Void-fraction measurements and sound-speed fields in bubble plumes generated by breaking waves

Abstract

Recent field and laboratory experiments have confirmed that low-frequency sound (10 to 300 Hz) is generated under breaking waves. It has been proposed that collective oscillations of the bubble plume generated by breaking may be the mechanism responsible for the generation of this sound. Confirmation of this process requires independent measurement of the void fraction, and therefore sound speed, in the bubbly mixture. Detailed measurements are presented of the evolution of the void-fraction field in bubble plumes generated by large-scale three-dimensional (3-D) laboratory breaking waves. Various moments of the void-fraction field are calculated and compared with results from two-dimensional (2-D) laboratory breaking waves [Lamarre and Melville, Nature 351, 469–472 (1991)]. The kinematics of the bubble plume reveals that the initial horizontal velocity of the plume is approximately 0.7C, where C is the wave phase speed. The centroid of the bubble plume is found to deepen at a speed of approximately 0.2H/T, where H and T are the wave height at breaking and the wave period, respectively. The radial dependence of the void-fraction and sound-speed field is characterized in terms of simple functions of time. Finally, the void-fraction measurements described here, along with independent measurements of the pressure fluctuations under breaking waves [Loewen and Melville, J. Acoust. Soc. Am. 95, 1329–1343 (1994)], support the hypothesis that low-frequency sound is generated by the collective oscillations of the bubble plume.