Phase-resolved analysis of velocity field structure and vorticity dynamics under colinear swell and wind-waves

The objective of this study is to analyze the turbulence field generated by the interaction between mechanical waves and colinear wind-waves in the liquid domain just below the free surface. Detailed three-dimensional velocity measurements close to the surface are decomposed into mean, swell, wind-waves, and turbulence contributions. In this work, we treat wind-waves as macroscale turbulence with a pseudo-random velocity field. Advanced data analysis yields phase-resolved and vertical examinations of wind-waves and turbulence stresses, kinetic energy, and vorticity. The results indicate that near the surface, the spanwise energy dominates both the wind-wave and the turbulence kinetic energy. The wind-waves and turbulence stress tensors exhibit a large anisotropy when swell waves are present, as a consequence of the interaction between swell and wind-waves. Furthermore, we present the spatio-temporal distribution of vorticity, and we elucidate the non-trivial interaction between vorticity and the flow field. This interaction results in body forces that contribute to the local variation in inertia, as described by the Navier–Stokes equation. It is observed that in all combinations, a body force acts, on average, downward, modifying the gradient pressure in the vertical direction.