Near-Field and Acoustic Far-Field Response of a High-Speed Jet to Excitation

The near-field and acoustic far-field response of an unheated Mach 0.9 jet with Reynolds number 6.2x105 was investigated. The study included both the baseline and controlled jets utilizing plasma actuators. Simultaneous acquisition of the near- and far-field signals with the actuation phase enabled the use of phase averaging to reconstruct the signature of the large-scale coherent structures generated by excitation of instabilities in the shear layer of the jet. Decomposition of the near-field pressure into its constitutive hydrodynamic and acoustic fields is accomplished via the application of a filter in the frequency/wave-number domain. The results showed that both the hydrodynamic and acoustic response to excitation for Strouhal numbers StDF less than 0.50 could be well predicted by a simple linear superposition of the impulse response of the jet. The results appear to indicate that the dominant acoustic radiation reaching the far-field aft angles is being generated over an extended region of the jet mixing layer: the upstream portion of the jet, just before the end of the potential core, with highly temporally intermittent bursts occurring downstream of the potential core. Significantly lower temporal coherency of the acoustic response was found when compared to the hydrodynamic response. Finally, excitation for StDF>0.15, which produces coherent interactions between the generated large-scale structures, was found to increase the temporal coherency of the acoustic response