Spray Generation by Naturally and Mechanically Forced Wind-Waves*

The production of spray droplets by wind-waves is studied in laboratory experiments at the University of Delaware's Air-Sea Interaction Laboratory. Wind-waves are produced at free-stream wind speeds of 9.7, 10.7, and 11.7 m/s with the addition of mechanically forced waves with central frequencies of 1.0, 1.2, 1.4, 1.6 and 1.8 Hz and sidebands at +/- 0.05 Hz as well as a case with wind only forcing. Drop positions and diameters (d > 60 μm) are measured for each experimental condition and over multiple wave periods using an in-line holography system positioned 6.4 cm above the mean water level and at a fetch of approximately 23 meters. Droplet statistics including total number, mean diameter, and size distributions are reported at the different experimental conditions. The relative importance of spray production mechanisms is discussed and correlated to wave-field characteristics including the frequency of breaking events and the wind-wave properties.