Title: Droplet and Bubble Measurements in Turbulent Free-Surface Flows
Defense Date and Time: Friday, April 17 starting at 10:00 am.
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Professor James H. Duncan, Chair/Advisor
Professor Kenneth Kiger
Associate Professor Johan Larsson
Emeritus Professor Peter Bernard
Associate Professor Anya Jones, Dean’s Representative
In this dissertation, the generation and dynamics of drops and bubbles in breaking waves and turbulent free-surface boundary layer shear flows, respectively, are studied in laboratory scale experiments. In the drop experiment, breaking waves are generated by a programmable wavemaker and measurements of breaker profile evolution and spatio-temporal distribution of drops are reported. The drop and breaking profile measurements are used synergistically to relate drop production to breaker characteristics and sub-processes in two related studies. In the first study, spray generation mechanisms by a weak plunging breaker are explored. Three distinct time zones of drop production are found, first when the jet impacts the free surface, second when the large air bubbles trapped by the plunging jet impact reach the surface and pop, and third when smaller bubbles reach the surface later in the breaking process and pop. In the second study, drop production by three plunging breakers is correlated to mean wave characteristics such as surface features, plunging jet impact velocity, and wave crest speed. The number of drops produced per breaking event is found to increase with breaker intensity. The relative importance of breaker intensity on breaking sub-processes, identified in the first study, is reported.
In the bubble experiment, air entrainment in a turbulent free-surface boundary layer shear flow is studied in a laboratory-scale experimental facility. The boundary layer is created by a horizontally moving surface-piercing stainless steel belt that travels in a loop between two rollers. One length of the belt between the two vertically oriented rollers is exposed to water (with a free surface). The belt accelerates suddenly from rest until reaching constant speed and creates a temporally evolving free-surface boundary layer analogous to the spatially evolving boundary layer that would exist along a surface-piercing towed flat plate. Air entrainment mechanisms and bubble statistics like bubble size, number, and speed, are reported and qualitatively compared to direct numerical simulations of a similar problem conducted by a different research group.