ACOUSTIC CHARACTERIZATION OF AN ADULT BUBBLE INJECTED INTO A FULLY-DEVELOPED TURBULENT-FLOW FIELD

The acoustical characteristics of bubbles injected into a fully develo ped turbulent flow field are studied. By injecting an ''adult'' bubble into a flow, generated by a submerged axisymmetric horizontal water j et, the acoustic reexcitation of the bubble with and without breakup m ay occur in the shear-induced flow region. Bubbles of various sizes we re introduced into jets of various speeds by means of interchangeable hypodermic needles. Results of the role of the turbulent flow characte ristics in determining the acoustic bubble response are discussed. The characterizations of both the acoustical and the dynamics of the bubb les encountering a turbulent flow field depend upon the estimated inte gral and microlength scales, the corresponding Reynolds numbers, and t he critical Weber numbers for both bubble distortion and breakup. A cr itical Weber number was estimated to be 0.52 for an ''adult'' bubble a coustic reexcitation without breakup and 1.10 for bubble breakup. A si mple model is given to account for the bubble acoustic radiation. The Rayleigh-Plesset equation was altered by incorporating the turbulent p ressure fluctuation of the flow as a driving force of the bubble. The turbulent energy spectrum obtained experimentally was used to predict the monopole acoustic emission by the bubble. Some conclusions are giv en about the nature of the turbulent flow field and conditions under w hich the bubble acoustic reexcitation may occur. (C) 1997 Acoustical S ociety of America.