Free surface fluctuations behind microbreakers: space--time behaviour and subsurface flow field

The unsteady development toward breaking of the wavy flow generated by a sub- merged hydrofoil starting from rest is investigated numerically by solving the two-dimensional Navier-Stokes equations for the two-phase flow of air and water and by using a level-set technique to capture the interface. The study, carried out for hydrofoils with various chord lengths, shows that, when passing from longer to shorter scales, the role played by surface tension becomes more and more relevant and different flow regimes are recovered ranging from intense plunging jet, eventually leading to a large amount of entrapped air, down to microscale breakers, in which the jet is replaced by a bulge growing on the wave crest and the breaking event takes place without air entrapment. The 'toe' of the bulge slides down upon the forward face of the wave and large downstream-propagating surface fluctuations are observed. Wavenumber and frequency spectra of the computed free-surface profiles, aimed at understanding the downstream motion of free-surface fluctuations, are evaluated and found in good agreement with similar investigations carried out experimentally and available in literature. A careful inspection of the instantaneous vorticity field under the microbreakers reveals the presence of an intense shear flow originating at the toe, instability of which eventually gives rise to coherent vortex-structures. These structures are convected downstream by the flow at a growing speed, remaining confined in a thin layer just beneath the free surface. This continuous interaction is responsible for the generation of the free-surface fluctuations experimentally found.