An Experimental Study of Droplet Generation by Plunging Breaking Water Waves

The production of droplets generated by strong, moderate, and weak plunging breakers is studied experimentally in a laboratory wave tank that is 14.8 m long, 1.15 m wide, and 2.2 m tall, with a water depth of 0.91 m. The water waves are generated mechanically using a dispersively focused wave packet technique with an average wave packet frequency of 1.15 Hz for all three waves. Surface profile histories of the breaking wave crests are measured using a high-speed cinematic laser-induced fluorescence technique at a frame rate of 650 Hz. The temporal evolution of the phase averaged surface profiles, obtained from 10 runs for each wave, are used to characterize the breakers. Droplets are measured using a cinematic high-speed digital in-line holographic system positioned at 28 streamwise locations along a horizontal plane (herein called the measurement plane) that is positioned 1 cm above the maximum wave crest height. The droplet measurements are taken at a frame rate of 650 Hz and are synchronized with the surface profile measurement images. The streamwise droplet measurement locations cover a region from just upstream to one meter downstream of the jet impact point. At each location, 10 experimental runs were performed for a total of 140 unique experimental runs for each wave. Droplet diameters (for d ≥ 100 µm), positions, and trajectories are determined from the holograms. Counting only the droplets that are moving up across the measurement plane, the spatio-temporal distribution of droplet generation by the breakers is obtained. Droplet statistics including total number, mean diameter, and speed are presented and discussed in context to the different phases of breaking. The relative importance of the various droplet generation mechanisms in the three waves are discussed and correlated with the mean wave profile characteristics.