An experimental study of deep water plunging breakers

Abstract

Plunging breaking waves are generated mechanically on the surface of essentially deep water in a two-dimensional wave tank by superposition of progressive waves with slowly decreasing frequency. The time evolution of the transient wave and the flow properties are measured using several experimental techniques, including nonintrusive surface elevation measurement, particle image velocimetry, and particle tracking velocimetry. The wave generation technique is such that the wave steepness is approximately constant across the amplitude spectrum. Major results include the appearance of a discontinuity in slope at the intersection of the lower surface of the plunging jet and the forward face of the wave that generates parasitic capillary waves; transverse irregularities 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 essentially irrotational until incipient breaking occurs; and the magnitude of the largest water particle velocity is about 30% greater than the phase speed of the (equivalent) linear wave.