Effect of microjets on a high-subsonic jet. Parametric study of far-field noise reduction.

A system of 36 impinging microjets was implemented on a Mach=0.9 round jet, and the noise reduction was studied 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 flow parameters, the global jet-noise reduction varied from 0 to 1.8 dB, showing some non-monotonic behaviors due to the change between subsonic and supersonic regimes of the microjets. The study of the layout of the microjets shows that the noise reduction decreases when the microjets are too close to each other and that some configurations of microjet pairs could be favourable, which can be linked to the flow structures induced by the microjets. Spectral analyses pointed out different mechanisms involved in the control, with a highfrequency regeneration for high injections, a local behavior of the control at the mid-frequencies and a global behavior at the low-frequencies.