W. M. Carey
Defense Advanced Res. Projects Agency, 3701 N. Fairfax Dr., Arlington, VA 22203-1714
Dynamic bubble distributions in liquids and their acoustic properties are important in a large variety of applications ranging from the measurement of nitrogen bubbles in blood, fish schools, to clouds from breaking waves. Unlike individual bubbles, these distributions maintain their compactness by viscous and hydrodynamic vorticity. The specific case of compact distributions that have complex shapes may be treated by the use of multipole expansions. This paper discusses the application of multipole expansions to complex shapes in general and compares the first-order radiation and scattering fields from simple geometrical shapes, spheres, cylinders, and ellipsoids. The paper shows that when the regions are acoustically compact the lowest-order radiation from these features can be described by a modified Minneart formula describing the net volume fluctuation. In the case of noncompact distributions scattering regimes are shown to be delineated by several nondimensional numbers.