Experimental study of acoustical emission from bubble cloud excitation

Abstract

A theoretical and an experimental study of sound emission processes by active and passive bubble cloud excitation have been performed, aimed at demonstrating the possibility of frequency transformation of an emitted sound signal by changing the physical qualities of a gas-water mixture. Theoretical studies have shown that the lowest natural mode of vibration for the bubble cloud lies far below the resonance frequencies of the individual bubbles. The fundamental frequency of vibration of a bubble cloud is governed by two parameters: 1, the void fraction and 2, the characteristic dimensions of the bubble cloud. The resonance frequency response of the active bubble region on excitation by an acoustical signal was shown to depend strongly on a scaling parameter defined in this paper which, in turn, depends, among other things, on the value of the void fraction. Theoretical calculations have shown the possibility of sound signal transmissions from hundreds of kilohertz (which correspond to the resonance frequencies of individual bubbles) to hundreds of hertz (which correspond to the frequencies of the vibration modes of the bubble cloud) by changing the scaling parameter. Experimental verifications of the theoretical predictions have been made in a water test tank with a bubble cloud produced by electrolysis of fresh water. The void fraction was changed during the experiments and was controlled by using resonance acoustical methods to determine the physical properties of the bubbly liquid. Active excitation of the bubble cloud vibration by probe generated noise signals led to the emission of sound at low frequencies. Analysis of the experimentally obtained sound emission spectra has confirmed that the bubble clouds may be a potential source of noise in the low frequency bands in natural environments.