Analytical predictions and measurements of the noise radiated from supersonic coaxial jets

The noise radiated from perfectly expanded coaxial jets was measured in an anechoic chamber for operating conditions with the same total mass flow and thrust and with the same temperature ratio. The shape of the measured noise spectrum at different angles to the jet axis was found to agree with spectral shapes for single, axisymmetric jets. Based on these spectra, the sound was characterized as being generated by large-scale turbulent structures or fine-scale turbulence. Modeling the large-scale structures as instability waves, a stability analysis was conducted for the coaxial jets to identify the growing and decaying instability waves in each shear layer and to predict their noise radiation pattern outside the jet. When compared to measured directivity, the analysis identified the region downstream of the outer potential core, where the two shear layers were merging, as the source of the peak radiated noise where instability waves, with their origin in the inner shear layer, reach their maximum amplitude. The changes in the measured noise directivity that occurred when the operating conditions were changed, holding mass flow and thrust constant, followed the trend predicted by the instability wave analysis.