Murray S. Korman
Dept. of Phys., U.S. Naval Acad., Annapolis, MD 21402
The interaction of two, mutually perpendicular crossed continuous wave ultrasonic beams, overlapping (at a common focal point) and interacting in the presence of turbulence in water is demonstrated by the detection of the far-field radiated sum frequency component. This scattered sum frequency pressure p[sub +] exhibits a time averaged Doppler shift and frequency broadening due to scattering from turbulent eddies (in the overlap region) that are not stationary in time. The angular dependence on the frequency spectrum of the scattered sound makes it possible to measure the mean flow velocity and the so-called higher order turbulent velocity correlations. The audience will be able to hear the frequency demodulation of the sum frequency signal by using an FM radio receiver. The radio receiver is coupled to the acoustic receiving transducer and is tuned to detect wideband FM centered on f[sub +]=f[sub 1]+f[sub 2]. Here, f[sub 1]=2.1 MHz and f[sub 2]=1.9 MHz are the respective primary frequencies. The demodulation signal will change as the scattering angle is changed or the overlap region is moved to a new point in the turbulent flow (which is generated by a submerged jet). [Work supported in part by the Naval Academy Research Council.]