NUMERICAL UNDERSTANDING OF BREAKING AND MAKING PHENOMENA AROUND GAS-LIQUID INTERFACES PRODUCING MULTISCALE ENTITIES

Abstract

The numerical investigation has been conducted to explore the underlying physics of interfacial entities like film, drop and bubble in the gas-liquid two-phase association. In this context, fluidics related to jet impingement onto a rigid flat surface, entrainment of the droplet in wavy-annular flow inside a vertical conduit, bubble bursting at free surface in the presence of neighbors, and spreading dynamics of inner gas of a bubble after bursting at the free surface are studied. Eulerian volume of fluid (VOF) method is used in multiscale dynamic mesh framework to study the liquid film produced after the impact of a liquid jet onto a solid surface. The formation process of circular hydraulic jump (CHJ) as a consequence of the vertical impact of a liquid jet on a horizontal surface and subsequent deviation of CHJ from the azimuthal symmetry due to non-orthogonal impingement are discussed. To understand the role of wall adhered liquid film/sheet on a jump, flow physics has been studied, revealing backflow in the jump region causing circulation. Moreover, equipotential and heterogeneous jump-jump interactions are simulated to predict fluidic features like fountain formation and upwash. At a higher ratio of jet strengths, complete engulfment of a smaller hydraulic jump as an eye of a larger one has been obtained from careful numerical simulation. The numerical investigation has also been carried out to understand the fluidic phenomena of asymmetric rim bounded thin liquid film/sheet produced due to the horizontal impact of a circular jet onto a vertical flat surface. The effect of jet strength over the radial and axial stretching of the sheet is studied. Further, different plate inclinations are provided, keeping the jet orthogonal to the plate, to understand the influence of gravity assistance on the liquid sheet dynamics. An obstruction on the impinging surface is modelled to understand the bifurcation of liquid film/sheet and the development of the inner rim. In a subsequent effort, wavy annular flow and droplet dispersion in air-water two-phase flow has been studied numerically using multiscale Eulerian VOF solver and coupled Eulerian-Lagrangian approach. The VOF based investigation has been reported to highlight the formation of droplet swarm and its population dynamics. Coupled Eulerian-Lagrangian method has also been shown to replicate similar features with lesser computational effort. Entrainment, deposition, fragmentation, and unification are traced from the numerical simulation, which ultimately predicts the volume filling behaviour of the droplets inside the tube. Flow kinematics around droplets is critically assessed for finding out reasons behind deposition, fragmentation, and unification. An assessment of the droplet population in the coupled Eulerian-Lagrangian method shows the smaller-sized spherical droplet generation by entrainment and fragmentation route. A numerical investigation of a bubble bursting phenomenon at the free surface under the influence of the symmetric and asymmetric pattern of neighboring bubbles is performed subsequently. A thorough comparison of a bubble bursting with symmetric neighbors (BBSN) and similar bursting of a solitary bubble (BSB) has been shown in terms of cavity collapse rate, jet formation, and consequential fragmentation of jet into drops. Bubble bursting under the influence of asymmetric neighboring presence is compared with BBSN. Bubble with absent neighbors (BWAN) turns into the formation of the bent jet, which is lying in the plane of the angular bisector of absent neighbors. The bending of a jet is found to nullify as time progresses, and the same turns into a vertical one like jet dynamics observed in BBSN. Simulation of BSB is also performed to capture the dynamics of evacuating inner gas using the three-phase Eulerian volume of fluid (VOF) method. The rate by which surrounding air rushing inside the bubble cavity through the inner gas evacuation is estimated. Further, the reachability of inner gas at different horizontal planes over the unperturbed free surface is estimated. The evacuating inner gas is seen to accompany vortex rings, which entrains the surrounding air. The axial growth of rising inner gas over the free surface and the radial expansion of vortex rings of a bubble bursting at the free surface is compared in situations with the quiescent surrounding air and under the respiration process. In this context, the effect of miscibility on the spreading dynamics of inner gas has been found to be minimal at the early stage of the bursting process.