Abstract:
Continuous plumes of bubbles were entrained by a jet of water impinging on a horizontal surface in both fresh and salt water environments. Several hours of acoustic time series were digitally recorded in both cases. Many of the prominant features of the sound generated in salt water were explained by modeling the plume as a cylindrical resonant cavity at the surface of a shallow-water waveguide. However, for salt water, the extreme low-frequency limit, the dominant mechanism for creating sound, was determined to be hydrodynamic turbulence. In addition to these two mechanisms of noise generation, in the fresh water case there were also large emissions resulting from the growth and break up of bubbly structures around the edges of the main plume---a mechanism that was absent in the salt water environment. Individual bubble oscillations could also occasionally be heard in fresh water at frequencies below 10 kHz. Details of these findings and the theoretical models will be presented, highlighting the important differences between noise emissions from bubble plumes in both fresh and salt water.