Fluidic Injection Effects on Acoustics of a Supersonic Jet at Various Mach Numbers

Abstract

The effect of fluidic injection on a convergent-divergent supersonic jet operating at design and imperfectly expanded conditions was investigated. The nozzle had flow contours simulating those of tactical military jet engines and twelve pairs of fluidic injectors were incorporated into the nozzle lip. The effect of fluidic injection was studied at the design Mach number of 1.56, under-expanded, and over-expanded conditions. Extensive far-field acoustic measurements were conducted to determine the optimum injection momentum ratios and the effect on imperfectly expanded jets. SPIV was used to resolve three velocity components and compute the mean and turbulent flow field. Modification of the turbulence distribution was related to the acoustic results. At all conditions, increasing the injector momentum flux ratio resulted in greater reduction of OASPL. At over-expanded conditions, OASPL of 4 dB and 3 dB were observed at forward and downstream angles, respectively. At nozzle design conditions, up to 15 dB reduction in BBSN was observed in the narrowband spectra. OASPL reduction of up to 3.5 dB and 2 dB was achieved at the upstream and sideline angles, respectively. A study of the number of injection points determined that optimum noise reduction could be achieved with less injection points. The interaction of the fluidic injectors with the shock structure was the main noise reduction mechanism observed. Shear layer spreading rate was increased, but peak turbulence levels were effectively the same downstream.