Experimental and Computational Approach for Jet Noise Mitigation by Mixing Control Devices

The notched nozzle as a new concept has been investigated for conventional nozzle design together with the Chevron nozzle and Micro-jets, through feasibility studies. The notched nozzle has a plurality of triangular pyramid-shaped dent positioned in a circumferential direction along the nozzle exit. These studies include acoustic experiments that utilize a lab-scale simple model in an anechoic chamber and numerical approaches. The results of the Large Eddy Simulation are compared with the results of either acoustic or aerodynamic experiments. The objective of these investigations is to verify the effects of noise mitigation and to gain understanding of the physics of fluid dynamics around the nozzle exit, especially within the shear layer between high velocity jet flow and external flow/or ambient air. One concept of conventional noise mitigation devices involves mixing enhancements in the shear layer, but this sometimes produces high frequency self noise. Moreover it will result in a penalty in terms of thrust loss, additional weight and extra manufacturing cost due to the complicated shapes around the nozzle exit. It is difficult to produce a nozzle design without affecting high frequency self-noise and decreasing low-frequency noise towards to down stream of the jet engines even though there is no thrust loss. Most of this study, the experimental data were physically validated by three kinds of nozzle concepts designed to be equal to the conventional model in terms of size of nozzle exit diameter and Mach number. Essentially far-fields noise measurements and pressure measurements are conducted by polar angle microphones and arch-shaped pitot tubes are located downstream of the jet. The noise benefit which is produced by the notched nozzle as a lab-scale in far-fields noise measurements is up to 1.3dB at the side of the jet and 0.5dB at downstream, in terms of size of small-engine. Furthermore this provided an advantage over the chevron nozzle due to the decreasing self-noise production when the Mach number of the jet was lower than 0.9. Moreover, numerical predictions which are provided by the Large Eddy Simulation were used to estimate the noise mitigation by performing turbulence statistical analysis. Numerical results which refer to the turbulent statistics are discussed in order to define how they can be affected to the acoustic results at the side of the jet. This shows how each device can deform the shear layer without producing additional streamwise and small scale vortices.