Numerical study of wind-wave interfacial phenomena

A DNS simulation has been performed in order to study the wind-wave interaction, especially focusing the attention on the interfacial phenomena. The numerical model is founded to reproduce gravity waves generation under the action of a turbulent wind. The VOF (volume of fraction) method is used in order to capture and model the free surface, while the wind is generated by imposing an external pressure gradient. This last choice, in contrast with the usual ways of simulating wind by a moving wall, turned out to be very effective from both the computational and the physical point of view. The simulation can be described by two fundamental parameters: the friction Reynolds number is about 330 and the wave age is 35. The wave age has been calculated by a spectral analysis of the free surface: through this analysis the wavelength of the main ripple is found to be about 5 cm, while its phase speed results to be 0.7 m/s. This wavelength and phase speed seems reasonable by comparing it with experiments and observations. The turbulent boundary layer is significantly modified by waves in a way that resembles rough turbulence. The spatially and time averaged mean velocity profile is affected by the roughness of waves as much as the fluctuating field. In this study a sea surface roughness of 0.017 is founded by observing the behaviour of the wind close to the free surface. This similarity between a rough wall and waves, can be relevant for modelling flows over wavy walls or predicting wind and wave currents, especially in the Geophysical fluid dynamic field. Finally, the Phillips mechanism (turbulent pressure fluctuations along the free surface) and the sheltering mechanism (positive and negative pressure fluctuations respectively in front and on the leeside of the wave due to the boundary layer separation) have been clearly observed.