Supersonic Jet Noise Reduction by Microjet Injection

Abstract

The effect of microjet (μjet) injection on the noise from supersonic jets is investigated. One convergent and three convergent-divergent (C–D) nozzles, covering design Mach numbers (M_D) 1.0 to 2.2, are used in the study; all nozzles have the same exit diameter (D). The μjets are injected perpendicular to the primary jet at the nozzle lip from six equally-spaced ports; each port has a diameter of 0.0054D. Effects in the over-expanded and under-expanded regimes as well as one case of fully expanded condition are explored. Relative to the effect on subsonic jets, larger reduction in the overall sound pressure level (OASPL) is achieved in most supersonic conditions. While the injection readily eliminates screech tones with the convergent nozzle it becomes increasingly ineffective in screech reduction with nozzles of higher M_D. With all nozzles, the injection is often found to amplify the broadband shock-associated noise; the overall noise reduction occurs due to a suppression of broadband levels especially at lower frequencies. With the shock still within the divergent section of the C–D nozzles, there is ‘excess broadband noise’ and sometimes there are ‘transonic tones’; the injection is found to affect the tones very little but reduce the broadband noise component significantly. For shock-free, fully expanded condition, the OASPL reduction in the peak noise radiation direction is comparable to that in a subsonic case; the same correlation, found earlier for subsonic cases, applies.