Bubbles in the close vicinity of breaking waves: Statistical characteristics of the generation and dispersion mechanism

A direct investigation focussed on the vicinity of the interface of bubble populations generated by different breaking wave fields in a wind‐water simulation facility is presented. Bubble size spectrum over a diameter range 60 μ ≤ d < 3000 μ, obtained from a laser‐based single‐particle technique is given for different depths in the vicinity of the interface and different conditions of wave age and wind. Self‐consistent results are obtained if the dimensionless depth z * = z/H ⅓ , where H ⅓ is the significant wave height, is used and two partly overlapping zones are apparent: (1) next to the surface the spectral shape is found to be invariant, and the global bubble concentration decreases rapidly with depth and (2) at lower levels the concentration of small bubbles is roughly constant, and the slope of the size spectrum increases with depth. Distributions of time intervals between bubble arrivals in the sampling volume and bubble speed estimations obtained from bubble time of residence in this volume further suggest bubble clusters in the upper zone that fade out in the lower zone. In addition, bubble concentration is found to increase very rapidly at a given depth with wind speed and more rapidly for large bubbles than for small ones. Bubble cluster, speed, and size distribution variations with depth furnish evidence for the identification of the upper zone with a generation zone and the lower with a dispersion zone.