion in the drop are simplified with the lubrication theory approximation and then solved for both zero and small but nonzero contact angles by the method of matched asymptotic expansions. Although the largest pressure gradients and velocity gradients occur near the contact line at the drop periphery, behavior in this region is not singular as found in previous analysis of spreading on perfectly smooth surfaces. The reason no singularities exist is that flow occurs in the “porous medium” underlying the drop, i.e., the region of surface irregularities which is present for all real surfaces. Because the solution is not valid in the initial stages of spreading where experimental data on spreading rates are available, a quantitative comparison of theory and experiment cannot be made at present. The theory does, however, explain all qualitative features observed for spreading drops, e.g., the increase in spreading rate with increasing roughness and the frequent appearance of apparent contact angles significantly different from equilibrium contact angles.

Liquid bridges with thermocapillarity

Abstract: An axisymmetric liquid bridge is subjected to an axial temperature difference. Thermal variations in surface tension drive a motion that is described using lubrication theory for slender bridges. At leading order in aspect ratio, a class of similarity solutions is obtained valid in the core region away from the endwalls of the bridge. These solutions describe the flows, temperature fields, and interfacial shapes of bridges that may sustain substantial interfacial deflections.

Sliding sheets: lubrication with comparable viscous and inertia forces

Abstract: A rigid plane thin sheet is sliding steadily at speed U close to a plane wall, in a fluid of kinematic viscosity v. The sheet is infinitely wide and is of length L in the direction of motion, and its leading edge is a distance h0 [Lt ] L from the wall. A solution is sought for arbitrary finite values of R = Uh20/νL. In the limit as e = h0/L→0, the problem reduces to that of solving the boundary-layer equation in the gap region between sheet and wall, and this is done here both by an empirical linearization, and by direct numerical methods. The solutions have the property that they reduce to those predicted by lubrication theory when R is small, and to those predicted by an inviscid small-gap theory when R is large. Special attention is paid to the correct entrance and exit conditions, and to the location of the centre of pressure.

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.

Aerodynamically-driven condensate layer thickness distributions on isothermal cylindrical surfaces

Abstract: A simple yet rather general mathematical model is presented for predicting the distribution of condensate layer thickness when aerodynamic shear is the dominant mechanism of liquid flow along the surface. The Newtonian condensate film is treated using well-known thin-layer (lubrication theory) approximations, and condensate supply is taken to be the result of either convective diffusion or inertial impaction. Illustrative calculations for a circular cylinder in a crossflow at Re = 100,000 reveal the consequences of alternate condensate arrival mechanisms and the existence of thicker reverse-flow films behind the position of gas boundary-layer separation. The present formulation is readily generalized to include transient liquid layer flows on noncircular objects of variable surface temperature, as encountered in turbine-blade materials testing or operation.

Optimum design of one-dimensional journal bearings

Abstract: When two surfaces in relative motion are separated by a thin layer of viscous fluid, they are kept apart by the pressure generated in the fluid film due to viscous forces. The magnitude of the resultant of this pressure is the load capacity of the bearing. We find the shape of a one-dimensional journal bearing which supports the greatest load.

Static and Dynamic Analysis of Hydrodynamic Bearings in Laminar and Superlaminar Flow Regimes by Finite Element Method

Abstract: Adopting the nonlinear, turbulent lubrication theory of Elrod and Ng, the classical Navier-stokes equations in cylindrical coordinates have been suitably modified. These modified equations have been solved by finite element method using Galerkin's technique and a suitable iteration scheme. Performance characteristics of a finite circular hydrodynamic journal bearing have been studied in terms of Sommerfeld number, attitude angle, stiffness and damping coefficients, and critical journal mass at various eccentricities for Reynolds numbers up to 13 300. Computed results have been compared with the published results obtained by linearized theory of Ng and Pan.

Chemical Films and Mixed Lubrication

Abstract: This paper relates studies of film behavior in mixed lubrication in a concentrated contact. In the simulator, 52100 steel specimens and TCP in hexadecane as lubricant are used. When the pure EHL regime is studied, no solid film is formed on surfaces, but within the boundary lubrication regime, the chemical decomposition of the additive contributes to a continuous film formation. In the mixed regime (ML): EHL- boundary lubrication, these films are formed and their thickness increases with time and with the slidelroll ratio. Surface damage can also be related with this ratio. A slight reduction of the traction force is found when these reaction films are present on the rolling tracks. This result is discussed. Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference in Washington, D.C., October 5–7, 1982

Hydrofilm Extrusion of Tubes Through Optimized Curved Dies

Abstract: An analytical method for hydrofilm extrusion of axisymmetric tubes through optimized curved dies is proposed. The upper-bound method and hydrodynamic lubrication theory are used in combination. The work-hardening effect of metal and the variation of fluid viscosity are taken into consideration. The optimal film thickness and the optimal die profile are found through energy minimization, and also pressure distributions of the fluid are obtained. A new mandrel system is devised for the film formation at the mandrel-billet interface. Experiments were carried out at room temperature for various diameter ratios of the tube, using the axisymmetric curved die. The theoretical prediction of extrusion pressure shows good agreement with experimental measurements for mild steel specimens, using castor oil as the lubricant.