An Experimental Study of Droplet Generation by Plunging Breaking Water Waves*

The production of droplets by strong and weak plunging breakers generated using dispersively focused wave packets is studied experimentally. The profile histories of the breaking wave crests are measured using a cinematic laser-induced fluorescence technique. The droplets are measured using a cinematic digital in-line holographic system positioned at 26 locations along a horizontal plane that is 1 cm above the maximum wave crest height. The droplet radii (r ≥ 50 μm), positions and velocities are determined from the reconstructed holograms. Counting only the droplets that are moving up across the measurement plane, the spatio-temporal distribution of droplet generation by the breaking waves is obtained. Three distinct regions of droplet production are found, first when the wave’s plunging jet impacts the free surface, second when the back face of the turbulent splash created by the jet impact collides with the top surface of the plunging jet, and finally, later in the breaking process, when bubbles rise to the free surface and pop. The probability distribution of radii for the droplets in different phases of the breaking process are presented and discussed.