Shadow Zone Boundary Limitation of the Effective Acoustical Turbulence Scattering Volume Using the Turbule Ensemble Model
Technical rept. 1994-1995
ARMY RESEARCH LAB ADELPHI MD
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The Turbule Ensemble Model TEM was developed to handle acoustical scattering from anisotropic inhomogeneous turbulence. A turbule is a localized atmospheric inhomogeneity. TEM, then, represents a turbulent region by a collection of turbules of different sizes. Since acoustic sources and sensors are omnidirectional, the scattering volume of the TEM region is ill defined. Scattering properties of individual turbules in TEM show that the majority of scattering originates from a constricted volume, called the effective scattering volume. This is true for a region of homogeneous turbulence and is anticipated to be true for a region of inhomogeneous turbulence, although no calculations have been made. Estimates have been given of the size and shape of the effective scattering volume as a function of turbule size for an experiment conducted in homogeneous turbulence and a uniform atmosphere. In this report, homogeneous turbulence is retained, but an upwardly refracting atmosphere is assumed, resulting in a shadow zone. Inclusion of the shadow zone boundary further limits turbule sizes and locations from which significant signals reach the detector. The finding is that large turbules are less effective scatterers and that effective scattering volumes are large enough for the number of small turbules to be large. Implications of this finding are discussed.