Chasapis Tassinis, Konstantinos; (2023) On the evolution of oceanic bubble statistics: an experimental study. Doctoral thesis (Ph.D), UCL (Universtiy College London).

Text On the evolution of oceanic bubble statistics, an experimental study.pdf - Accepted Version Access restricted to UCL open access staff until 1 December 2024. Download (76MB)

Abstract

The bubbles generated by breaking waves in the open ocean are thought to have a significant influence on air-sea gas transfer, aerosol production, and the optics and acoustics of the top few meters of the ocean . Short-lived larger bubbles ( r  0.1 mm) dominate the transfer of less soluble gases such as carbon dioxide ( CO2). However, a lack of mechanistic understanding remains a n obstacle to improving parametrizations of ocean bubble effects. Laboratory studies have enabled significant progress, but breaking wave crests can have different geometries and there has been relatively little discussion of the extent of this influence on bubble evolution. In this thesis the development and validation of an experimental set-up was specifically designed to study the bubbles in the laboratory formed by breaking waves with different geometries, and the detailed methods used to integrate wave and bubble measurements and assess their relationship. The proposed experimental framework includes a number of novel elements , including bubble size spectra which are evolving referenced to the breaking location and to the geometry of their breaker for various strong or weak spillers. The experimental set-up allows for a breakdown of the active and quiescent stage of bubble evolution for breakers of different intensity and in different types of water. This leads to an analysis of the bubbles that dominate the total gas volume underwater at different breaking stages. The experimental program consists of three parts: breaking wave generation, breaking wave profile extraction and bubble detection. The breaking wave generation uses an iterative dispersive focusing technique to generate phase-shifted breaking waves with different wave group steepness and different amplitude spectra. The successive wave profiles of those breakers are extracted using a laser fluorescent imaging technique. Breaking location, shape evolution and bulge volume (the part of the wave profile that changes shape during breaking) are measured for all breakers. Bubble plume evolution is monitored for all breakers in freshwater and in saltwater. Bubble size sp ectra evolution is demonstrated for the three main breaker categories: overturning bulge, collapsing bulge and pure spiller.viii Results show that approximately 10% of the breaker’s bul ge volume becomes bubbles. This novel result allows to predict the total maximum bubble volume by measuring the geometry of each breaker. Further analysis demonstrates how breaker geometries relate to bubble evolution. Bubble volume of pure spillers is dominated by small � = 1 − 3 mm bubbles that go shallow and live short lives (less than a quarter of wave period), whereas bubble volume of overturning bulges is dominated by large � > 4 mm bubbles that go deep and live for several wave periods. These relationships can be used to incorporate differences in maximum total bubble volum e and � > 1 mm slope in bubble generation models by measuring breaker geometry. Therefore, these relationships can be used to make more accurate predictions of aeration caused by different types of breakers in the open ocean

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