Acoustic and flow fields of an excited high Reynolds number axisymmetric supersonic jet

Abstract

An axisymmetric perfectly expanded Mach 1.3 jet, with a Reynolds number based on the nozzle exit diameter (ReD) of 1.1 × 106 and turbulent boundary layer at the nozzle exit, was excited using localized arc filament plasma actuators over a wide range of forcing Strouhal numbers (StDF). Eight actuators distributed azimuthally were used to excite azimuthal modes m = 0–3. Far-field acoustic, flow velocity and irrotational near-field pressure were probed with a three-fold objective: (i) to investigate the broadband far-field noise amplification reported in the literature at lower speeds and ReD using excitation of m = 0 at low StDF; (ii) to explore broadband far-field noise suppression using excitation of m = 3 at higher StDF; and (iii) to shed some light on the connection between the flow field and the far-field noise. The broadband far-field noise amplification observed is not as extensive in amplitude or frequency range, but still sufficiently large to be of concern in practical applications. Broadband far-field noise suppression of 4–5 dB at 30° polar angle peak frequency, resulting in approximately 2 dB attenuation in the overall sound pressure level, is achieved with excitation of m = 3 at StDF ~ 0.9. Some of the noteworthy observations and inferences are (a) there is a strong correlation between the far-field broadband noise amplification and the turbulence amplification; (b) far-field noise suppression is achieved when the jet is forced with the maximum jet initial growth rate frequency thus limiting significant dynamics of structures to a shorter region close to the nozzle exit; (c) structure breakdown and dynamic interaction seem to be the dominant source of noise; and (d) coherent structures dominate the forced jet over a wide range of StDF (up to ~ 1.31) with the largest and most organized structures observed around the jet preferred mode StDF.