Acoustic emission mechanisms of bubble detachment and pinch-off.

Abstract

The evolution of bubbles released from a submerged nozzle involves coupled hydrodynamic and acoustic effects throughout growth, detachment, and ascent. This study investigates the acoustic emission mechanisms of millimeter-scale bubbles using high-speed imaging and hydrophone measurements over a range of gas flow rates. A spherical-cap-cone correction is introduced into the classical force-balance model to characterize the non-spherical morphology of bubbles detaching from large orifices at low flow rates, improving detachment-diameter prediction. With increasing gas flow rate, the classical Minnaert frequency increasingly overestimates the dominant detachment acoustic frequency. A coupled-bubble oscillator model accounting for interactions between newly detached and rising bubbles reproduces this downward frequency shift. Under stable gas supply and nearly axisymmetric detachment, three satellite-bubble pinch-off modes are observed during bubble ascent. Statistical analysis reveals that the dominant pinch-off mode is governed by the competition between inertial-buoyancy effects and surface tension. These events generate distinct acoustic pulses associated with satellite-bubble volume oscillations and re-excitation of the parent-bubble breathing mode, driven by Laplace-pressure perturbations induced by abrupt changes in interfacial curvature. This study clarifies the unsteady dynamics and acoustic emission mechanisms of nozzle-generated bubbles, supporting acoustic monitoring of gas-liquid two-phase flows.