Turbulence and noise suppression of a high-speed jet by water injection

An experimental investigation has been carried out on a supersonic jet of air issuing from an M=1.44 converging–diverging rectangular nozzle of aspect ratio 4. Particle image velocimetry measurements of the flow field along with near field acoustic measurements were made. The effect of injection of a small amount of water (5 % of the mass flow rate of the jet) into the shear layer of the jet, on the unsteady flow structure and sound generation were examined. The presence of water droplets in the jet modified the turbulence structure significantly, resulting in axial and normal r.m.s. velocity reductions of about 10% and 30%, respectively, as compared to that of a normal jet. An even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The near-field noise levels (OASPL) were found to reduce by about 2–6 dB depending on the location of the injection and the water mass flow rate. Far-field acoustic measurements carried out on a heated M=0.9 (jet exit velocity=525 m s^{-1}) jet show significant (6 dB) reductions in the OASPL with moderate amounts of water injection (17% of the mass flow rate of the jet) suggesting that the techniques is viable at realistic engine operating conditions.