VOID-FRACTION MEASUREMENTS AND SOUND-SPEED FIELDS IN BUBBLE PLUMES GENERATED BY BREAKING WAVES

Recent field and laboratory experiments have confirmed that low-freque ncy sound (10 to 300 Hz) is generated under breaking waves. It has bee n proposed that collective oscillations of the bubble plume generated by breaking may be the mechanism responsible for the generation of thi s sound. Confirmation of this process requires independent measurement of the void fraction, and therefore sound speed, in the bubbly mixtur e. Detailed measurements are presented of the evolution of the void-fr action field in bubble plumes generated by large-scale three-dimension al (3-D) laboratory breaking waves. Various moments of the void-fracti on field are calculated and compared with results from two-dimensional (2-D) laboratory breaking waves [Lamarre and Melville, Nature 351, 46 9-472 (1991)]. The kinematics of the bubble plume reveals that the ini tial horizontal velocity of the plume is approximately 0.7C, where C i s the wave phase speed. The centroid of the bubble plume is found to d eepen at a speed of approximately 0.2H/T, where H and Tare the wave he ight at breaking and the wave period, respectively. The radial depende nce of the void-fraction and sound-speed field is characterized in ter ms of simple functions of time. Finally, the void-fraction measurement s 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 .