Oxygen transfer from bubble-plumes

Abstract

Air-bubble plumes produced beneath ocean breaking waves have important roles in gas transfer between atmosphere and ocean because the gas within the bubbles, convected by breaking-wave-induced turbulence, is dissolved at a deeper level. In particular, oxygen dissolution supports all biological activities in a marine ecosystem. Oxygen transfer from bubbles to bulk water depends on the dynamics of local bubble flows. As lateral bubble motion associated with vortex wakes generates turbulence in the ambient fluid, depending on the bubble size, the dissolved gas in the plume is transported via complex convection and diffusion processes. Although analytical and empirical models of gas transfer from a small, rigid bubble have been proposed previously, the effects of bubble size on gas concentrations in the turbulence field remain poorly understood. In this study, we examine the explicit effects of bubble size on bubble plume turbulence and the dissolved oxygen (DO) concentration field, and propose a new empirical gas transfer model that is applicable to large deformable bubbles on the basis of experimental imaging analysis. The gas transport process in the plume was identified using bubble turbulence coupled flow computations with the proposed gas source model, which well explained the variation in experimental DO concentration. The proposed transfer velocity model extended the applicable bubble size range and predicted the major features of the increasing oxygen concentration within the plume. We expect these findings to serve as a starting point to improve our understanding of the dynamics of practical bubble plume flows with wider bubble size ranges, as typically formed under breaking waves, and to predict oxygen concentrations in marine environments.