Magnetic-field- and thermal-radiation-induced entropy generation in a multiphase nonisothermal plane Poiseuille flow

Kelvin-Helmholtz Instability Augmented by von Kármán Vortex Shedding during an Oil Droplet Impact on a Water Pool

The impact of an oil droplet on a water surface has been explored with the aid of computational fluid dynamics simulations. The study reveals the details of the spatiotemporal evolution of such a ternary system with a triplet of interfaces, e.g., air-water, oil-water, and oil-air, when the impact velocity of the oil droplet with the water surface is high. The oil droplet is found to flatten, spread, stretch, and eventually dewet on the water surface of the deep crater to show a host of interesting post-impact flow morphologies. Furthermore, at higher impact velocities, the formation of a biphasic oil-water crown is observed followed by the ejection of secondary water droplets from the crown tip due to capillary instability. The rapidly spreading oil film on the "crater" of the water surface is found to undergo Kelvin-Helmholtz instability before dewetting the same due to cohesion failure. Subsequently, the formation of an array of secondary oil droplets is observed during the process of dewetting. The dominant wavelength evaluated from the linear stability analysis of a representative flow system could faithfully predict the simulated spacing of dewetted oil droplets floating on the crater. Importantly, the variations in Laplace pressure around the curvatures of the undulatory interfaces along with sharp viscosity gradients across the three-phase contact line is found to engender interesting recirculation patterns, which eventually shed to form a coherent wake region in air near the crater. We also uncover the conditions under which the counter-rotating vortices shed along the oil-water interface resembling a von Kármán vortex street.

Influence of the pre-impact shape of an oil droplet on the post-impact flow dynamics at air-water interface

We computationally explore the effects of pre-impact shape of an oil droplet on the spatiotemporal dynamics after the droplet impacts an air-water interface. Simulations reveal that the initial shape of the impacting oil-droplet alters the post-impact transient flow structures during the evolution. The spherical and oblate drop spreads over the crater to manifest interesting flow morphologies including the formation of oil-toroids and compound oil-droplets. However, the prolate drop impinges much deeper into the water pool after impact to create a few more exclusive flow features, such as, interface overturning, vortex shedding and formation of secondary droplets. The temporal variation of the crater depth shows distinct three stage dynamics, which can be explained by the generic energy analysis of the entire system. The combined theoretical and numerical energy analyses reveal the influences of the pre-impact drop shape and their effects on the subsequent energy conversion after the impact takes place. The analysis also reveals that the initial surface and kinetic energies are different for non-spherical droplets than for the spherical ones. The conversion of such excess surface energy due to the non-spherical curvature into kinetic energy dictates the impact and subsequently the crater dynamics of such systems. Such influences largely lead to the exclusive flow patterns demonstrated here. Concisely, this study presents a tri-phasic computational model, which is capable of analyzing the salient features of the impact and splash dynamics of the non-spherical droplets into a water continuum.