Jet-noise reduction by impinging microjets:: An acoustic investigation testing microjet parameters

A system of 36 impinging microjets was implemented on around jet of Mach number 0.9, and the noise reduction was Am lied as a function of the microinjection mass flux, the number of microjets blowing, the layout of the blowing microjets, and the microjet diameter. Depending on the microinjection flux parameters, overall jet-noise reduction varied from 0 to 1.5 dB, showing some nonmonotonic behavior due to the change between subsonic to supersonic microjets regimes. The study of the microjet layout showed that the noise reduction decreased when the microjets were too close to each other and that certain configurations of microjet pairs could be favorable; this can be related to the flow structures induced by the microjets. Spectral analysis disclosed different control mechanisms, with high-frequency regeneration for high-injection flux, local control behavior at midfrequencies, and global behavior at low frequencies.