Characteristic Features of a Wind Wave Field with Occasional Breaking, and Splashing Droplets at High Winds

Abstract

The characteristics of intermittent and sporadic wind wave breaking and the splashing of droplets along a wave are investigated through wind-wave tank experiments (wind-wave tank: 20m long, 0.6m wide, 1.2m high, with fresh water 0.6m deep). For investigating the characteristics of wind wave breaking, theintensity of breaking of individual wind-wave crests has been related to the parameters of wave form (e.g. height, period, steepness and skewness), and their process of change has been studied (reference wind speed 15ms−1, fetch 16m). Distributions of the wave form parameters are different for breaking and nonbreaking waves. Fully breaking waves seem to be ones for which the relation HαT2, between the individual wave height H and period T, holds. The condition of breaking is more complicated than a simple criterion of Stokes’ limit. The quantities wave height and steepness for a breaking wave are not always larger than these quantities for a nonbreaking wave. This may suggest that there is an overshooting nature to the breaking wave. The wave form parameters change cyclically during wave propagation. The period of the cycle in the present case is longer than 4 wave periods. An intermittency in the wave breaking is associated with this cyclic process. Roughly speaking, two or three successive breakingwaves sporadically exist among a series of non-breaking waves moving downwind. The direct production of droplets from breaking individual waves is described, based on experimental results. The spreading of the droplets (diameter > 0.8mm) under the direct action of the wind field along a breaking wind wave is also described, based on the comparison of the actual distribution of droplet velocities with the wind field (reference wind speed: 16ms−1, fetch 16m). Both the movement of the droplets and that of styrofoam flakes used as tracers for the air flow visualization are measured by a simple photographic technique involving the use of multi-colored overlapping exposures. This comparison gives a concrete picture of the acceleration of droplets by the wind.