Monopole acoustic radiation by a bubble encountering a turbulent flow field

Abstract

The acoustical characteristics of an ‘‘adult’’ bubble encountering a turbulent flow are studied both exprimentally and theoretically. By injecting an ‘‘adult’’ bubble in a flow, generated by a submerged axisymmetric water jet, the acoustic reexcitation of the bubble with and without breakup may occur in the shear-induced flow region. Bubbles of various sizes were introduced into jets of various speeds by means of interchangeable hypodermic needles. Results of the role of the turbulent flow characteristics in determining the acoustic bubble response are discussed. The characterizations of both acoustical and the dynamics of bubbles encountering the turbulent flow field depend upon the estimated integral and microlength scales, the corresponding Reynolds numbers, and the cirtical Weber numbers for both bubble distortion and breakup. These parameters are examined both in fresh and salt water. A simple model is given to account for bubble acoustic radiation. The Rayleigh–Plesset equation was altered by incorporating the turbulent pressure fluctuation of the flow as a driving force of the bubble. Some conclusions are given about the nature of the turbulent flow field and conditions under which the bubble acoustic reexcitation may occur. [Work supported by ONR.]