Correlations of Jet Noise Azimuthal Components and Their Role in Source Identification

Abstract

In this paper, we study the subsonic cold jet noise using the azimuthal decomposition technique (ADT). The results of measurement of correlations for jet noise azimuthal components are reported. It is shown that the correlations for tone-excited jet strongly differ from those for unexcited jet; for example, an unexpectedly high value of correlation for tone-excited jet noise azimuthal components has been obtained for observation angles close to 90o to the jet axis. This behavior of the correlation renders it quite promising for the jet noise mechanism identification and source localization. An analytical model based on qudrupole source distribution is proposed for description of the correlations for unexited jet noise azimuthal components and its applicability is validated experimentally. I. Introduction t present the process of noise generation by turbulent subsonic cold jets is thought to be related with different mechanisms: fine-scale turbulence (Refs. 1 and 2), eigen-oscillations of large-scale vortex structures (Ref. 3), instability waves (Refs. 4 and 5) etc. Identification of these mechanisms in the process of jet noise generation and assessment of their contribution to the total noise is an important task. To achieve this on the basis of the measurements of only far-field total noise directivity is difficult, however. The azimuthal decomposition of the far field noise allows us to obtain more detailed characteristics of the sound sources; the previous studies (Refs. 3 and 6) on modeling the azimuthal components show that the observed experimental data for cold subsonic jet noise can be explained if both large-scale vortex structures (vortex rings) and fine-scale turbulence (modeled as moving point quadrupoles) are accounted for as sound sources, the vortex rings’ contribution to the total noise being about 40%. An excellent collapse of the modeling and experimental curves has been observed, which evinces that this is a plausible framework to model turbulent cold subsonic jet noise. To further validate this model and get new insights into the structure of the sound sources, the analysis has been expanded to include correlation characteristics of the azimuthal components. In Ref. 7 the first results of correlation measurements of azimuthal harmonics for unexcited cold subsonic jets (i.e. the correlation between the simultaneous measurements for the same azimuthal mode at two different points) have been reported that experimentally verify the absence of correlation between different modes, which is what should be expected from the theoretical considerations (azimuthal components are orthogonal). A The cross-correlation curve for tone excited jet has characteristic peculiarities, albeit it is generally similar in shape to the unexcited jet curve. These curves demonstrate that the large-scale structure sound radiation is concentrated in the region at the right angle to the jet, because in this region the cross-correlation curve for the tone excited jet is significantly higher than the curve for the unexcited jet. This property of the curve is somewhat unexpected from the general view that large-scale structures in jet radiate in the downstream direction, whereas at the right angle to the jet direction it is so-called fine scale turbulence that radiates. In Ref. 7 the spatial correlations of the azimuthal components have been obtained, but no attempt has been made to propose a theoretical model to explain the observed spatial correlation curves. Such an attempt is made in the present work. The sound sources are modeled as a distribution of moving point quadrupoles. The results of modeling are compared with the results of measurements of cross-correlation function for the far sound field of unexcited cold jet with velocity 120 m/s. The comparison is performed for the zeroth harmonics a0 in two frequency bands 600. II. Experimental Setup