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.

Thin film lubrication dynamics of a binary mixture: Example of an oscillatory instability

Abstract: We study thin film instabilities in liquid films with deformable surface using the lubrication theory. An externally applied vertical temperature gradient may give cause to an instability (Marangoni instability) of the flat motionless film. Contrary to the earlier work where mostly pure fluids were discussed, the focus of the present paper lays on instabilities in mixtures of two completely miscible liquids. We show that the normally found monotonic long-wave instability may turn into an oscillatory one if the two components have a different surface tension and if the Soret coefficient establishes a stabilizing vertical concentration gradient. A systematic derivation of the basic equations in long-wave approximation is given. The character of instabilities is studied using linear stability analysis. Finally, a real system consisting of a water-isopropanol mixture is discussed in some detail.

Evaporation and contraction of a droplet that wets a surface monitored by photoacoustic detection.

Abstract: The evaporation and contraction of a droplet wetting a flat metallic surface is monitored using photoacoustic detection. The results are interpreted in terms of an effective backing model together with the lubrication theory for droplet dynamics

Time-dependent analysis of electroosmotic fluid flow in a microchannel

Abstract: The present work models electroosmotic induced fluid flow in a microfluidic device. For theoretical analysis, the geometry of this device is considered as an asymmetric, narrow, wavy channel with charged surface. It is assumed that the length of the channel is finite and the characteristic wavelength is very large compared to the half width of the channel. The flow is assumed to be governed by Navier–Stokes equations augmented with electric body force. A transient two-dimensional flow analysis is presented by employing lubrication theory. Debye–Huckel linearization is adopted to obtain a general solution of Poisson–Boltzmann equation. The flow rate, velocity profile, pressure distribution, and wall shear stress are analyzed as functions of various parameters involved like zeta-potential ratio, Debye–Huckel parameter, etc. It is noted that the fluctuations in pressure and shear stress increase with the increasing zeta-potential ratio when the Helmholtz–Smoluchowski velocity is positive. The streamline pattern and the particle trajectories are analyzed to understand the trapping phenomenon and the retrograde motion. It is observed that the electroosmosis phenomenon drastically modulates the fluid flow in microchannels. Although the asymmetric nature of the wavy channel does not support active particle transport, the applied electric field enhances the particle motion favoring optimal conditions. It is observed that the extreme asymmetry of the wall motility reduces the net flow rate. Further, it is noticed that the asymmetry reduces the amplitudes of the pressure. This model can help toward designing artificial organs based on microfluidic devices which can also be applicable to analyze lumenal flow inside arteries and flow inside intrauterine system, and to implant the embryo at the best location in the uterus for human-assisted reproduction.

Transient lubrication of piston compression ring during cold start-up of SI engine

Abstract: Cold start-up conditions differ from the normal operating conditions for an engine, and this discrepancy adversely affects the tribological performance of interfaces. The majority of health-hazardous engine exhaust emissions occur during the cold start-up of an engine; consequently this condition generates a major environmental concern. This study numerically investigates the transient hydrodynamic lubrication of the first compression ring during initial start-up of a cold engine by combining lubrication theory with the realistic oil rheology of multigrade oils. Specifically, in this study transient speed, variation in cylinder pressure, and high lubricant viscosity during start-up are considered to determine developing pattern of the lubricating film and of other tribological performance parameters. The cycle-by-cycle developments of lubricating film, friction force, power loss, and oil transport are analyzed as well. The start-ups of a cold and a warm engine are also compared in terms of frictional losses and oil transport. Different monograde and multigrade engine oils are used to evaluate oil-dependent performance. Results show that in all cases, performance during cold start-up is of greater significance than that of a warm engine. Low lubricant temperature and stated start-up conditions substantially increase amount of energy loss and net oil transport. Temperature-dependent variations in performance are also more significant in high viscosity-grade oils.