Vortex-Ring-Induced Internal Mixing Upon the Coalescence of Initially Stationary Droplets

TL;DR

This paper uses advanced numerical methods to analyze how vortex rings induce internal mixing during the coalescence of stationary droplets, revealing key mechanisms and criteria that govern jet formation and mixing efficiency.

Contribution

It introduces a vortex-ring-based Reynolds number criterion to predict internal jet formation and mixing behavior in droplet coalescence.

Findings

Vortex-ring growth and detachment are essential for internal jet formation.

A Reynolds number criterion predicts jet occurrence across different conditions.

Stronger internal jets lead to increased mixing rates.

Abstract

This study employs an improved volume of fluid method and adaptive mesh refinement algorithm to numerically investigate the internal jet-like mixing upon the coalescence of two initially stationary droplets of unequal sizes. The emergence of the internal jet is attributed to the formation of a main vortex ring, as the jet-like structure shows a strong correlation with the main vortex ring inside the merged droplet. By tracking the evolution of the main vortex ring together with its circulation, we identified two mechanisms that are essential to the internal-jet formation: the vortex-ring growth and the vortex-ring detachment. Recognizing that the manifestation of the vortex-ring-induced jet physically relies on the competition between the convection and viscous dissipation of the vortex ring, we further developed and substantiated a vortex-ring-based Reynolds number criterion to interpret the occurrence of the internal jet at various Ohnesorge numbers and size ratios. For the merged droplet with apparent jet formation, the average mixing rate after jet formation increases monotonically with the vortex-ring Reynolds number, which therefore serves as an approximate measure of the jet strength. In this respect, stronger internal jet is responsible for enhanced mixing of the merged droplet.