Void fraction measurements beneath plunging and spilling breaking waves

The temporal and spatial variations of the void fraction fields beneath deepwater breaking waves were investigated in the laboratory. There were a total of 13 measurement positions along the plunging wave; the peak void fractions measured varied from 0.024 to 0.96 and the time‐averaged void fractions varied from 0.012 to 0.37. For the spilling wave, there were four measurement positions, and the mean void fractions varied from 0.17 to 0.29. It was found that an energetic spilling breaker may entrain approximately the same volume of air as a steeper, larger‐amplitude plunging breaker. The speed of advance of the air cavity entrained beneath the plunging wave was estimated to be 70% of the phase speed of the breaking wave. The speed of the third bubble cloud entrained by the splash‐up was found to be approximately 90% of the phase speed. Beneath the spilling breaker, the speed of advance of the bubble cloud was estimated to be 100% of the phase speed. These measurements have lead to the identification of four significant events during the breaking of a plunging wave: the plunging water jet impacting the forward face of the wave, the air cavity collapse, the splash‐up impact on the forward wave face, and the location and timing of the peak void fraction in the splashing zone. Numerical models of plunging breakers should be able to accurately predict the timing and nature of these events.