Direct numerical simulation of a temporally-developing subsonic round jet and its sound field

A temporally-developing isothermal round jet at a Mach number of 0.9 and a diameterbased Reynolds number of 3125 is computed by direct numerical simulations in order to investigate its turbulent development and its generated noise. The simulations are performed using high-order finite differences on a grid of 940 million points extending up to 120 jet radii in the axial direction. Snapshots and statistical properties of the jet flow and acoustic fields are shown. The latter are calculated from five runs using different initial random perturbations in the jet shear layers. They include mean, rms, skewness and kurtosis values and auto-correlations of flow velocity and near-field pressure, as well as flow-noise cross-covariances. It is found that, when the jet potential core closes, mixinglayer turbulent structures intermittently intrude, accelerate and merge on the jet axis. Simultaneously, strong low-frequency acoustic waves, significantly correlated with the centerline flow fluctuations, are emitted in the downstream direction. The present results for a temporally-developing jet are very similar to those obtained at the end of the potential core in spatially-developing jets. This suggests the presence in both cases of the same sound source on the jet axis due to the potential-core closing, radiating mainly in the downstream direction.