An experimental investigation of breaking wave-induced turbulence

Breaking waves are central to the interaction between the ocean and the atmosphere. Although a reasonably large body of literature exists describing the mechanisms which lead to wave breaking, significantly less is known about the flow induced by breaking. This dissertation presents velocity measurements in the flow induced by a breaking wave from which a physical description of that flow is attained.; A wave focussing technique was used to generate a repeatable, unsteady breaking wave in the laboratory. Full field velocity measurements of the flow induced by breaking were recorded using the recently developed digital particle tracking velocimetry (DPTV) technique. The breaking is first apparent in the instantaneous flow fields as a highly turbulent, mixed air-water, jet-like structure propagating across the surface of the ambient fluid. This structure shears the ambient fluid, inducing a mixing layer with its associated surface current and eddies. Flow originating in the air-water structure has a distinctive turbulence structure which is conjectured to form as a consequence of bubbles propagating through the fluid. After the jet structure has passed, the growth of the induced mixing layer slows abruptly. Downstream the shear structure is still present in the flow. The bubble generated turbulence structure, however, is no longer apparent.; The induced mean surface current and change in turbulent structure were further confirmed by the turbulent properties, calculated by ensemble averaging over approximately 140 breaking events. As the breaking structure propagates across the surface, turbulence is diffused down into the ambient fluid below. Once the structure has passed, however, the shear layer becomes the dominant influence on the turbulence, reorienting the fluctuations. The induced shear layer also acts to reposition the maximum turbulent kinetic energy to the depth of the largest induced shear. These transitions in the turbulent structure are apparent in the changes in the measured anisotropy and turbulent structure parameters of the flow. These values indicate that the turbulent structure of the flow tends to a jet-like shear structure once the breaking structure has passed.