Gregory J. Orris
Naval Res. Lab., Washington, DC 20375
Bubble plumes of various void fractions and sizes were produced by varying the flow velocity of a water jet impinging normally on a water surface. The bubbles entrained at the surface were carried downwards by the fluid flow to depths ranging from roughly 1.2 to 2.0 m, and formed roughly cylindrical plumes with diameters ranging from roughly 0.5 to 1.0 m. The acoustic emissions from the plumes were recorded onto digital audio tape using a hydrophone placed outside the cloud at distances ranging from 0.5 to 16.0 m. Closeup video images of the individual bubbles within the plume were also taken in order to gain knowledge of the bubble size distributions. The experiments were performed in both freshwater and saltwater environments. Most of the acoustic energy of this system is broadband and confined to frequencies below 1200 Hz, yet bubble size distributions do not show any appreciable number of bubbles large enough to account for the strong spectral signature at these low frequencies. The bubble entrainment is itself a nonlinear process. To investigate the possibility that the collective excitation (with fundamental frequencies characteristic of the gross dimensions of the plume) is driven by the broadband nonlinear entrainment, methods of nonlinear signal processing recently developed have been applied.