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.

Time-resolved imaging of a compressible air disc under a drop impacting on a solid surface

Abstract: When a drop impacts on a solid surface, its rapid deceleration is cushioned by a thin layer of air, which leads to the entrapment of a bubble under its centre. For large impact velocities the lubrication pressure in this air layer becomes large enough to compress the air. Herein we use high-speed interferometry, with 200 ns time-resolution, to directly observe the thickness evolution of the air layer during the entire bubble entrapment process. The initial disc radius and thickness shows excellent agreement with available theoretical models, based on adiabatic compression. For the largest impact velocities the air is compressed by as much as a factor of 14. Immediately following the contact, the air disc shows rapid vertical expansion. The radial speed of the surface minima just before contact, can reach 50 times the impact velocity of the drop.

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 of non-Newtonian fluids on hydrophilic surfaces

Abstract: The spreading of Newtonian fluids on smooth solid substrates is well understood; the speed of the contact line is given by a competition between capillary driving forces and viscous dissipation, yielding Tanner's law . Here we study the spreading of non-Newtonian liquids, focusing on the two most common non-Newtonian flow properties, a shear-rate dependence of the viscosity and the existence of normal stresses. For the former, the spreading behaviour is found not to deviate strongly from Tanner's law. This is quite surprising given that, within the lubrication approximation, it can be shown that the contact line singularity disappears due to the shear-dependent viscosity. The experiments are compared with the predictions of the lubrication theory of power-law fluids. If normal stresses are present, again only small deviations from Tanner's law are found in the experiment. This can be understood by comparing viscous and normal stress contributions to the spreading; it turns out that only logarithmic corrections to Tanner's law survive, which are nonetheless visible in the experiment.

Minimum film thickness in elliptical contacts for different regimes of fluid-film lubrication

Abstract: The film-parameter equations are provided for four fluid-film lubrication regimes found in elliptical contacts. These regimes are isoviscous-rigid; viscous-rigid; elastohydrodynamic of low-elastic-modulus materials, or isoviscous-elastic; and elastohydrodynamic, or viscous-elastic. The influence or lack of influence of elastic and viscous effects is the factor that distinguishes these regimes. The film-parameter equations for the respective regimes come from earlier theoretical studies by the authors on elastohydrodynamic and hydrodynamic lubrication of elliptical conjunctions. These equations are restated and the results are presented as a map of the lubrication regimes, with film-thickness contours on a log-log grid of the viscosity and elasticity parameters for five values of the ellipticity parameter. The results present a complete theoretical film-parameter solution for elliptical contacts in the four lubrication regimes.

Coefficient of restitution for wet particles.

Abstract: The influence of a liquid film on the coefficient of restitution (COR) is investigated experimentally by tracing freely falling particles bouncing on a wet surface. The dependence of the COR on the impact velocity and various properties of the particle and liquid is presented and discussed in terms of dimensionless numbers that characterize the interplay between inertial, viscous, and surface forces. In the Reynolds number regime where lubrication theory does not apply, the ratio of the film thickness to the particle size is found to be a crucial parameter determining the COR.