Noise-related shear-layer dynamics in annular jets

Abstract

Suppression of low-frequency noise has been observed for subsonic annular jets with extended centerbodies. The dynamics of annular jets both with and without centerbody extensions were investigated experimentally to determine the noise-reduction mechanism. For small annular heights, the dynamics of the shear layer are modified by the presence of the centerbody extension and the jet instability mode is effectively suppressed. When the centerbody is truncated at the nozzle exit plane, the flowfield quickly reverts to a form similar to a jet from a round nozzle with equivalent cross-sectional area. Configurations with large annulus heights also generate shear layers with characteristics comparable to a round jet. Detailed spectral contour maps are used to illustrate the spatial development of the velocity spectrum, and the limitations of single-point measurements are discussed. NOZZLE with an extended porous centerbody was in- troduced by Maestrello1'2 primarily as a device for reduc- ing supersonic jet noise. Detailed acoustic measurements for a parametrically varying series of nozzles were obtained by Bauer,3 and flowf ield measurements were conducted by Kibens and Wlezien4 to determine the noise-suppression mechanism. It was determined that shock-associated noise at supersonic pressure ratios is reducd by attenuation of the core flow shock system. A more detailed description of these results is given in Ref. 5. Maestrello2 and Bauer3 also observed significant sound reduction at subsonic velocities, for which the noise-reduction , mechanism clearly must be different. In both investigations far-field microphone correlations were found to have maxima at zero time delay, in contrast to a round jet for which the maxima occur over a wide range of delays. The correlations imply that the sound sources for a jet with an extended center- body are located over a compact segment of the shear layer. Maestrello2 attributed the source concentration to a reduction in the length of the potential core. Bauer3 proposed that the turbulent structure of the jet is modified by the presence of the centerbody and that noise is reduced because vortex pairing is inhibited. This paper summarizes an investigation of the noise- suppression mechanisms in subsonic annular jets. The role of large-scale turbulence in the generation of jet noise69 and the recent shift of attention to the control of turbulence as a basis for noise reduction provide motivation to link flow characteristics to the acoustic far field. The acoustic results presented in detail in Ref. 5 are summarized briefly here. In this paper the authors focus on the shear-layer dynamics of annular jets. The degree to which the shear layer is modified by the presence of a centerbody is measured, and it is deter- mined whether the modifications are consistent with the observed noise reduction. Hot-wire spectra, mean-flow distributions, and a novel spectral mapping technique are used to document the dynamic characteristics of flows having a variety of centerbody geometries.