Nonlinear scattering of crossed ultrasonic beams in the presence of turbulence: Multiple scattering effects

Abstract

The nonlinear scattering of two finite‐amplitude mutually perpendicular crossed ultrasonic beams—interacting in the presence of turbulence—generates a scattered sum frequency component that radiates outside the interaction region. In the absence of turbulence, virtually no scattering, at the sum frequency, is observed [M. S. Korman and R. T. Beyer, J. Acoust. Soc. Am. 84, 339–349 (1988); 85, 611–620 (1989)]. Here, two primary cw (f1=2.05 MHz and f2=1.95 MHz) beams are generated by 2.54‐cm‐diam circular plane array piston transducer units (T1 and T2). A 4‐MHz receiving unit (R) is of similar construction. All beam axes form a common plane and overlap region with the axis of a submerged circular water jet, which generates the turbulence. With R fixed, T1 and T2 rotate on radius arms—always keeping the beams perpendicular. Symmetry suggests an angle θ*, where θ*=0° defines forward scattering. This geometry allows a nonlinear forward scattering intensity component to exist without concern for a coherent component. Here, multiple scattering effects are needed to predict their results. Supplemental scattering experiments will be presented in an attempt to identify a transition from single (Born approximation) to multiple scattering. [Work supported by Naval Academy Research Council.]