A complete model describing broadband sea surface scattering at high wind speeds has not been developed. One difficulty is accounting for scattering from near surface bubble clouds. This problem has been addressed in the literature for low frequencies. To first order, an acoustically compact bubble cloud can be modeled as a compressible sphere, where the scattering strength depends only on spherical cloud volume and mean void fraction, not the bubble size distribution or cloud shape. This hypothesis has been experimentally tested using freely rising artificial bubble clouds [J. Acoust. Soc. Am. 92, 2993--2996 (1992)]. The measured low frequency monopole target strength of the cloud agreed with theory but higher frequency results did not. To further understand scattering from these objects, laboratory scattering experiments are underway using geometrically well-characterized bubbly fluid targets. Initial measurements of scattering from a bubbly-fluid-filled latex tube are presented and compared to an effective medium theory. These initial results lack independent void fraction determination but good qualitative agreement is found, even above the monopole resonance frequency. A new method used in these experiments to generate large volumes of nearly monodisperse bubbly fluid samples will also be described. [Work supported by ONR.]
Table of ContentsAcoustic scattering from an elastic tube filled with bubbly fluid. Motivation for Work Overview of Theoretical Approach Theoretical Scattering Model Mass Production of Microbubbles Experimental Setup and Laser Alignment System Schematic of Experiment (side view) Experimental Results: Typical Pressure Signature ñ 12 kHz Preliminary Model: CW Plane Wave, Infinite Fluid Cylinder, No Shell A More Realistic Model: Account for Elastic Shell A More Realistic Model: Account for Elastic Shell Even Better: Account for Finite Pulse Length Convolve Incident Pulse with Scattering Response Accounting for Shell and Finite Pulse Length Reduces High Void Fraction Error Overview of Experimental Results Effect of Void Fraction on Scattering Strength Summary / Future Work |
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