Interaction between large-scale vortical structures and quasi-streamwise vortices in Langmuir turbulence

Langmuir circulations arise through the interaction between the Lagrangian drift of the surface waves and the wind-driven shear layer. The high shear rate alone is sufficient for generating quasi-streamwise vortices within the shear layer. Despite the different formation mechanisms, both vortical structures manifest themselves by inducing wind-aligned streaks on the surface. Numerical simulations of a stress-driven turbulent shear layer bounded by monochromatic surface waves are conducted to reveal the mutual interaction between the large-scale vortical structures of Langmuir circulations and the small-scale quasi-streamwise vortices in Langmuir turbulence. The averaged structure of Langmuir circulations is educed from conditional averaging guided by the signatures of predominant surface streaks obtained from empirical mode decomposition. The width of the averaged vortex pair of Langmuir circulations is found to be comparable to the most unstable wavelength of the wave-averaged Craik–Leibovich equation. Small-scale coherent vortical structures are identified using a detection criterion based on local analysis of the velocity-gradient tensor and their topological geometry. Quasi-streamwise vortices accumulated beneath the windward surface are found to dominate the distribution of small-scale coherent vortical structures. Employing the variable-interval spatial average to the identified quasi-streamwise vortices reveals that they tend to form in the edge vicinity of the high-speed surface jets induced by the Langmuir cells. The tilting of vertical vorticity at the outer edges of surface jets by shear current and wave drift enhances the formation of quasi-streamwise vortices. The results highlight the differences in the coherent vortical structures between the Langmuir turbulence and the turbulent wall layer.