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 (RANS) flowfield showed a good agreement with those based on a Large Eddy Simulation (LES) 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 optimising jet noise reduction concepts and could contribute towards the development of quieter nozzles for future aircraft.