The Dynamics of Plunging Breakers and the Generation of Spray Droplets

Abstract

In this dissertation, drop generation by plunging breaking waves is studied in laboratory-scale experiments. The breaking waves are generated by a programmable wavemaker that is set to produce a dispersively focused wave packet. Breaker profile and drop measurements are obtained for three breaking waves of increasing intensity, called herein the, weak, moderate, and strong plunging breakers, respectively. Drops, with radius >= 50 microns, are measured using a cinematic in-line holographic system positioned at 28 streamwise measurement locations, arranged in a horizontal plane, called herein the measurement plane, positioned 1 cm above the maximum wave crest height, during many repeated breaking events. From the holograms, the radius, three-dimensional location, and velocity of the drops is determined. The evolution of the breaker profile is measured at the center plane of the tank using a cinematic laser-induced fluorescence (LIF) technique. Drop and breaker profile movies are taken simultaneously and recorded at 650 frames per second.

The breaker profile and drop measurements from the weak breaker are used to identify and quantify spray generation processes in plunging breakers. Two spatially and temporally separated regions of drop production are found. The first region (I) of drop production is associated with the active phase of wave breaking and begins with jet impact. In this region, drops are produced by jet impact, large bubble bursting events, and splashing. The second region (II) of drop production occurs after the active phase of wave breaking, approximately one wave period after jet impact. In this region, drops are generated by small air bubbles, initially entrained by the breaker, that rise to the free-surface and burst.

Various features of the breaker profiles and drop production are compared and contrasted between the three waves. The temporal evolution of the breaker profile is measured in 10 unique realizations for each of the three breakers. At every instant in time, the phase averaged mean breaker profile and the distribution of standard deviation (SD) in breaker height along the streamwise direction of the mean profile is computed. Using the mean profiles, the three breakers are physically characterized based on their wave crest and jet impact speed. When aligned to the plunging jet impact location in space and time, the breaker profiles are found to be highly repeatable throughout the non-linear wave breaking process. Profile regions with high SD in height correspond to regions of high drop production. The number of drops generated per breaking event is found to scale exponentially with jet impact speed. The drop probability distribution from each of the three waves follows a power law scaling where large and small drop regions obey power laws with different coefficients.