Measurements of air/water interfaces in plunging breaking waves

Of the energy dissipation that takes place in violent breaking waves, some is accounted for by work done against the buoyancy of bubbles, and some by work done in generating splashes that may rise to elevations far above the crest. In deep water the probability of encountering air in such conditions varies from almost zero at large submergences to almost unity at high elevations. Between these extremes in regular long-crested waves, there is a two dimensional continuum of time-dependent ensemble-averaged void fractions. A detailed knowledge of this distribution would represent a major contribution to a better understanding of the process of wave breaking, but largely for practical reasons, little of this information exists. Previous studies of air entrainment beneath breaking waves have made use of a variety of techniques including local conductivity probes (Cox & Shin, 2003; Hoque 2002), global conductivity probes (Lamarre & Melville, 1994), acoustic techniques and laser methods (Hwung & Jih, 1993). Many of these have shortcomings such as large measurement volumes, limited sensitivity at at least one end of the range of void fractions, or probes that are significantly intrusive. This paper describes detailed measurements made with an instrument that detects individual air/water interfaces over extremely small areas at high frequency. The results allowed us to compute time-dependent void fractions, not only in that part predominantly occupied by water, but in the region above, including (probably for the first time) the splash-up created by the plunging jet