Gentle spilling breakers: crest flow-field evolution

The flow fields at the crests of mechanically generated gentle spilling breakers are studied experimentally with particle image velocimetry (PIV). During the initial development of the breaker, a bulge forms on the forward face of the wave near the crest. The present measurements show that the flow field in this bulge is wavelike in nature and that the maximum horizontal component of the fluid velocity, u(max), is less than the wave crest speed. No vorticity is found during this stage of the breaking process. After a short while, the leading edge (called the toe) of the bulge begins to move rapidly down the wave crest; u(max) reaches the speed of the wave crest just as the toe begins to move. Concurrent with the beginning of the toe motion, a vortical region appears at the free surface just downstream of the toe. At about the point in the wave development where the measurements end, the vortical region extends from the toe to the crest.