Author: Dingfeng Deng

Publisher:

Published: 2007

Total Pages: 329

ISBN-13:

"The relationship between the onset of Taylor instability and appearance of what is commonly known as 'turbulence' in narrow gaps between two cylinders is investigated. A question open to debate is whether the flow formations observed during Taylor instability regimes are, or are related to the actual 'turbulence' as it is presently modeled in micro-scale clearance flows. This question is approached by considering the viscous fluid in narrow gaps between two cylinders with various eccentricity ratios. The computational engine is provided by CFD-ACE, a commercial multi-physics software. The flow patterns, velocity profiles and torques on the outer cylinder are determined when the speed of the inner cylinder, clearance and eccentricity ratio are changed on a parametric basis. Calculations show that during the Taylor vortex regime velocity profiles in the radial direction are sinusoidal with pressure variations in the axial direction even for the case of the 'long journal bearing' (L/D>2). For the concentric case, both velocity and pressure profiles are axisymmetric and time-independent during the Taylor vortex regime. During the wavy vortex regime the radial velocities maintain their sinusoidal profiles, while pressure varies in both axial and circumferential directions. Both velocity and pressure profiles are non-axisymmetric and time-dependent. An order of magnitude analysis of the Navier-Stokes equation terms shows that the inertia, viscous, pressure and the Reynolds stress terms are equally significant during the transition regime (Taylor to wavy vortex regimes). Based on these findings, a new model for predicting the flow behavior in long and short journal bearing profiles are sinusoidal and depend on the local Reynolds number (or Taylor number) and the position in the axial direction. Unlike the modified turbulent viscocity of the most accepted models (Constantinescu, Ng-Pan, Hirs and Gross et al), the viscosity used in the new model is kept at its laminar value. A comparison is made between the results of this model and the four most accepted turbulence models mentioned above. Experimental torque measurements and flow visualization are performed for three kinds of oils with different viscosities. It is shown that in general there is a good agreement between the numerical and experimental torques except those in turbulent regime. The different flow patterns in the turbulent regime accounts for the different between the numerical and experimental torques. Comparison between numerical and experimental flow patterns is also made and it shows that they match well in the Couette, Taylor and Wavy regimes. There is a discrepancy between numerical and experimental flow patterns in Pre-wavy regime. The reason is that the vorticity is strong enough to destroy the organized Taylor vortices but it is not strong enough to hold the particles in the center areas of the vortices during experiments in this regime. In general there is a good agreement between the numerical and experimental results including torque measurements and flow pattersn. The new model for predicting the flow behavior in journal bearing films in the transition regime is justified."--Abstract.