Air flow separation over unsteady breaking waves

The evolution of the airflow instantaneous structure over an unsteady break ing wave propagating in a group is measured in detail using the digital par ticle image velocimetry technique. It is found that the boundary-layer over a breaking wave, the steepest in the group, separates at a point close to the sharp crest and reattaches in the front slope of the following wave. Du ring breaking, the evolution of the, turbulent vorticity is essentially uns teady and the recirculation zone of the separated flow takes the form of a large well-organized vortex. Links between the wave-crest geometry and geom etrical features of the separation bubble have been established. (C) 1999 A merican Institute of Physics. [S1070-6631(99)01607-4].