The “Bag Breakup” Spume Droplet Generation Mechanism at High Winds. Part I: Spray Generation Function

This paper describes the results of an experimental and theoretical investigation into the mechanisms by which spume droplets are generated by high winds. The experiments were performed in a high-speed wind-wave flume at friction velocities between 0.8 and 1.5 m s −1 (corresponding to a 10-m wind speed of 18–33 m s −1 under field conditions). High-speed video of the air–water interface revealed that the main types of spray-generating phenomena near the interface are “bag breakup” (similar to fragmentation of droplets and jets in gaseous flows at moderate Weber numbers), breakage of liquid ligaments near the crests of breaking surface waves, and bursting of large submerged bubbles. Statistical analysis of these phenomena showed that at wind friction velocities exceeding 1.1 m s −1 (corresponding to a wind speed of approximately 22.5 m s −1 ), the main mechanism responsible for the generation of spume droplets is bag breakup fragmentation of small-scale disturbances that arise at the air–water interface under the strong wind. Based on the general principles of statistical physics, it was found that the number of bags arising at the water surface per unit area per unit time was dependent on the friction velocity of the wind. The statistics obtained for the bag breakup events and other data available on spray production through this type of fragmentation were employed to construct a spray generation function (SGF) for the bag breakup mechanism. The resultant bag breakup SGF is in reasonable agreement with empirical SGFs obtained under laboratory and field conditions.