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Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;141(3):031101-031101-12. doi:10.1115/1.4041120.

New apparatus is described to simulate a compliant seal interface, allowing the percolation of liquid to be viewed by a fluorescence microscope. A model, based on the boundary element (BE) methodology, is used to provide a theoretical explanation of the observed behavior. The impact of contact pressure, roughness, and surface energy on percolation rates are characterized. For hydrophilic surfaces, percolation will always occur provided a sufficient number of roughness length scales are considered. However, for hydrophobic surfaces, the inlet pressure must overcome the capillary pressure exerted at the minimum channel section before flow can occur.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;141(3):031201-031201-7. doi:10.1115/1.4040972.

Starting flow due to a suddenly applied pressure gradient in a circular tube containing two immiscible fluids is solved using eigenfunction expansions. The orthogonality of the eigenfunctions is developed for the first time for circular composite regions. The problem, which is pertinent to flow lubricated by a less viscous near-wall fluid, depends on the ratio of the radius of the core region to that of the tube, and the ratios of dynamic and kinematic viscosities of the two fluids. In general, a higher lubricating effect will lead to a longer time for the starting transient to die out. The time development of velocity profile and slip length are examined for the starting flows of whole blood enveloped by plasma and water enveloped by air in a circular duct. Owing to a sharp contrast in viscosity, the starting transient duration for water/air flow can be ten times longer than that of blood/plasma flow. Also, the slip length exhibits a singularity in the course of the start-up. For blood with a thin plasma skimming layer, the singularity occurs very early, and hence for the most part of the start-up, the slip length is nearly a constant. For water lubricated by air of finite thickness, the singularity may occur at a time that is comparable to the transient duration of the start-up, and hence, an unsteady slip length has to be considered in this case.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031202-031202-14. doi:10.1115/1.4041121.

Laboratory experiments were conducted to study the dynamics of particle clouds in viscous fluids. Different shapes of frontal head and trailing stems were observed, and particle clouds were classified using data mining methodology. The stability of the frontal head of particle clouds was found to be correlated with the nozzle diameter and mass of sand particles in the form of an initial aspect ratio. The formation of particle clusters into a torus and the split of the frontal head into two or three clusters were investigated in detail. The cluster of particles flow through viscous fluid experienced partial separation due to the release of air bubbles from the rear of frontal head. It was observed that the time and location of major particle separation increase linearly with the aspect ratio. The oscillatory motion of the frontal head, caused by an uneven release of air bubbles from the rear of the frontal head, was found to be correlated with the initial aspect ratio. Both amplitude and wavelength exhibited a linear relationship with nondimensional time. The average drag coefficient of particle clouds Cd in viscous fluids was calculated for different aspect ratios, and the results were compared with the drag coefficient of individual particles. It was found that the averaged drag coefficients of particle clouds were smaller than the drag coefficient of individual particles, and Cd slightly increases with the increasing initial aspect ratio.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2018;141(3):031301-031301-11. doi:10.1115/1.4041065.

An accurate and practical approach is necessary for predicting oil fraction in horizontal oil–water flows. In this study, a concept of a drift-flux model is adopted to develop a predictive method for the oil fraction in the horizontal oil–water flows due to its simplicity and practicality. A new drift-flux correlation for the horizontal oil–water flows is developed based on the least square method using collected experimental data. The distribution parameter is determined to be 1.05 for the data with the ratio of oil density to water density ranging from 0.787 to 1.00, whereas the oil fraction weighted mean drift velocity is set at 0 m/s due to the flow direction perpendicular to the gravity direction. The physical meaning for the order of unity of the distribution parameter is explained by introducing a simple model. The predictive capability of the new drift-flux correlation is examined using the collected database of oil–water flows in horizontal pipes under a variety of test conditions. It is demonstrated that the new drift-flux correlation can predict the existing oil fractions in the horizontal pipe channels with the mean absolute error, standard deviation, mean relative deviation, and mean absolute relative deviation being −0.0124, 0.0338, −3.25%, and 9.57%, respectively.

Commentary by Dr. Valentin Fuster

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