The dynamics, energetics, and air entrainment of unsteady breaking waves in the absence and presence of direct wind stress
File(s)
Author(s)
Cao, Rui
Type
Thesis
Abstract
Wave breaking is a discrete process occurring at steep wave crests, playing a fundamental role in weather, climate, and marine renewable energy. Notwithstanding their importance, the onset, severity, and air entrainment of individual breaking waves within a given population remain poorly understood, particularly under joint influences of wind forcing and non-linear wave-wave interactions. This knowledge gap arises largely from challenges in tracking breaking waves within discrete wave groups in the ocean and practical limitations of laboratory studies that often exclude wind effects.
This thesis presents a comprehensive analysis of the breaking process and subsequent two-phase flow evolution of uni-directional, individual breaking waves generated via dispersive focusing under controlled laboratory conditions. A wide range of wave group conditions, with and without wind forcing, were explored through three dedicated experimental campaigns (SIREN, BUBER, and EURUS). Data were collected using a combination of in-situ and remote sensing techniques, including wave gauges, high-resolution cameras, hydrophones, and wind velocimeters. These experimental set-ups, combined with novel image processing techniques developed as part of this research, provide a carefully-controlled environment in which to study key breaking wave characteristics.
The thesis first details the geometric, kinematic, and dynamic behaviours of breaking crests under varying wave scales, spectral bandwidths, non-linearities, and wind speeds. The findings help to de-convolve the roles of these variables on the spectral energy distribution, energy dissipation and its rate, and the breaking strength, thereby offering new insights into breaking-wave energetics. Then, the evolution of breaking-induced two-phase flows, including air cavity, bubble plumes, and surface whitecaps, is examined, highlighting the wind's role in air entrainment and its link to wave group energy budgets. Moreover, methods and physical models are proposed or refined to quantify breaking-wave energy dissipation, parameterise dissipation rates and breaking strength, and predict surface foam time-series, all of which are discussed in detail herein.
This thesis presents a comprehensive analysis of the breaking process and subsequent two-phase flow evolution of uni-directional, individual breaking waves generated via dispersive focusing under controlled laboratory conditions. A wide range of wave group conditions, with and without wind forcing, were explored through three dedicated experimental campaigns (SIREN, BUBER, and EURUS). Data were collected using a combination of in-situ and remote sensing techniques, including wave gauges, high-resolution cameras, hydrophones, and wind velocimeters. These experimental set-ups, combined with novel image processing techniques developed as part of this research, provide a carefully-controlled environment in which to study key breaking wave characteristics.
The thesis first details the geometric, kinematic, and dynamic behaviours of breaking crests under varying wave scales, spectral bandwidths, non-linearities, and wind speeds. The findings help to de-convolve the roles of these variables on the spectral energy distribution, energy dissipation and its rate, and the breaking strength, thereby offering new insights into breaking-wave energetics. Then, the evolution of breaking-induced two-phase flows, including air cavity, bubble plumes, and surface whitecaps, is examined, highlighting the wind's role in air entrainment and its link to wave group energy budgets. Moreover, methods and physical models are proposed or refined to quantify breaking-wave energy dissipation, parameterise dissipation rates and breaking strength, and predict surface foam time-series, all of which are discussed in detail herein.
Version
Open Access
Date Issued
2024-09-05
Date Awarded
2024-12-01
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Callaghan, Adrian
Sponsor
Skempton Scholarship
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)