Experimental study on insertion loss of air bubble layer in freestream flow

Abstract

Air injection in water has several engineering advantages, such as an air lubrication system, cavitation control, and noise reduction. The present study investigates the effect of air injection rate and stream velocity on bubble layer volume fraction, thickness, and noise mitigation. The air bubble volume fraction is controlled through the injection rate of 2 to 25 SLPM at different stream velocities. The Froude number is in the range of 30.30 to 70.7. The image processing technique quantifies the air bubble projected void fraction and thickness. The air is injected through a multi-hole pipe placed on a modified NACA 0010 hydrofoil with a 100 mm chord length. The acoustic measurement system consists of a sound transmitter that projects sound waves at the frequency range of 7 to 50.5 [kHz], and on the opposite side of the air bubble layer, a hydrophone is placed to receive the transmitter sound wave. The results showed that the air bubble projected void fraction increased as the air injection and Froude number increased. Higher stream velocity promotes air bubble breaking and the formation of diverse air bubble patterns such as mono-dispersion bubbles and clustering bubbles. As the Froude number increases, the size of the bubble decreases. At various air injection rates and stream velocities, the thickness of the air bubble layer is found to be nearly constant. The bubble layer effectively reduces the noise in all frequency ranges of the present study. The sound pressure level (SPL) inhibition rate increases as the air bubble projected void increases. The insertion loss increases with stream velocity as higher flow velocity leads air bubbles to break and form a cluster of air bubbles, which is found to be more effective in reducing noise than a single bubble. Regardless of injection rate, the maximum insertion loss is 44 [dB] in a frequency of 20 [kHz] and Froude number 70.7.