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

Abstract

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.

The Study of Momentum, Mass, and Heat Transfer in a Droplet‐Laden Turbulent Airflow Over a Waved Water Surface by Direct Numerical Simulation

“…Thus, in the present study, we follow Andreas and Emanuel () and Andreas () and inject droplets with initial velocities equal to the velocities of fluid particles in the surface wave, and the results are consistent with those obtained by Druzhinin et al (). Recent study by Troitskaya, Kandaurov, et al () suggests that the dominant mechanism of spume droplets generation is that of a bag‐breakup fragmentation. Troitskaya et al () and Troitskaya, Druzhinin, et al () developed a statistical description of the bag‐breakup phenomena, but PDF of the drops velocities at injection still remains unknown and requires further research.…”

A Laboratory Study of the Effect of Surface Waves on Heat and Momentum Transfer at High Wind Speeds

“…First of all, the fetch for wind waves in the ocean is dramatically higher than in a laboratory tank. Besides, the amount of sea spray ejected to the atmosphere, which is also fetch dependent, also can be an order of magnitude higher (see, e.g., Troitskaya, Kandaurov, et al, 2018). The advantage of laboratory experiments is in focusing on a certain physical process behind the phenomenon (in our case, the air‐water fluxes) under control conditions.…”

“…The problem of dependency of the exchange coefficients in MABL on wind speed is closely connected with the question of the influence of surface waves and spray, that is, the products of wind‐forcing on the exchange of the momentum and mass between the atmosphere and ocean. The effect of waves on the momentum transfer has been studied intensively (see, e.g., Ayet et al, 2020; Husain et al, 2019; Janssen, 1989; Makin et al, 1995; Makin & Mastenbroek, 1996; Makin & Kudryavtsev, 1999; Moon et al, 2004; Takagaki et al, 2012; Takagaki, Komori, & Suzuki, 2016; Takagaki, Komori, Suzuki, et al, 2016; Troitskaya et al, 2012, 2017; Troitskaya, Druzhinin, et al, 2018; Troitskaya, Kandaurov, et al, 2018; Troitskaya & Rybushkina, 2008, and references therein). It has been shown that the momentum transfer by wave disturbances, or the form drag, increases with increasing wind speed due to the widening of the wave spectra.…”

On the physical mechanism of front–back asymmetry of non-breaking gravity–capillary waves

“…At present, such an ambitious undertaking is far beyond the art of the possible. There are works which tackle some aspects of the listed elements, for example, viscous decay due to molecular viscosity (Fedorov & Melville 1998), generation of vorticity at the free surface (Hung & Tsai 2009), exact three-dimensional simulation of the Navier-Stokes equations in the absence of wind and turbulence (Hung & Tsai 2009), interaction of waves with a turbulent surface layer (Fedorov & Melville 2009) and the generation of spay (Troitskaya et al 2018).…”