VELOCITY, ACCELERATION AND VORTICITY UNDER A BREAKING WAVE

The fluid particle velocities in the overturning jet of a breaking wav e have been measured by the Particle Image Velocimetry (PIV) technique . Monochromatic waves with wave height of 14.5 cm and wavelength of 12 1 cm were generated in the water depth of 20 cm. The measured fluid pa rticle velocity at the tip of the overturning jet reached 1.68 times o f the phase velocity calculated from the linear wave theory. Fluid par ticle accelerations were estimated from the velocity data with the fol lowing results: The overturning jet enters the horizontal water surfac e with an acceleration of 1.1 g at an angle of 88 degrees downward. Th e PIV technique was also used to measure the instantaneous vertical vo rticities generated by breaking waves. The number and locations of the vortices on the horizontal plane appear to be random. The maximum ins tantaneous vorticity was in the order of magnitude of 20-30 s(-1), whe reas the ensemble-averaged vorticity was quite small. (C) 1998 America n Institute of Physics.