Scattering of an acoustic field by a free jet shear layer

The angle and amplitude changes for acoustic waves refracted by a circular open jet shear layer were determined. The generalized refraction theory was assessed experimentally for on axis and off axis acoustic source locations as source frequency varied from 1 kHz to 10 kHz and free stream Mach number varied from 0.1 to 0.4. Angle and amplitude changes across the shear layer show good agreement with theory. Experiments confirm that the refraction theory is independent of shear layer thickness, acoustic source frequency, and source type. A generalized theory is, thus, available for correcting far field noise data acquired in open jet test facilities. The effect of discrete tone scattering by the open jet turbulent shear layer was also studied. Scattering effects were investigated over the same Mach number range as frequency varied from 5 kHz to 15 kHz. Attenuation of discrete tone amplitude and tone broadening were measured as a function of acoustic source position and radiation angle. Scattering was found to be stronger at angles close to the open jet axis than at 90 deg, and becomes stronger as the acoustic source position shifts downstream. A scattering analysis provided an estimate of the onset of discrete tone scattering.

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Refraction and Scattering of Sound by a Shear Layer

Spectral Broadening Effects in Open Wind Tunnels in Relation to Noise Assessment

A free-jet anechoic facility is used for quantitative determination of the effect of motion on the pure jet mixing noise from subsonic jets. The farfield measurements obtained outside the free jet are subjected to amplitude and angle corrections due to free-jet shear layer refraction; in addition, corrections are made to account for the distributed nature of the jet noise source as a function of frequency. The corrected results, which provide the changes in the jet mixing noise as a result of simulated jet motion, are presented for a range of jet velocities with a fixed free-jet velocity. Comparisons are made between the findings obtained and those related to other simulation and flight measurements. The results indicate that the effect of motion is to reduce jet mixing noise at all angles of measurement, and the reduction is broadband with the largest magnitude occurring around the spectral peak.

Experimental Study of Sound Radiation from a Subsonic Jet in Simulated Motion

Shear layer effects on pure tone sound propagation in open jet wind tunnels using 1/3 octave analysis

An experiment has been conducted in NASA Langley's free-jet anechoic facility to assess the effect of a free-jet shear layer on sound radiation from an acoustic source placed inside the jet. The nozzle diameter was 1.22 m and operated at an exit velocity of 35 m/s. Two different types of sources were used; one was a compact source with a nearly uniform radiation pattern, the other was a noncompact source with highly directional radiation pattern. The acoustic source was driven by pure tone and 1/3-octave band white noise. Acoustic measurements were made both inside the jet flow in the potential core region and outside the jet in the acoustic farfield. The effects of mean flow convection and mean shear refraction on sound radiation were determined. The experimental results compared favorably with analytical predictions. The effect of free-jet turbulence on sound scattering was found to be insignificant.

J. C. Yu

Papers

An experimental investigation of acoustic radiation from a source inside a large turbulent free jet