Abstract
As a jet of water penetrates a free water surface, air is entrained in the form of bubbles which produce sound by various mechanisms, including both individual and collective oscillations. The low-frequency acoustic emissions of the bubble clouds entrained and dispersed by vertical continuous and transient fresh water jets impinging from varying heights into a fresh water receiving pool in a large laboratory tank were studied both theoretically and experimentally for jet velocities ranging to about 10 m/s, and jet diameters of millimeter order. The acoustic signals were measured with hydrophones in different positions, amplified, and digitally sampled. The measured power spectra revealed well-defined, nonuniformly spaced resonance peaks at frequencies below about 1 kHz. A theory based on a conical bubble plume, with a nonuniform sound speed profile, acting as a resonant cavity predicts the frequencies of the resonance peaks and shows remarkable agreement with the experimentally measured values. Further, time-frequency analysis gives bubble size distributions and shows the driving source for the resonant cavity to be large secondary bubbles formed through coalescence, which become acoustically active through deformation or break-up as they are re-entrained by the downstream flow in a failed attempt to rise to the surface.
Published Version
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