Swell and wind-waves interaction under partial reflection conditions

The main aim of this thesis is to shed light about the influence that partial reflection, which is the most common case encountered both in field and laboratory studies, exerts on a wave field where regular waves are present alone and under the action of following or opposing wind. We have analysed experimentally and theoretically the flow field of paddle waves, paddle waves plus following wind and paddle waves with opposing wind under partial reflection conditions in laboratory, in terms of free surface elevations, velocities and stresses. For the theoretical and experimental analyses of the flow field, we consider velocities and free surface elevations as a superposition of a mean (current), a periodic (wave) and a turbulent (fluctuating) component. A theoretical model for the periodic (wave) component is developed by means of a perturbation scheme which considers the wave field as a superposition of an incident, a reflected and a bound wave. The phase shift between the incident and the reflected wave is included in the analysis. The theory yields an analytical formulation of the wave components of velocities and stresses, and a spatial variation of the mean water level. The model also predicts the principal stresses, as well as the angle of the principal axes, of both the Reynolds wave and turbulent stress tensors. The wave flume used for the experimental activity provides a complete control of the generated and reflected wave conditions, and is used to investigate the influence of partial reflection on the main variables of interest, i.e. velocity and free surface elevation, and the combined effect of regular waves and wind action. We measure velocity and free surface elevation in two different sets of experiments: in the first series, paddle waves and paddle waves plus following wind are observed and studied; in the second series, paddle waves with opposing wind are observed and studied. Data analysis allows a complete representation of the experimental velocities, stresses and free surface characteristics of the flow field. Time average yields the mean components, phase average yields the wave (periodic) components, while the residual part (after time and phase average) represents the fluctuating components. A spectral filter is also used for the separation of the free surface elevation component attributed to the paddles (wave) and to the wind (turbulent). Several analyses of velocity and free surface data are performed to quantify the experimental contributions of the separated components. The discussions of the two series of experiments show separately the main results of the activities, and the final conclusions offer a unified vision of the advances brought by this thesis in the wide scenario of swell and wind waves interaction under partially-reflective conditions.