A review on rising bubble dynamics in viscosity-stratified fluids

Systems with a bubble rising in a fluid, which has a variation of viscosity in space and time can be found in various natural phenomena and industrial applications, including food processing, oil extraction, waste processing and biochemical reactors, to name a few. A review of the aspects studied in the literature on this phenomenon, the gaps that exist and the direction for further numerical and experimental studies to address these gaps is presented

Dynamics of an air bubble rising in constant and varying viscosity media

The dynamics of a gas bubble rising in a liquid is observed in many natural phenomena, and also in industrial applications. Due to its practical relevance, this has been studied from many centuries and it continues to be a problem of great interest even today. The main objectives of the thesis are to investigate some novel problems on bubble rising in viscosity varying systems. In several situations one could observed viscosity stratified media. Two of them are considered in this work. (a) When viscosity stratification is inherently present in the surrounding medium (say, viscosity of the outer fluid is increasing linearly with height). (b) When the outer fluid is a non-Newtonian fluid, wherein the surrounding fluid viscosity varies due to the shear caused by the motion of the bubble. Three-dimensional numerical simulations and experiments are performed to study these flows. The flow dynamics is governed by mass, momentum and energy conservation equations. A volume-of-fluid (VoF) approach is used to track the interface separating the fluids. The computational and experimental approaches used in this study are discussed in Chapter 2. The validations of the present numerical solver are also performed in the same chapter. In order to compare the bubble behaviour in a medium with varying viscosity with that observed in a constant viscosity medium (standard system), the dynamics of a rising bubble in a standard air-water system is also investigated

Evaporating Falling Drop

There-dimensional numerical simulations are carried out to investigate the dynamics of a drop undergoing evaporation and falling due to gravity. In order to accurately capture the interfacial phenomena dynamic adaptive grid refinement has been incorporated. The results are presented in terms of spatio-temporal evolution of the shape of the drop, along with the contours of the vapour concentration generated due to evaporation. This study has implications in natural phenomena, such as rainfall, dew formation and several industrial applications undergoing phase change. A parametric study of this phenomenon will be presented at the conference

Motion of an air bubble under the action of thermocapillary and buoyancy forces

A novel way to handle surface tension gradient driven flows is developed in the volume-of-fluid (VoF) framework. Using an open source Navier-Stokes solver, {\it Basilisk}, and the present formulation, we investigate thermocapillary migration of drops/bubbles in a surrounding medium. Several validation exercises have been performed, which demonstrate that the present solver is a robust one to investigate interfacial flows with variable surface tension. It is well known that it is a challenging task to numerically model the tangential and normal surface forces arising due to interfacial tension. We have shown that the present method does not require the artificial smearing of surface tension about the interface, and thus predicts the theoretical value of the terminal velocity of bubble/drop migrating due to an imposed temperature gradient very well. It is also demonstrated that the present solver provides accurate results for problems exhibiting the gravity and thermocapillary forces simultaneously, and useful for systems with high viscosity and density ratios.Comment: 30 pages, 16 figures, submitted to Computers and Fluid

Super-linear speed-up of a parallel multigrid Navier-Stokes solver on Flosolver

In parallel computing, scalability is an important issue and getting linear speed-ups is difficult for most codes. Super-linear speed up has been achieved on an eight-processor Flosolver system for a multigrid Navier-Stokes code. The physical problem solved, the parallelization method, the speed-ups obtained and possible explanations for this result are discussed here

Physics of drying complex fluid drop: flow field, pattern formation, and desiccation cracks

Drying complex fluids is a common phenomenon where a liquid phase transforms into a dense or porous solid. This transformation involves several physical processes, such as the diffusion of liquid molecules into the surrounding atmosphere and the movement of dispersed phases through evaporation-driven flow. As a result, the solute forming a dried deposit exhibits unique patterns and often displays structural defects like desiccation cracks, buckling, or wrinkling. Various drying configurations have been utilized to study the drying of colloids, the process of their consolidation, and fluid-flow dynamics. This review focuses on the drying of colloids and the related phenomena, specifically the drying-induced effects observed during sessile drop drying. We first present a theoretical overview of the physics of drying pure and binary liquid droplets, followed by drying colloidal droplets. Then, we explain the phenomena of pattern formation and desiccation cracks. Additionally, the article briefly describes the impact of evaporation-driven flows on the accumulation of particles and various physical parameters that influence deposit patterns and cracks.Comment: 20 pages, 28 figures, Accepted in Physics of Fluids. arXiv admin note: substantial text overlap with arXiv:2011.1402

A study of pressure-driven displacement flow of two immiscible liquids using a multiphase lattice Boltzmann approach

The pressure-driven displacement of two immiscible fluids in an inclined channel in the presence of viscosity and density gradients is investigated using a multiphase lattice Boltzmann approach. The effects of viscosity ratio, Atwood number, Froude number, capillary number, and channel inclination are investigated through flow structures, front velocities, and fluid displacement rates. Our results indicate that increasing viscosity ratio between the fluids decreases the displacement rate. We observe that increasing the viscosity ratio has a non-monotonic effect on the velocity of the leading front; however, the velocity of the trailing edge decreases with increasing the viscosity ratio. The displacement rate of the thin-layers formed at the later times of the displacement process increases with increasing the angle of inclination because of the increase in the intensity of the interfacial instabilities. Our results also predict the front velocity of the lock-exchange flow of two immiscible fluids in the exchange flow dominated regime. A linear stability analysis has also been conducted in a three-layer system, and the results are consistent with those obtained by our lattice Boltzmann simulations

Physics of drying complex fluid drop: Flow field, pattern formation, and desiccation cracks

Drying complex fluids is a common phenomenon where a liquid phase transforms into a dense or porous solid. This transformation involves several physical processes, such as the diffusion of liquid molecules into the surrounding atmosphere and the movement of dispersed phases through evaporation-driven flow. As a result, the solute forming a dried deposit exhibits unique patterns and often displays structural defects like desiccation cracks, buckling, or wrinkling. Various drying configurations have been utilized to study the drying of colloids, the process of their consolidation, and fluid-flow dynamics. This review focuses on the drying of colloids and the related phenomena, specifically the drying-induced effects observed during sessile drop drying. We first present a theoretical overview of the physics of drying pure and binary liquid droplets, followed by drying colloidal droplets. Then, we explain the phenomena of pattern formation and desiccation cracks. Additionally, the article briefly describes the impact of evaporation-driven flows on the accumulation of particles and various physical parameters that influence deposit patterns and cracks