Quantifying the Threshold for Fragmentation of Newtonian and Non-Newtonian Drops

This dissertation investigates how the properties of liquid drops influence the minimum aerodynamic forces needed to cause the drop to fragment (breakup). For this work, we focus only on aerodynamic forces that are applied on a stationary drop by a sudden, uniform flow, a process called impulsive acceleration. Previous studies have largely focused on the fragmentation of spherical drops of Newtonian fluids (liquids with constant viscosity, like water), when impulsively accelerated in air-like ambient medium. Such systems only capture a limited range of drop and ambient properties (described by non-dimensional groups). Many real-world applications, such as aerial fire retardant delivery, involve conditions beyond this typical range. Fire retardants, for instance, can exhibit shear-thinning behavior (their viscosity decreases under stress), and the drops undergoing fragmentation often start with a non-spherical shape. The ambient medium could also be one with much higher densities and viscosities, common in cases where liquid drops fragment in other liquids. Through this dissertation, we aim to utilize computational models, specifically interface-tracking multiphase flow direct numerical simulations (DNS), to systematically examine how various drop properties affect the threshold of secondary fragmentation. The research is divided into five main parts. First, a model was developed to predict the trajectory and coverage of aerially delivered fire retardants, motivating the need for a deeper understanding of drop fragmentation. Second, we expanded the study of spherical, Newtonian drops beyond existing parameters, identifying a more robust non-dimensional parameter to predict the breakup threshold. Third, we performed the first systematic investigation of how initial drop shape affects fragmentation, quantifying its significant influence. Fourth and Fifth, we characterized the influence of shear-thinning or a viscoplastic nature of the drop fluid.