Acoustic characterization of an adult bubble injected into a fully developed turbulent flow field

The acoustical characteristics of bubbles injected into a fully developed turbulent flow field are studied. By injecting an “adult” bubble into a flow, generated by a submerged axisymmetric horizontal 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 the acoustical and the dynamics of the bubbles encountering a turbulent flow field depend upon the estimated integral and microlength scales, the corresponding Reynolds numbers, and the critical Weber numbers for both bubble distortion and breakup. A critical Weber number was estimated to be 0.52 for an “adult” bubble acoustic reexcitation without breakup and 1.10 for bubble breakup. A simple model is given to account for the 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. The turbulent energy spectrum obtained experimentally was used to predict the monopole acoustic emission by the bubble. Some conclusions are given about the nature of the turbulent flow field and conditions under which the bubble acoustic reexcitation may occur.