Inertial fragmentation processes associated with ocean wave breaking

Thesis

Ocean sprays are liquid droplets ejected from the sea surface, which enhance mass, momentum and energy transfer at the air-sea interface. Depending on their sizes, ocean sprays interact with their environment and influence the global climate system differently. They are produced from breaking ocean waves through a few dominant mechanisms, including film and jet drops associated with bursting surface bubbles, spume drops sheared from wave crests under high winds, and splash drops due to the collapse of corrugated wave fronts. Here, we use the open-source numerical solver Basilisk to investigate the latter two spray generation mechanisms, which remain poorly understood due to difficulties in the detailed analysis of breaking waves. Due to the extremely high computational cost of resolving all spray production events during wave breaking, we instead investigate two canonical fragmentation configurations as toy models for ocean spray generation, namely the bag breakup of droplets in airflows and the transverse collision of liquid rims.

For droplet aerobreakup, we utilise a recently proposed Manifold Death (MD) algorithm to artificially perforate thin liquid films in a controlled manner, establishing numerical grid convergence of the statistics of large fragments for the first time. We then analyse different fragmentation mechanisms leading to bag film rupture and discuss their contribution to fragment statistics. We further implement the Synthetic Turbulence Generation Method to investigate the influence of air-phase turbulence on the early-time evolution of bag morphology, where the decrease in droplet aspect ratio, late-time tilting of the bag and formation of small-scale surface corrugations are systematically measured and analysed.

As for rim collision, we identify ligament merging as the dominant underlying mechanism causing the increase of the size of splash drops over time, and establish a quasi-steady theoretical framework capable of predicting the long-term fragment size and velocity distributions. Gravity arrests the ligament merging process by pulling back the liquid lamella arising from rim coalescence. The size and velocity distributions of the splash drops compare favourably with available wave-breaking data, and we propose a novel theoretical model accounting for the time evolution in the rim fragment size distribution. We further propose sea spray generation functions corresponding to splash drops produced from realistic sea states. Our discovery of fragmenting large splash drops challenges the previous speculation that wave splashing is an inefficient sea spray generation mechanism, thus bearing far-reaching implications for modelling efforts in air-sea interaction. Overall, the numerical results and physical insights obtained here serve as a stepping stone towards fully understanding ocean spray formation with the aid of two phase direct numerical simulations.

Authors: Tang, K

• DOI: 10.5287/ora-5rrajd4ge
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: thin film breakup; ligament dynamics; droplet statistics; fragmentation; ocean sprays
• Subjects: Fluid mechanics