An experimental study on turbulent coherent structures near a sheared air-water interface

The turbulence structures near a sheared air-water interface were experimen tally investigated with the hydrogen bubble visualization technique. Surfac e shear was imposed by an airflow over the water flow which was kept free f rom surface waves. Results show that the wind shear has the main influence on coherent structures under air-water interfaces. Low- and high- speed str eaks form in the region close to the interface as a result of the imposed s hear stress. When a certain airflow velocity is reached, "turbulent spots" appear randomly at low-speed streaks with some characteristics of hairpin v ortices. At even higher shear rates, the flow near the interface is dominat ed primarily by intermittent bursting events. The coherent structures obser ved neat sheared air-water interfaces show qualitative similarities with th ose occurring in near-wall turbulence. However, a few distinctive phenomena were also observed, including the fluctuating thickness of the instantaneo us boundary layer and vertical vortices in bursting processes, which appear to be associated with the characteristics of air-water interfaces.