Azimuthal Decomposition of Turbulent Fluctuations in a Round Jet near the Nozzle Exit and Far Downstream

As interest grows in innovating commercial supersonic aircraft, accurate modeling of jet noise from engine exhaust is crucial for manufacturers and regulatory bodies. Existing prediction tools for jets from conical nozzles typically use simplified noise source models based on statistical averaging and symmetry assumptions of turbulence fluctuations. However, these assumptions may not adequately account for the aeroacoustic effects of instability waves in the shear layer near the nozzle or the intermittent bursts of fluctuations downstream of the potential core. This could explain why noise prediction models are less accurate for upstream noise emission and spectra outside the mid-St range (St < 0.2 or St > 1).

Using LES data of round turbulent jets at Mj = 0.5, 0.8, and 0.9, this work aims to shed light on the spatiotemporal fluctuations near the nozzle exit (0-2 nozzle diameters, De) and downstream of the jet potential core (8-20 De). This work examines the flow’s prime variables, followed by the Reynolds stress tensor decomposed into different azimuthal modes. Preliminary results verify statistical homogeneity in the azimuthal plane and self-similarity in the fully mixed region of the jet. The analysis will then highlight flow structures near the initial shear layer and downstream of the jet potential core, focusing on high azimuthal modes in the early shear layers for upstream noise emission and the characteristics of extreme intermittent fluctuations near the end of the jet potential core.