Modelling underwater noise mitigation of a bubble curtain using a coupled-oscillator model

Abstract

Bubbles can inhibit underwater sound transmission, owing to density and acoustic-impedance mismatches and associated reflection and absorption of sound waves, and also scattering due to bubble-acoustic resonances. Thus, bubble curtains have been used to reduce underwater noise propagation. For the present study, a discrete bubble model (DBM) utilising self-consistent coupled-oscillator theory was developed to model noise mitigation by bubble curtains. The DBM is inherently able to handle polydisperse-sized and anisotropically-distributed bubbles. The DBM was shown to accurately predict the bubble collective resonance frequencies of 1D line, 2D planar and 3D complex bubble cloud configurations. Subsequently, the model demonstrated that the collective resonance frequency decreases as the inter-bubble spacing was reduced in 2D planar bubble screens and 3D cylindrical bubble curtains. Modal analysis indicated that the fundamental mode in both configurations dominated the vibration of bubble clouds when subjected to an external acoustic source. The broadband acoustic pulse levels were able to be reduced by approximately 17 dB using realistic cylindrical bubble curtain configurations with a diameter of 5 m and two different gas volume fraction values of 1% and 2%. Variations in the sizes of bubbles within a curtain were shown to have a significant impact on curtain performance, whereas variations in inter-bubble spacing had no significant impact, implying that bubble-curtain operators should focus on controlling the size of the bubbles.