Large eddy simulation of passive noise reduction in subsonic jets by using chevrons

Abstract

Chevrons are widely used passive noise reduction devices that have emerged as an significant breakthrough for aicraft industry in enabling substantial noise reduction without sacrificing thrust. However, the conventional testing of different chevron designs necessitates costly experimental facilities. This challenge can be circumvented through computational validation using CFD. Hence, this study employs a hybrid computational aeroacoustics approach to assess the viability of chevrons as a passive noise reduction technique within free subsonic jets using the commercial CFD software StarCCM+. Two sets of numerical simulations performed with and without chevrons applied at the end of the nozzle were examined. The dynamic Smagorinsky model was utilized to resolve the sub-grid scale stresses in these simulations of turbulent flows, which were run using large eddy simulation at an exit Mach number of 0.75. Using Ffowcs Williams Hawkings acoustic equations and the Fourier transform, the far-field analysis was performed on the acquired flow field to calculate the jet noise distribution in terms of the Sound Pressure Levels (SPL). The simulation results for free jets show good agreement with the published experimental data in terms of capturing the mean flow field and the acoustic levels in farfield. The simulations with chevrons show a reduction of approximately 2-3 dB in the farfield which results from a reduction in low-frequency mixing noise due to the creation of vortices in the shear layers. This result substantiates the capability of the computational aeroacoustics technique to evaluate chevron designs for effectively mitigating jet noise, particularly at high Mach numbers.