Modeling of ocean-atmosphere interaction phenomena during the breaking of modulated wave trains

Air water interaction phenomena taking place during the breaking of ocean waves are investigated here. The study is carried out by exploiting the combination between a potential flow method, which is used to describe the evolution of the wave system up to the onset of the modulational instability, and a two-fluids Navier-Stokes solver which describes the strongly non-linear air-water interaction taking place during breaking events. The potential flow method is based on a fully non-linear mixed Eulerian-Lagrangian approach, whereas the two-fluid model uses a level-set method for the interface capturing. The method is applied to study the evolution of a modulated wave train composed by a fundamental wave component with two side band disturbances. It is shown that breaking occurs when the initial steepness exceed a threshold value. Once the breaking starts, it is not just a single event but it is recurrent with a period associated to the group velocity. Results are presented in terms of free surface shapes, velocity and vorticity fields, energy and viscous dissipation. The analysis reveals the formation of large vortex structures in the air domain which are originated by the separation of the air flow at the crest of the breaking wave. The form drag associated to the flow separation process significantly contributes to the dissipation of the energy content of the wave system. The energy fraction dissipated by each breaking event is distinguished.