Flow field and near and far sound field of a subsonic jet

Space-Time Correlation Measurements in Nearfields of Jets

A large body of careful experimental measurements of two-point correlations of far field jet noise was carried out The model of jet-noise generation is an approximate version of an earlier work of Ribner, based on the foundations of Lighthill The model incorporates isotropic turbulence superimposed on a specified mean shear flow, with assumed space-time velocity correlations, but with source convection neglected The particular vehicle is the Proudman format, and the previous work (mean-square pressure) is extended to display the two-point space-time correlations of pressure The shape of polar plots of correlation is found to derive from two main factors: (1) the noncompactness of the source region, which allows differences in travel times to the two microphones - the dominant effect; (2) the directivities of the constituent quadrupoles - a weak effect The noncompactness effect causes the directional lobes in a polar plot to have pointed tips (cusps) and to be especially narrow in the plane of the jet axis In these respects, and in the quantitative shapes of the normalized correlation curves, results of the theory show generally good agreement with Maestrello's experimental measurements

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Theory of two-point correlations of jet noise

Two point correlations of jet noise

Continuation of Near-Acoustic Fields of Jets to the Far Field. Part I: Theory

By measurement and analysis, the relationship between the distribution of the outflow of acoustic energy over the jet boundary and the far-field intensity is considered. The physical quantity used is the gradient of the pressure evaluated on a geometrical plane at the smallest possible radial distance from the jet axis, but outside the vortical region, in the area where the homogeneous wave equation is reasonably well satisfied. The numerical and experimental procedures involved have been checked out by using a known source. Results indicate that the acoustic power output per unit length of the jet, in the region from which the sound emanates, peaks at approximately 9 diameters downstream. The acoustic emission for a jet Strouhal number of about 0.3 exceeds the emission for all other Strouhal numbers nearly everywhere along the measurement plane. However, the far-field peak intensity distribution obtained from the contribution of each station was found to depend on the spatial extent of the region where sound emanates from the jet, which, in turn, depends more on the far-field angle than on the Strouhal number.

L. Maestrello

Papers

On the relationship between acoustic energy density flux near the jet and far field acoustic intensity