Experiments were performed to exploit the dispersive properties of unsteady surface waves and to induce breaking by using a modified chirp pulse technique to focus the wave energy at a specific location in the Naval Research Laboratory deep water wave channel. The experiments have resulted in a highly resolved archive of breaking events ranging from wave steepening and incipient breaking to spilling and to plunging. The potential energy density, the crest front steepness, the horizontal asymmetry, and other geometric properties of an incipient breaker vary only within a moderate band about their mean values over the extent of these experiments. Thus the properties of an incipient unsteady breaker are well defined. The application of the phase-time or Hilbert transform method to the data set provides new insights into the local properties of the unsteady wave breaking. Recently, spectral and piecewise-linear algoritbans for two-dimensional potential flow were developed and used by Schultz et al. [1994] to compare the onset of breaking for several methods of energy input to the unsteady wave system. The computations show that steep plunging waves occur when energy input rates are large. The various energy input methods exhibit similar breaking trends in the limit as the energy input rate becomes small in that incipient spilling breakers form when the potential energy is approximately 52 to 54% of the energy for the most energetic Stokes wave, with the formation of a singularity immediately before the crest. 16,515 16,516 GRIFFIN ET AL.: EVOLLrHON OF DEEP WATER BREAKING WAVES Background The fundamental experiments for studying twodimensional wave breaking (apart from wind-generated waves which are discussed briefly later) fall into three main categories: (1) the focusing of essentially two-dimensional waves in the lateral direction [Van Dom and Pazan, 1975; Ramberg et al., 1985; Ramberg and Griffin, 1987]; (2)the towing of a submerged object, such as a hydrofoil to produce steady breakers [Duncan, 1981, 1983]; and (3)the focusing of variable-length waves from a modulated or chirped wave maker to produce unsteady breakers [Dornmermuth et al., 1988; Duncan et al., 1987; Rapp and Melville, 1990; Peltzer et al., 1993; Duncan et al., 1994; Sletten and Savtchenko, 1996] or the overturning of an irregular wave train [Ochi and Tsai, 1983; Bonrnarin, 1989] to produce unsteady breakers. One of the most extensive laboratory studies of unsteady deep water breaking thus far is the work of Rapp and Melville [1990]. The dispersive character of the deep water waves was used to focus a wave packet and to generate a single unsteady breaking event at a controlled location in the wave channel. Losses of excess momentum and energy flux during breaking within a wave group ranged from 10% for spilling breakers to 25% for plunging breakers. Rapp and Melville found that the growth rate of the waves prior to breaking was an important factor in predicting breaking. This had been found previously by Van Dom and Pazan [1975] and, to a lesser e...
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