Swell and wind-waves interaction under partial reflection conditions

Abstract

Esta investigación se refiere a un estudio experimental de la interacción entre el viento y las olas generadas por las palas afectadas por la reflexión. El viento y las ondas de palas regulares se generan, juntas y por separado, en un canal de interacción océano-atmósfera, que permite la absorción activa para obtener la reflexión deseada para cada experimento. La posibilidad de generar una reflexión deseada en el campo de onda permite una evaluación y cuantificación completas del efecto reflexivo en el campo de onda. En particular, el surgimiento de tensiones adicionales, la modulación espacial del nivel medio del agua y otros fenómenos se observan experimentalmente y se derivan teóricamente.

The main aim of this thesis is to shed light about the influence that partial reflection (one of the most common cases encountered both in field and laboratory studies of water waves) exerts on a wave field where regular waves are present alone and under the action of following or opposing wind. We have analyzed experimentally and theoretically the ow 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 ow 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 considering the wave field as a superposition of an incident, a re ected and a bound wave. The phase shift between the incident and the re ected 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 ume used for the experimental activity provides a complete control of the generated and re ected wave conditions, and is used to investigate the influence of partial re ection 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 di erent sets of experiments: in the rst 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 ow 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 uctuating 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-re ective conditions.