<div> Underlying physics in bubble coalescence is critical for understanding bubble transportation. It is one of the major mechanisms of microfluidics. Understanding the mechanism has benefits in the design, development, and optimization of microfluidics for various applications. The underlying physics in bubble coalescence is investigated numerically using the free energy-based lattice Boltzmann method by massive parametrization and classification.</div><div><br></div><div> Firstly, comprehensive GPU (Graphics Processing Unit) parallelization, convergence check, and validation are carried out to ensure the computational efficiency and physical accuracy for the numerical simulations.</div><div><br></div><div> Then, the liquid-gas system is characterized by an Ohnesorge number (Oh). Two distinct coalescence phenomena with and without oscillation, are separated by a critical Oh (~0.477)number. For the oscillation cases(Oh<0.477), the mechanism of damped oscillation in microbubble coalescence is explored in terms of the competition between driving and resisting forces. Through an analogy to the conventional damped harmonic oscillator, the saddle-point trajectory over the entire oscillation can be well predicted analytically. Without oscillation in the range of 0.50r^-n </div><div><br></div><div> After that, the liquid-gas-solid interface is taken into consideration in the liquid-gas system. Six cases based on the experiment set-ups are simulated first for validation of the computational results. Based on these, a hypothesis is established about critical factors to determine if coalescence-induced microbubble detachment (CIMD) will occur. From the eighteen experimental and computational cases, we conclude that when the radius ratio is close to 1 and the father bubble is larger, then it will lead to CIMD.</div><div><br></div><div> Lastly, the effects of initial conditions on the coalescence of two equal-sized air microbubbles (R₀) in water are investigated. In both initial scenarios, the neck bridge evolution exhibits a half power-law scaling, r/R₀=A₀(t/t_i)^1/2 after development time. The development time is caused by the significant bias between the capillary forces contributed by the meniscus curvature and the neck bridge curvature. Meanwhile, the physical mechanism behind each behavior has been explored.</div>

Numerical simulation of cell motility is one of the difficult problems in computational science. It belongs to a class of problems which involve moving interfaces between flowing or deforming media. Different numerical techniques are being developed for different application areas and in this work an attempt is made to apply two popular numerical techniques used in the field of computational multiphase flows to a cell motility problem. An unsteady cell motility problem is considered to simulate numerically based on a two-dimensional mathematical model. Two important numerical methods, the Level set method and the Front tracking methods are applied to the cell motility problem to study several cases and to verify the convergence of the solution. With the assumption of no mechanical or physical obstructions to the cell, the results of the numerical simulations show that the domain shapes converge to a circular shape as they reach the steady state condition. The final steady state velocities with which the domains move and the final steady state area to which they converge are observed to be independent of domain shapes. Moreover all shapes converge to exactly same radius of circle and move with same velocity after reaching steady state condition.

Modelling of laminar multiphase flow is extremely important in a wide range of engineering and scientific applications. The particle phases are often difficult to model, especially when particles display a range of sizes and velocities at each location in space. Lagrangian methods can be too expensive and many Eulerian methods, though often computationally more affordable, suffer from model deficiencies and mathematical artifacts that lead to non-physical results. For example, efficient Eulerian models that can accurately predict the crossing of multiple streams of non-interacting particles in laminar flow have traditionally been lacking. The predictive capabilities of modern techniques from the kinetic theory of gases to the treatment of disperse multiphase flows are investigated. In particular, several moment-methods, including a recently proposed fourteen-moment approximation to the underlying kinetic equation describing particle motion, are considered and their abilities to predict particle-stream crossing are assessed. Furthermore, a new polydisperse model has been proposed for treatment of flows that display a range of particles sizes. The proposed model is an extension of the well-known maximum-entropy ten-moment model from rarefied gas dynamics with an addition for the treatment of a range of particle diameters. This model allows for anisotropic variance of particle velocities in phase space and directly treats correlations between particle diameter and velocity. The derivation and mathematical structure, of the proposed models are presented. A fine-volume discretization solution procedure for the resulting moment equations is described and used for performing numerical experiments. Results for flow problems that are designed to demonstrate the fundamental behaviour of each model are presented. It is shown that the new models offer clear advantages in terms of accuracy as compared to traditional Eulerian models for multiphase flows.

Physics of immiscible two-phase flow in porous media is relevant for various industrial and environmental applications. Wettability defined as the relative affinity of fluids with the solid surface has a significant impact on the dynamics of immiscible displacement. Although wettability effects on the macroscopic fluid flow behaviour are well known, there is a lack of pore-scale understanding. Considering the crucial role of wettability in a diverse range of applications; this research aims to provide a pore-scale picture of interface configuration induced by variations in the wetting characteristics of porous media. Besides, this study also relates the pore-scale interfacial phenomena with the macroscopic response of fluids. High-resolution direct numerical simulations (DNS) at multiscale (single capillary and a highly heterogeneous porous media) were performed using computational fluid dynamics (CFD) approach in which the Navier-Stokes equation coupled with the volume of fluid method is solved to represent immiscible displacement. Numerical results demonstrate that at pore scale as the wettability of porous media changes from strong to intermediate wet the effects of pore geometry (that includes corner angle and orientation angle) on the interfacial dynamics also enhances. This was demonstrated by the non-monotonic behaviour of entry capillary pressure at the junction of pore, curvature reversal in the converging-diverging capillary and the co-existence of concave and convex interfaces in heterogeneous porous media with uniform contact angle distribution. In addition to simulations, theoretical argument is also presented that rationalize the underlying physics of complex, yet intriguing interfacial phenomena shown by DNS. Overall this research extends the fundamental understanding of multiphase flow in porous media and paves the way for future studies on porous media.

The effects of bubbles on a jet issued below and parallel to a free surface are experimentally studied. The jet under study is isothermal and in fresh water, with air injectors that allow variation of the inlet air volume fraction for 0% to 13%. Measurements of the jet exit conditions, water velocity, water entrainment, Reynolds stresses and surface currents have been performed using LDV, PIV and surface PIV. Air volume fraction, bubble velocity, chord length and free surface elevation and RMS have been obtained using local phase detection probes. Visualization was performed using laser-induced fluorescence. Measurements show that water entrainment decreases up to 22% with the presence of bubbles, but surface current strength increases up to 60% with 0.4 l/min of air injection. The mean free surface elevation and turbulent fluctuation significantly increase with the injection of air. The water normal Reynolds stresses are damped by the presence of bubbles in the bulk of the liquid, but very close to the free surface the effect is reversed and the normal Reynolds stresses increase slightly for the bubbly flow. Flow visualizations show that the two-phase jet is lifted with the presence of bubbles and attaches to the free surface sooner. Significant bubble coalescence is observed, leading to an increase of 20% in mean bubble size as the jet develops. The coalescence near the free surface is particularly strong, due to the time it takes the bubbles to pierce the free surface, resulting in a considerable increase in the local air volume fraction.

Drinking water distribution networks such as South East Water Ltd. (SEWL), Melbourne Water, Sydney Water, etc. are supposed to transport only dissolved matter rather than a few visible particles. However, it is almost impossible to make the drinking water free from suspended solid particles. The ability to determine the origins of these particles varies between different water supply systems, with possible sources being from catchment, treatment processes, biofilm growth within the water supply pipes, and corrosion products. Improvement of our understanding of the complex hydrodynamic behavior of suspended and/or deposited particles involved in these distribution pipe networks requires mathematical and physical models. Computational Fluid Dynamics (CFD) along with analytical turbulent model is one of the most popular mathematical techniques, which has the ability to predict the behavior of complex flows for such multiphase flow applications. This study has been completed mainly in two steps. A CFD investigation was carried out to predict the hydrodynamic behavior of turbid particle flowing through a horizontal pipe networks including loop consist of bends and straight pipes. Furthermore, an extended analytical model was re-developed for the liquid-solid system to look at the similar behavior of the solid particles flowing in a turbulent field. These two parallel studies will provide better understandings about the turbidity spikes movements in the distribution networks. A comprehensive CFD investigation was carried out for particle deposition in a horizontal pipe loop consisting of four 900 bends in a turbulent flow field. A satisfactory agreement was established with the experimental data as validation. This was a steady state multi-particle problem, which helped to understand the deposition characteristics for different particle sizes and densities at upstream and downstream sides of the bends as well as its circumference. Particle concentration was seen high at the bottom wall in the pipe flow before entering the bends, but for the downstream of bend the deposition was not seen high at the bottom as seen in upstream of bend rather inner side of the bend wall (600 skewed from bottom). The larger particles clearly showed deposition near the bottom of the wall except downstream. As expected, the smaller particles showed less tendency of deposition and this was more pronounced at higher velocity. Due to the high stream line curvature and associated centrifugal force acting on the fluid at different depths the particles became well mixed and resulted in homogeneous distribution near the bend regions. The hydrodynamic behavior of particles flowing in a turbulent unsteady state flowing through a horizontal pipe was also studied to compare with the drinking water distribution networks data. In this numerical simulation six different flow-profiles and particle-load profiles were used to compute particles deposition and re-entrainment into the systems and to identify the conditions of the deposition and suspension mechanisms. Results showed that after a certain length of pipe and period of time after downward velocity gradient, when the velocity was constants over time, the shear stress was sufficiently high enough to cause the particle deposition on and roll along the bottom wall of pipe wall and created a secondary group of particle peak (called kink). Finally, an extended analytical Turbulent Diffusion Model for liquid-solid phase was developed following an existing gas-liquid turbulence model. This turbulent diffusion model was then compared with the results of the CFD investigation making use of the same boundary conditions. The comparison established good agreement between these two models. The influence of velocity on the particle size distribution was dominant over the influence of the superficial liquid velocity, which was also explained by using the new parameter, velocity ratio. This velocity ratio was defined as the ratio between the free flight and gravitational velocity. Due to some inevitable assumptions used in the analytical model, the results showed typically less deposition as compared with the CFD investigation.

The development of a cost-effective multiphase flow meter to determine the individual phase flow rates of oil, water and gas was investigated through the exploitation of a single clamp-on gamma densitometer and signal processing techniques. A fast-sampling (250 Hz) gamma densitometer was installed at the top of the 10.5 m high, 108.2 mm internal diameter, stainless steel catenary riser in the Cranfield University multiphase flow test facility. Gamma radiation attenuation data was collected for two photon energy ranges of the caesium-137 radioisotope based densitometer for a range of air, water and oil flow mixtures, spanning the facility’s delivery range. Signal analysis of the gamma densitometer data revealed the presence of quasi-periodic waveforms in the time-varying multiphase flow densities and discriminatory correlations between statistical features of the gamma count data and key multiphase flow parameters. The development of a mechanistic approach to infer the multiphase flow rates from the gamma attenuation information was investigated. A model for the determination of the individual phase flow rates was proposed based on the gamma attenuation levels; while quasi-periodic waveforms identified in the multiphase fluid density were observed to exhibit a strong correlation with the gas and liquid superficial phase velocity parameters at fixed water cuts. Analysis of the use of pattern recognition techniques to correlate the gamma densitometer data with the individual phase superficial velocities and the water cut was undertaken. Two neural network models were developed for comparison: a single multilayer-perceptron and a multilayer hierarchical flow regime dependent model. The pattern recognition systems were trained to map the temporal fluctuations in the multiphase mixture density with the individual phase flow rates using statistical features extracted from the gamma count signals as their inputs. Initial results yielded individual phase flow rate predictions to within ±10% based on flow regime specific correlations.

No description available.

Supersaturated water injection (SWI) is a novel remediation technology which is able to remove entrapped residual NAPLs from saturated porous media by both volatilization (partitioning of volatile contaminants into the gas phase) and mobilization (displacement of isolated NAPL residuals by gas clusters). The character of gas saturation evolution in-situ in saturated porous media during SWI results in high sweep efficiency. This work focuses on studying the recovery of entrapped residual NAPL by the mobilization mechanism during SWI, thus low-volatility NAPL residuals, kerosene and a kerosene-hexadecane mixture, are used as contaminants. A series of SWI recovery experiments are conducted to investigate the influence of grain size, low-permeability layering, and physical properties of the contaminants on the recovery behavior. For columns contaminated with kerosene, the residual saturation can be reduced to around 4% from an initial value of 16%, and over 70% of the residual kerosene is recovered by a combination of mobilization and volatilization in homogeneous sand packs. For columns contaminated with a kerosene-hexadecane mixture, the final residual saturation is 7.4% and the final NAPL recovery is lower than that in kerosene columns. Grain size has little influence on NAPL recovery, but low permeability layering has a significantly negative influence. Experiments designed to compare SWI to sparging, and water-gas co-injection showed that water-gas co-injection was able to effectively recovery residual NAPLs albeit not as efficiently as SWI, while steady gas sparging is completely ineffective at recovering residual NAPL by mobilization. Based on these experimental observations, a conceptual model, involving double displacements and NAPL bank formation, is purposed to explain the experimental observations.

Water-in-oil (W/O) emulsions consist of water droplets dispersed in continuous oil phase. They are encountered at various stages of oil production. The oil produced from an oil-well usually carries a significant amount of water in the form of droplets. In enhanced oil recovery techniques involving the injection of polymer solution, the aqueous phase of the water-in-oil emulsions produced from the oil well consists of polymeric additive. A good understanding of the flow behavior of emulsions in pipelines is essential for the design and operation of oil production-gathering facilities and emulsion pipelines. A number of studies have been reported on simultaneous flow of oil and water in pipelines. However, the studies reported in the literature are mainly focused on either oil-water flow patterns and separated flows (annular and stratified flow of oil and water phases) or oil-in-water (O/W) emulsion flows. The pipeline flow of water-in-oil (W/O) emulsions has received less attention. Also, little work has been carried out on the effect of additives such as polymer. In this study, new experimental results are presented on the pipeline flow behavior of water-in-oil (W/O) emulsions, with and without the presence of polymeric additive in the aqueous phase. The emulsions were prepared from three different oils, namely EDM-244, EDM-Monarch, and Shell Pella of different viscosities (2.5 mPa.s for EDM-244, 6 mPa.s for EDM-Monarch, and 5.4 mPa.s for Shell Pella, at 25 0C). The water-in-oil emulsions prepared from EDM-244 and EDM-Monarch (without any polymeric additive in the dispersed aqueous phase) exhibited drag reduction behavior in turbulent flow. The turbulent friction factor data of the emulsions fell well below the standard Blasius equation for smooth pipes. The water-in-oil emulsions prepared from EDM-244 exhibited stronger drag reduction as compared with the EDM-Monarch emulsions. The Shell Pella emulsions (w/o type) did not exhibit any drag reduction in turbulent flow; the friction factor data followed the Blasius equation. The Shell Pella emulsions were more stable than the EDM-244 and EDM-Monarch emulsions. When left unstirred, the EDM-244 and EDM-Monarch emulsions quickly coalesced into separate oil and water phases whereas the Shell Pella emulsions took significantly longer time to separate into oil and water phases. The Shell Pella oil emulsions were also milkier than the EDM emulsions. The addition of polymer to the dispersed aqueous phase of water-in-oil emulsions had a significant effect on the turbulent drag reduction behavior. Emulsions were less drag reducing when polymer was present in the aqueous droplets. The effect of surfactant on the pipeline flow behavior of water/oil emulsions was also investigated. The surfactant-stabilized water-in-oil emulsions followed the single phase flow behavior. The presence of surfactant in the emulsions caused the dispersed droplets to become significantly smaller. It is believed that the droplets were smaller than the scale of turbulence when surfactant was present and consequently no drag reduction was observed.