Correlation analysis of flow and sound in non-isothermal subsonic jets based on large eddy simulations

Abstract

The sound radiation mechanism is investigated in non-isothermal subsonic jets at acoustic Mach number 0.9 and at temperature ratios of 0.86, 1.0, and 2.7, using causality methods on the results predicted by large eddy simulations. Turbulence signals in the jet flows are rearranged according to the acoustic analogies, such as the streamwise and radial shear signals, [Formula: see text] and [Formula: see text], the self-signal, [Formula: see text], the entropy fluctuation, [Formula: see text], and the vortex norm, [Formula: see text]. The ray-tracing method is applied to locate the acoustically correlated regions, and the two-point correlation is employed to analyze the acoustic correlation. The results show that the cancelation between the [Formula: see text] and [Formula: see text], and between the [Formula: see text] and [Formula: see text], vary with temperature ratio, and, thus, may affect the radiation directionality. The density fluctuation, [Formula: see text], suppresses the acoustic correlations of the shear- and self-signals in the cold jet, respectively, but strengthens them in the hot jet; thus, the [Formula: see text] enhances the cancelation both in cold and in hot jets. The intense negative fluctuation of the [Formula: see text] is found to be associated with the cancelation mechanism. It induces the peaks of [Formula: see text] and [Formula: see text] by disturbing the average fields. The jet temperature changes the averaged density and entropy so as to change the cancelation mechanism. Two large-scale acoustically correlated parts of the coherent structures are visualized by the zonal correlation of the [Formula: see text]. Their acoustic correlations cancel each other in the hot jets, but enhance each other in the cold jets. The acoustically correlated regions represented by the zonal correlations of the [Formula: see text] and [Formula: see text] are in small scale and gather around the end of the potential core region.