Jet noise reduction by impinging microjets: an aerodynamic investigation testing microjet parameters

The effects of microjets on the aerodynamic characteristics of a Mach 0.9 high-Reynolds axisymmetric jet are investigated and are interpreted in the light of previous acoustic re-sults. These measurements are obtained by means of Stereoscopic Particle Image Velocime-try in planes normal to the jet axis. Three parameters of the microjets system are varied: the outgoing mass flux per microjet, the number of microjets and their layout in the az-imuth of the main jet. The aerodynamic results indicate a strong correlation between the maximum level of turbulence just behind the nozzle exit and the high-frequency noise, pre-viously shown to potentially balance the acoustic benefits obtained for lower frequencies. The maximum level of turbulence measured at the longitudinal position corresponding to half the potential core length is also highly correlated to the jet noise reduction, which is highlighted by the similar evolution of these two quantities regarding the mass flux per microjet and the number of microjets. For low values of the number of microjets, the microjets are shown to act independently, and their contributions to the turbulence reduc-tion are retrieved far downstream the impinging point without any noticeable azimuthal diffusion. I