Void-fraction measurements in bubble plumes generated by 2-D and 3-D breaking waves.

Abstract

Recent experiments have confirmed that low-frequency sound (10 to 300 Hz) is generated under breaking waves. It has been suggested that volume oscillations of the bubble plume generated by breaking is 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 of the evolution of the void-fraction field in bubble plumes generated by 2-D and large scale 3-D laboratory breaking waves are presented and compared. Various moments of the bubble plume void-fraction field give information on the (a) volume of air entrained, (b) cross-sectional area of the plume, (c) mean void fraction and sound speed, (d) bubble plume resonant frequencies, (e) potential energy of the bubble plume, (f) horizontal and vertical displacement and velocity of the bubble plume. These measurements along with independent measurements of the sound radiated (Loewen and Melville, this meeting) support the hypothesis that low-frequency sound is generated by the volume oscillation of the bubble plume. The radial dependence of the bubble plume’s void-fraction (and sound speed) field and its effect on low-frequency sound generation is investigated and the influence of the void-fraction instrumentation detection threshold in estimating the above moments is also examined. [Work supported by ONR (Ocean Acoustics) and NSF (Physical Oceanography).]