Charles M. Loeffler
Appl. Res. Labs., Univ. of Texas, P.O. Box 8029, Austin, TX 78713-8029
Fluid-filled, thin spherical shells have been studied for use as passive sonar targets with applications in underwater navigation and calibration [see, e.g., D. L. Folds et al., J. Acoust. Soc. Am. 73, 1147--1151 (1983)]. The present research investigates the target strength of fluid-filled spheres as a function of shell and fluid parameters to aid in choosing alternative filling fluids. (Partial motivation is from the recent discontinuation of several chlorofluorocarbons which are commonly used.) Calculations based on 3-D elasticity theory describe the target strength dependence on the internal fluid density and sound speed. Here both time and frequency calculations of the form function and time domain scattering signatures describe favorable fluid characteristics. These features relate to the thickness and density of the confining spherical shell. It is shown that internal-fluid density requirements vary considerably for thin aluminum and stainless-steel shells. Several different filling fluids are tested in spherical shells with consideration given to effective target strength, safety considerations, and cost.