Study of an air bubble curtain along a wall in water and radiated noise mitigation

This work experimentally investigates the characteristics of air bubble injection into freestream flow using three different injector models. The effects of the bubble layer on the sound wave attention are studied. A wide range of air injection rates from 2-50 SLPM is injected into the freestream water at Froude numbers of 50.5, 70.7, and 90.9. The injector model is placed on the side wall, which is where the bubble layer is generated. High-speed cameras and an image processing technique are used to visualize and quantify the volume fraction of air bubbles. The sound measurement system consists of two hydrophones. The first hydrophone projects sound waves at discrete frequencies ranging from 17-50 kHz, and the second receives the transmitted sound waves. The bubble volume fraction is observed to increase with the air injection rate and the freestream velocity increases. There is no significant increment in thickness of the bubbly layer as the injection rate or freestream velocity increases. The different injector models produce various air bubble flow patterns, Model II provides the highest volume fraction. The bubble layer greatly suppresses sound waves at all frequency ranges. However, the sound attenuation rate varies along the frequency range. The insertion loss increases depending on the volume fraction of the bubbles, freestream velocity, and frequency range. At a high freestream velocity of FR = 90.9, the increased air injection rate does not affect the insertion loss. Model II has a relatively higher insertion loss rate at frequencies >26 kHz.