Anatomy of Coherent Vortical Structures in Droplet-Laden Turbulent Airflow over a Wavy Water Surface

This study investigates the effects of spume droplets on coherent vortical structures (CVSs) in turbulent airflow over a wavy water surface, focusing on the role of wave steepness and droplet injection velocity in modulating these interactions. Direct numerical simulations are performed using a droplet-laden Couette turbulent airflow over progressive surface waves as an idealized model. A Eulerian-Lagrangian approach is employed, considering low- and high-steepness wave conditions (ak =0.1 and 0.2, where a is wave amplitude and k is wavenumber) and two droplet injection velocities (surface and airflow velocities near wave crest). Results from droplet-free simulations reveal strong phase dependence in CVS formation, with forward and reversed horseshoe vortices near wave troughs and quasi-streamwise vortices aligning along the windward surface. Droplets injected at the surface velocity remain near wave crests, where CVS formation is weak, leading to minimal interaction. In contrast, droplets injected at the airflow velocity disperse broadly, increasing the likelihood of interactions with CVSs. For low-steepness waves, these droplets stay within the surface layer, attenuating CVSs and suppressing ejection and sweep events. However, as wave steepness increases, more droplets escape the near-surface layer, reducing their influence on CVSs.