On sound generated by gas-jet impingement on a bubbly gas–water interface, with application to supercavity self-noise

An analysis is made of the sound produced when a high speed turbulent gas jet impinges at normal incidence on a planar gas–water interface in the presence of a uniform, thin homogeneous bubble layer between the gas and the water. It is shown that the bubble layer exhibits a behaviour similar to a ‘quarter-wave’ resonator, storing energy supplied by gas impingement which is subsequently released as high amplitude sound into the water. In the absence of bubbles the radiation into the water has dipole characteristics, peaking strongly in the direction normal to the interface. The bubbles diffuse this sharp dipole lobe, and are predicted to increase the sound power in the water by up to 10 dB or more over a range of intermediate frequencies when the layer properties are similar to those encountered in experiments using a model scale supercavitating vehicle. At higher frequencies, in the range important for vehicle guidance and control, the bubble layer tends to reduce this source of self-noise. The same mechanism of resonant amplification should be effective more generally, when the bubbles lie within a more confined surface envelope, such as the surface ‘wake’ of the jet impact zone, whose shape defines a different set of interior eigenmodes.