Note on natural and forced breaking waves in deep water

Abstract

Two laboratory investigations of natural and forced breaking waves in a two-dimensional wave tank are reported. They are considered to be simple representations of breaking waves that exist in a real ocean, in moderate and rough sea states. The first type (Crescent Waves) is usually observed on a sea surface in relatively calm weather. Initially, for a nonlinear initial wave train of wave steepness ak =0.34, a three-dimensional instability dominates the side band instability, and stable Crescent Waves are observed. The direction angle of radiated oblique waves as a most unstable mode, and associated growth rate are found to be in excellent agreement with Mclean's theory. The second type (strongly overturning breaking waves) was generated on the surface of essentially deep water using a wave energy focusing technique. Time evolution of the transient wave was observed using a high speed video image, and the inherent complicated unsteady flow fields was measured by means of a two-dimensional Laser Doppler Anemometer. We found that, 41% of the wave momentum was lost by the extreme wave plunging breaker. The maximum horizontal velocity at the overturning top of the crest was found to be equal to 1.4 times the phase velocity of the wave, in its most developed stage. The two-dimensional velocity field reveals that the flow is essentially irrotational up to the top of the crest. Surface wave height measurements before and after both an isolated plunging and spilling breaking wave, reveal a frequency downshift phenomenon in the corresponding spectral shapes which seems not to have been previously seen.