Use of downstream fluid injection to reduce subsonic jet noise

A fluid injection scheme consisting of multiple radial microjets located downstream from the nozzle exhaust is analyzed for its ability to suppress far field jet noise. Microjets in cross flow are known to enhance turbulent mixing due to the induced stream-wise vortices. Contrary to previous studies which injected fluid either inside the nozzle or just at the nozzle exhaust, this injection scheme uses a coaxial injector tube to inject multiple equally spaced microjets perpendicular to the jet axis at an axial location downstream from the nozzle exhaust. Microjet injection closer to the jet axis leads to the formation of a counter rotating vortex pair (CVP) close to the injection location which further beaks down into stream-wise vortices as the microjet bends and follows the flow direction. Detailed Large Eddy Simulations are performed for a nozzle-injector setup operating at Mach 0.9 jet with Reynolds number ≈10⁶ to help understand the aerodynamic and acoustic features of the interaction of this fluid injection scheme with the main jet. Permeable Ffowcs Willaims Hawkings based formulation is used for computing the far field acoustic spectra for injector setups with various numbers of injection ports (n_p) at two microphone locations. It is observed that as the number of microjets is increased, it leads to reduction in the far field noise. Peak noise reduction of ≈ 4.5 dB is observed for both locations. However, beyond n_p > 4 there is a lower acoustic benefit per additional port due to spatial interference between the induced CVPs.