Effect of air injection on the far field pressure radiated from a jet at subsonic Mach numbers

This paper deals with an experimental study of the acoustic field resulting from the interaction between the air injected from 6 microjets and the flow issuing from a round jet at low and high subsonic Mach numbers. The displacement and inclination of the microjet axes with respect to the main stream direction, as well as the ratio between the microjets and the main jet flow rates, are varied parametrically and the effect on the far field sound is analyzed. In the low Mach number cases no relevant influence upon the acoustic field is observed. This is probably an effect of the limited number of microjets activated. On the contrary, when the flow becomes highly compressible, a significant alteration of the far field acoustic pressure with respect to the baseline conditions is detected. The most relevant effect is observed when the microjet axes are placed in direction perpendicular to that of the main stream. In these cases the pressure energy is considerably reduced and a significant overall noise abatement, of the order of more than 2dB, is achieved.