Scattering from bubbles rising in a vertical line—Comparison between theory and experiments

Abstract

In this paper, the experimental data from the sound field around similar sized air bubbles rising in a vertical chain are analyzed. The data reveal a strong anisotropy in the acoustic field. The transition from individual to continuum behavior in a bubble chain is not correctly described with classical theory especially when the bubbles are uniformly sized, discretely populated, and the frequency of interest is close to the natural frequency of the individual bubbles. Single compact scatterers initiated at frequencies near their resonant frequency in isolation act preliminarily as monopole sources, amplifying the local pressure field by a factor of order 1/(k*a), a being the scattering radius and k the wave number at the resonant frequency in the surrounding material [Tolstoy, 1986]. A laboratory investigation used air bubbles in fresh water and varied the bubble sizes and separation by carefully controlling bubble production rates. A linear coupled equation method was developed to explain the result. The model reproduced the acoustic pressure anisotropy along the chain and the change in pulse waveform along the chain. The results suggest that the enhancement of sound intensity along the chain can be explained by bubbles acting as resonant scatterers retransmitting the acoustic energy.