Bubble motion and break-up in turbulence: fluid mechanics affecting bubbles entrained by breaking waves

Abstract

This thesis examines the break-up and rise of air bubbles in turbulence. Understanding these physical phenomena is important for modeling gas transfer through the surfaces of bubbles that are created by breaking waves at the ocean surface. Further, the bubble dynamics studied are relevant to industrial applications in which bubbles facilitate gas or heat transfer, and multiphase turbulent flows are important components of other areas of environmental research such as bubble bursting at the ocean surface and oil spill mitigation. Laboratory experiments in which turbulence is generated in a tank of water are employed to study these phenomena in a setting that is less complex than the flow under a breaking wave, allowing for more detailed measurements to be made. The first problem we investigate is that of an air bubble, deformed by turbulence, pinching apart from a needle, for which we find that the effects of the turbulence persist even once the neck has shrunk beyond the relevant turbulent scales. Then, studying large bubbles breaking apart in turbulence, we find a broad distribution of bubble sizes is created. The smallest bubbles stem from the unstable collapse of elongated air ligaments involved in the deformations. This mechanism explains the separation of scales between the energetic turbulent motions and the very small bubbles observed in the ocean and in industrial flows. Further, we find that turbulence in the liquid decreases the rate at which a bubble rises due to buoyancy. The role of turbulence is elucidated using numerical simulations of bubbles in a turbulent flow, which show that the non-linearities in the drag force are the source of this slow-down for larger bubbles. Finally, we move from experiments on air bubbles in forced turbulence to larger-scale experiments involving air bubbles that are produced by breaking wind-waves, characterizing the dynamics and statistics of entrained bubbles over a range of conditions. The action of wave breaking is found to enhance the stream-wise transport of the entrained bubbles. Then, we invoke results from the studies on bubble break-up and rise to describe the time-averaged bubble size distributions measured in the various wave fields.