Near-field Pressure and Far-field Acoustic Response of Forced High-Speed Jets

Abstract

The near-field pressure of an unheated, Mach 0.9 jet with a ReD of 6.2x10 5 excited by plasma actuators has been investigated in order to evaluate the hydrodynamic and acoustic response of the jet, and the link between the two. Simultaneous acquisition of the far-field acoustic, the near-field pressure, and the actuation phase enables the use of phase-averaging of the pressure and acoustic signals and space-time correlations between the near field and the far field. By applying a filter in the frequency-wavenumber space, the nearfield pressure is decomposed into its constitutive hydrodynamic and acoustic components. Finally, wavelet analysis is utilized to assess the decomposed fields in the time domain. Both the hydrodynamic and acoustic response to forcing for StDF < 0.50 are found to follow a quasi-linear interaction model, in which the response to periodic forcing can be well predicted by a simple linear superposition of the impulsive response. Measurements of the pressure fluctuations and correlations to the far-field aft angle display an upstream shift in the structure saturation point as well as the dominant acoustic source region with increasing forcing frequency. Preliminary investigation in the time-domain found significantly lower temporal coherency of the acoustic response versus the hydrodynamic response; this difference was reduced by periodic forcing.