Acoustic Simulation of a High-Speed Jet with Localized Parallel Injection of Secondary Flow

Wall-Modeled Large Eddy Simulation (WMLES) of a high-speed jet with localized, parallel injection of secondary air is investigated. The injection scheme is designed to attenuate turbofan exhaust noise affecting launch crew personnel on an aircraft carrier deck. The focus of the computation is on understanding and eventually modeling the physical mechanisms of noise reduction measured experimentally. The simulated geometry is selected from previous experimental studies that yielded a 5.2 dBA reduction in the peak overall sound pressure level (OASPL) in the far field, in directions relevant to crew exposure. Analysis of the simulation data encompasses traditional statistics, axial space-time correlations that yielded convective velocities, spectral proper orthogonal decomposition (SPOD), and propagation to the far field using a Ffowcs-Williams and Hawkings surface. The simulated acoustic data compare favorably with experimental measurements for angular sectors of relevance. Focus is placed on the outer surface of peak Reynolds stress (OSPS), a location where the most energetic eddies are in contact with the ambient; and on the radiator surface, located in the transition between the hydrodynamic and acoustic fields and on which the axial convective velocity equals that on the OSPS. In the azimuthal direction of the injection, the OSPS is displaced outward radially, causing the convective velocity of the noise-generating eddies to drop from supersonic to subsonic values for an axial extent similar to the potential core length. In this region, the level of Reynolds stress on the OSPS declines by as much as 40%. On the radiator surface, the magnitude of pressure fluctuations drops by as much as 40% and the decline persists beyond the end of the potential core. SPOD of the simulated pressure fluctuations is used to extract the coherent structures in the flow. The radiative components of the leading SPOD mode in the direction of secondary injection are significantly reduced. The axial distribution of the leading mode along the radiator surface yields a wavepacket-like structure.