Experimental Study of the Scattering of Sound by Flowing Turbulence

Abstract

The method used in experimental study of the scattering of sound by sound reported earlier [J. Acoust. Soc. Am. 27, 1002(F3) (1955) and National Advisory Committee for Aeronautics Contract NAw-6405 report, March 23, 1955] is applied in a corresponding investigation of scattering of sound by a turbulent flow. The turbulent region is a “sheet” jet emanating from a diffuser. A monochromatic ultrasonic beam is passed through the sheet, and scattered energy having a continuous frequency spectrum is measured on the back side of the sheet. The angular distribution for various frequencies is measured. It is found that the angular location of the maxima of scattering can be predicted from a simple Doppler shift calculation assuming that the turbulent velocity field is not changing with time. In fact, the angle of a scattering maximum with frequency f0 (1 + Δ) is obtained from sin θ = [Δ/M (1 + Δ)], where M is the Mach number of the flow. The angle of scattering θ is positive for downstream angles, negative for upstream angles. Our work is essentially directed toward the experimental verification of these simple theoretical predictions. The relation above has been checked for various values of Δ and M with fairly good agreement between predicted values and experimental results. [Supported by Contract NAw-6405 with National Advisory Committee for Aeronautics.]