On the origin of bubbles in breaking waves: a numerical study

Air entrained by breaking waves in the ocean evolves within the surrounding turbulent flow field, resulting in a bubble-size distribution in the upper ocean by the end of the acoustically active phase of wave breaking. Understanding the origins of bubbles in this spectrum is crucial for studying bubble-mediated heat and mass exchange between the ocean and the atmosphere, as well as the production of sea spray aerosols. In this study, we perform high-fidelity direct numerical simulations of spilling and plunging breakers using our in-house, GPU-based, two-phase flow solver. By employing a novel bubble tracking tool, the Optimal Network algorithm, we detect events of bubble creation, extinction, fragmentation, and coalescence, spanning from the initial entrainment of a cylindrical air cavity to the formation of dense bubble plumes. From the constructed bubble genealogies, we investigate the nature of bubble fragmentation and coalescence and analyze their roles in the spatio-temporal evolution of bubble plumes in breaking waves with varying wave-steepnesses. Furthermore, we assess the importance of air cylinder fragmentation in generating super- and sub-Hinze scale bubbles.