Summary Laboratory models of geophysical magnetic field production require new experi‐mental characterization methods. Self‐generating liquid metal magnetic dynamos are explored using two new experiments. Kinematic dynamo studies lead us to charac‐terize the magnetic field dynamics in terms of eigenvalues and eigenfrequencies of the induction equation. Observing the decay of magnetic field pulses indicates the real part of the leading eigenvalue of the induction equation, while a chirp magnetic field diagnoses the imaginary part of the eigenvalue. Finally, a single‐frequency applied magnetic field characterizes the structure of the velocity field. These measurements provide a new means to characterize and measure the approach to self‐generation. We present data from numerical simulations and laboratory experiments using these techniques.

An experiment was designed to investigate the characteristics of sound produced by the turbulent excitation of bubbles. By injecting a bubble into a fully developed turbulent flow, generated by a submerged axisymmetric horizontal water jet, the excitation of bubbles and shear-induced fission/fusion may occur. In the case of shear-induced bubble distortion by a turbulent jet, nonlinear coupling between volume pulsation and surface oscillations occur at exactly the same frequency as the radial breathing mode of the bubble. Particle tracking velocimetry (PTV) is used to characterize the turbulent flow field. Two high-speed Kodak Ekta-Pro video cameras are used to capture, simultaneously, several 180-×180-×10-mm illuminated regions along the jet. An 8-W argon ion laser is utilized for illumination. The method provides not only a visualization of the various patterns and structures of the turbulent flow, but also yields quantitative instantaneous velocity data of the flow containing the bubble. The role of the turbulent flow characteristics in determining the acoustical bubble response is discussed. [Work supported by ONR.]

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.

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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.]