Numerical Study on the Generation and Transport of Spume Droplets in Wind over Breaking Waves

Abstract

Sea spray droplets play an important role in the momentum, heat and mass transfer in the marine atmospheric boundary layer. We have developed a new direct numerical simulation method to study the generation and transport mechanisms of spume droplets by wind blowing over breaking waves, with the wave breaking process taken into account explicitly. In this new computational framework, the air and water are simulated as a coherent system on fixed Eulerian grid with the density and viscosity varying with the fluid phase. The air-water interface is captured accurately using a coupled level-set and volume-of-fluid method. The trajectories of sea spray droplets are tracked using a Lagrangian particle-tracking method. The generation of droplets is captured by comparing the fluid particle velocity of water and the phase speed of the wave surface. From the simulation data, we obtain for the first time a detailed description of the instantaneous distribution of droplets at different stages of wave breaking. Furthermore, the time histories of the droplet number and its generation and disappearance rates are analyzed. Simulation cases with different parameters are performed to study the effects of wave age and wave steepness. The flow and droplet fields obtained from simulation provided a detailed physical picture of the problem of interest. It is found that plunging breakers generate more droplets than spilling breakers. Droplets are generated near the wave crest at young and intermediate wave ages, but at old wave ages, droplets are generated both near and behind the wave crest. It is also elucidated that the large-scale spanwise vortex induced by the wave plunging event plays an important role in suspending droplets. Our simulation result of the vertical profile of sea spray concentration is consistent with laboratory measurement reported in the literature.

A Unified Breaking Onset Criterion for Surface Gravity Water Waves in Arbitrary Depth

“…The laboratory work of Saket et al (2018) showed that B th ≈ 0.85 also segregates breaking from nonbreaking crests in the presence of wind forcing in deep water breaking. A number of high-fidelity two-phase flow simulations of breaking waves in the presence of wind forcing (e.g., Tang et al, 2017;Yang et al, 2018) have been recently performed. Detailed quantification of the effect of direct wind forcing on the proposed breaking onset criterion in shallow water is left for future study.…”

Spray Generation by a Plunging Breaker

“…By using a high-resolution simulation, Wang et al (2016) investigated the bubbles and droplets generated by plunging breakers. Tang et al (2017) developed a direct numerical simulation scheme to study the generation and transportation of spume droplets.…”

A Laboratory Investigation of Spume Generation in High Winds for Fresh and Seawater

“…Barenblatt et al () provided a mathematical model to corroborate Lighthill's theory and demonstrated, in a highly idealized scenario, that the presence of spray inhibits turbulence intensity and drastically reduces the aerodynamic drag coefficient of the ocean surface. Recently, high‐resolution model simulations have shown that the turbulent energy differs significantly for the spray‐laden versus spray‐free air flow over waves (Pan et al, ; Richter & Sullivan, ; Tang et al, ).…”