Resolving Langmuir Turbulence in a Coupled Wind-Wave System

Langmuir turbulence, arising from the nonlinear interaction between surface gravity waves and wind-driven shear currents, significantly contributes to ocean mixing and the air-sea transfer of mass, momentum, and energy. Previous studies have either prescribed surface wind stress in single-phase flow simulations or left surface waves indeterminate in two-phase flow simulations. To better understand the generation and evolution of Langmuir turbulence, and to quantify the momentum and energy transfer across the air-sea interface, a series of two-phase wave-resolved direct numerical simulations are conducted across various Langmuir numbers. In these simulations, fully developed pressure gradient-driven turbulence on the air side is acted upon prescribed surface gravity waves. The results reveal characteristic structures of Langmuir cells at varying scales, including pairs of counter-rotating vortices and elongated streamwise streaks on the water surface. By decomposing flow velocity into mean current, wave orbital motion, and turbulence fluctuation, the impact of wave-induced phase-dependent strain on underlying turbulence and the enhancement of streamwise vorticity are analyzed in detail. Additionally, the momentum flux across the air-sea interface is calculated and its transfer mechanism is discussed, providing insights for parameterization in climate models.