AN EXPERIMENTAL-STUDY OF DEEP-WATER PLUNGING BREAKERS

Plunging breaking waves are generated mechanically on the surface of e ssentially deep water in a two-dimensional wave tank by superposition of progressive waves with slowly decreasing frequency. The time evolut ion of the transient wave and the flow properties are measured using s everal experimental techniques, including nonintrusive surface elevati on measurement, particle image velocimetry, and particle tracking velo cimetry. The wave generation technique is such that the wave steepness is approximately constant across the amplitude spectrum. Major result s include the appearance of a discontinuity in slope at the intersecti on of the lower surface of the plunging jet and the forward face of th e wave that generates parasitic capillary waves; transverse irregulari ties occur along the upper surface of the falling, plunging jet while the leeward side of the wave remains very smooth and two dimensional; the velocity field is shown to decay rapidly with depth, even in this strongly nonlinear regime, and is similar to that expected from linear theory-the fluid is undisturbed for depths greater than one-half the wavelength; a focusing or convergence of particle velocities are shown to create the jet in the wave crest; vorticity levels determined from the measured, full-field velocity vectors show that the waves are ess entially irrotational until incipient breaking occurs; and the magnitu de of the largest water particle velocity is about 30% greater than th e phase speed of the (equivalent) linear wave. (C) 1996 American Insti tute of Physics.