Experimental aeroacoustics study on jet noise reduction using tangential air injection

Abstract

Aircraft jet exhausts are a source of undesirable noise and continue to be an area of investigation driven by increasingly stringent regulation. The noise is produced by the unsteady mixing of the jet with the surrounding air and is dominated by the effects of the shear layer. In this study, the mechanisms of noise suppression are investigated on an unheated Mach 1.3 jet through three distinct control techniques. The first consists of tangential steady flow injectors located upstream of the nozzle exit whereas the second involves an equal number of control jets spaced further downstream around the nozzle exit. The third technique pulses air tangential into the shear layer at a frequency coinciding with the preferred modes of the jet (St~0.17, f~2kHz). Near and far-field acoustic measurements were made in anechoic chamber with an array of 10 free-field microphones. All three forms of tangential air injection induced reductions in overall sound pressure levels (OASPL) across a range of observer angles. External tangential injection was found to be the most efficient technique, as it produced comparably similar noise reductions at only a fraction of the injection mass flow ratio. The most significant acoustic benefit was an 8dB SPL reduction at the sideline observer angle, subsequently eliminating both screech and broadband shock noise at Strouhal numbers of St~0.74 (f~9.6kHz) and St~1(f~13kHz) respectively. An OASPL reduction of up to 5dB was also recorded at a downstream angle of 15º. However, the low-frequency noise benefits from these control jets came at the expense of increased high frequency noise beyond St>2 (f~26kHz). The flow-fields of the jet were observed using stereoscopic Particle Image Velocimetry (PIV). The introduction of a swirling component of velocity downstream of the nozzle exit was found to have a stabilizing effect on the jet shear layer. Reductions in turbulence intensity and Reynolds stress were recorded towards the end of the potential core by up to 18% and 25% respectively. The ultimate objective of this study was to develop an injection configuration that is effective at reducing jet noise whilst minimising penalties in weight and thrust.