Investigation of noise source mechanisms in subsonic jets

The precise nature of the origin of jet noise remains in question to this day. Apart from the scientific importance of gaining a better understanding of the noise sources, there is the practical consideration of our ability to alter/ manipulate the sources so as to achieve noise reduction. In this study, the sources ofjet noise have been investigated with 1) the analyses of far-field spectra, and 2) the measurements of the far-field correlations, both in the polar and azimuthal directions. Two geometries were used: a round nozzle and a beveled nozzle. The acoustic characteristics of these nozzles exhibit interesting trends at different jet velocities (V-j/a), thereby permitting a better understanding of the nature of the noise sources. The far-field spectra from unheated round jets at extremely low Mach numbers conform to the large-scale similarity shape at angles close to the jet axis; there is a gradual progression from the finescale similarity shape to the large-scale similarity shape for all jet velocities. This seems to be a fundamental characteristic ofjet noise and is a new finding. The far-field correlations indicate that there is very little correlation at the lower polar angles; in the peak aft radiation sector, high correlations prevail. It is established with both the round and beveled nozzles that low correlations are associated with fine-scale-similarity-shaped spectra and high correlations with large-scale-similarity-shaped spectra. Experimental evidence for the importance of the noise from large coherent structures even at low velocities is offered. The data indicate the existence of two independent sources in both high-speed and low-speed jets, with the coherent large structures radiating to the aft angles and the random turbulence radiating to the lower angles. It is shown that the noise reduction of the beveled nozzle is due to the alteration of the noise from the coherent