Acoustic measurements of air entrainment by a plunging free water jet

Abstract

Low-frequency acoustic emissions of a free plunging water jet were studied to determine the volumetric rate of gas transfer across the pool surface. At the studied jet velocities, up to 10 m/s, air bubbles are entrained at the interface and form well-defined bubble clouds penetrating up to 20 cm into the pool. The resulting biphasic region of the submerged jet was found to be an efficient acoustic resonator. The radiated sound field was recorded with hydrophones close to the region of the submerged jet and strong resonances were found below 1 kHz. The frequencies of the lowest modes were measured for a given jet diameter as a function of the jet velocity. The observed acoustic eigenfrequencies are unique functions of the jet velocity and the gas flux, independent of the details of the gas entrainment. This functional dependence was investigated both experimentally and theoretically, allowing an inversion of the acoustic data for the air flux across the interface as a function of the jet velocity. Reference measurements of the air flux were made to calibrate the inversions. Using this technique, measurements of the air flux were made for a variety of jet diameters, lengths, and velocities. [Work supported by ONR.]