Jet-Noise-Prediction Model for Chevrons and Microjets

This study develops a jet-noise-prediction model for chevrons and microjets. A novel equation is proposed to express the amplitude of the fourth-order space-time velocity cross correlations, which represent the sources of noise emanated from unheated jets, in terms of mean-flow parameters and turbulence statistics, such as streamwise circulation, axial velocity, and turbulent kinetic energy. The cross correlations based on a Reynolds-averaged Navier-Stokes flowfield showed a good agreement with those based on a large-eddy-simulation flowfield. With the novel acoustic-source description, there is a good agreement between the model's jet-noise predictions and the experimental data for unheated jets for a wide range of frequencies and observer angles for both chevrons and microjets. As the model provides quick and accurate jet-noise predictions, a parametric study is performed to understand the impact of chevrons and microjets on jet noise. Chevron penetration is the underpinning factor for jet-noise reduction, and its optimum is found to be around one-seventh of the nozzle diameter. The number of chevrons has a considerable effect on jet noise, and six is found to be an optimum number of chevrons. The injected mass-flow rate of a system of microjets has a noticeable impact on jet noise, and for 18 microjets, its optimum is found to be around 0.0072 of the main-jet mass-flow rate. There is a good agreement between predicted and measured optimum values. This establishes that the model is indeed capable of assessing and optimizing jet-noise-reduction concepts, and could contribute toward the development of quieter nozzles for future aircraft.