The primary source of supersonic jet noise originates from the interaction of the turbulent flow with the ambient air. Tam et al. [AIAA Paper 96-1716 (1996)], proposed similarity spectra for a two-source model corresponding to omnidirectional fine-scale turbulence structures (FSS) and directional large-scale turbulent structures (LSS). These empirical similarity spectra agree reasonably with angular variation in mid and far-field spectra of both military and laboratory-scale jets. Near-field measurements of an ideally expanded, Mach 1.8 laboratory-scale jet from the Hypersonic and High-Enthalpy Wind Tunnel at Kashiwa Campus of the University of Tokyo were analyzed. Similarity spectra decompositions adequately describe the turbulent mixing noise as close as 10 jet diameters. Neglecting the effect of the hydrodynamic field, the LSS spectrum provides consistent fits at 15°-40° from the jet axis. A combination of LSS and FSS spectra match the measured spectra at 45°-55°. FSS spectrum matches the spectra at angles greater than 60°, except very close to the nozzle exit plane where there is an overprediction at high frequencies. Comparison of near and mid-field locations may provide insights into propagation radials.

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