Statistical Characteristics of Droplets Formed due to the "Bag Breakup" Fragmentation Event at the Interface between Water and High-Speed Air Flow

Recent studies indicate that the dominant mechanism for generating sprays in hurricane winds is a "bag breakup" fragmentation. This fragmentation process is typically characterized by inflation and consequent bursting of short-lived objects, referred to as "bags" (sail-like pieces of water film surrounded by a rim). Both the number of spray droplets and their size distribution substantially affect the air-sea heat and momentum exchange. Due to a lack of experi-mental data, the early spray generation function (SGF) for the bag breakup mechanism was based on the assumed similar-ity with resembling processes. Here we present experimental results for the case with a single isolated bag breakup fragmentation event. These experiments revealed several differences from similar fragmentation events that control the droplet sizes, such as secondary disintegration of droplets in gaseous flows and bursting of bubbles. In contrast to the bub-ble bursting, the film thickness of the bag canopy is not constant but is random with lognormal distribution. Additionally, its average value does not depend on the canopy radius but is determined by the wind speed. The lognormal size distribu-tion of the canopy droplets is observed in conjunction with the established mechanism of liquid film fragmentation. The rim fragmentation results in two types of droplets, and their size distribution has been found to be lognormal distribution. The constructed SGF is verified by comparing it with experimental data from the literature. The perspectives of transfer-ring the results from laboratory to field environment have also been discussed. SIGNIFICANCE STATEMENT: The "bag breakup" fragmentation is the dominant mechanism for generating spray in hurricane winds. The number and the sizes of the spray droplets substantially affect the heat transport from the ocean to the atmosphere and, thereby, the development of hurricanes. This paper presents experimental data and anal-ysis that demonstrate how droplet formation occurs during bag breakup fragmentation. It also shows analysis of the quantity and size of droplets formed during a single fragmentation event. This work demonstrates how obtained experi-mental results can be applied to real field conditions in the context of hurricane prediction models.