Numerical and experimental pressure fields of nozzle-generated underwater bubbles

Abstract: Since Minnaert's seminal work in 1933, researchers have tried to link the acoustic emissions generated by underwater air bubbles with their physical properties. Despite extensive research, the precise mechanism behind sound production remains elusive. Various models, including the neck-collapsing theory, have been proposed, but none fully explain the complexity of the sound pressure mechanism. To investigate this further, a new series of experimental tests and numerical simulations were conducted to examine the pressure oscillations during bubble pinch-off from underwater nozzles. Nozzles with diameters ranging from 3 to 6 mm produced bubbles of varying sizes, and their acoustic emissions were recorded using a hydrophone positioned 15 mm from the nozzle. The bubbles' physical properties were determined using Minnaert's frequency and frames captured using high-speed cameras. The experimental setup was replicated using the solver compressibleInterFoam to simulate bubble pinch-off under identical conditions. Void fraction, vertical velocity, and dynamic pressure fields were sampled at pinch-off. Numerical results were validated by comparing the simulated acoustic pressure readings with those from the hydrophone during the experimental tests. Analysis of the pressure field inside the bubble neck revealed oscillations between +-100 Pa immediately before and after pinch-off, supporting the neck-collapsing theory. Keywords: Bubble acoustics, Multiphase simulations, Compressible simulations, Underwater sound, Sound measurement