A Study of Subsonic Turbulent Jets and Their Radiated Sound Using Large-Eddy Simulation

Doktorsavhandling, 2005

Stricter noise regulation for near-ground operations has made noise reduction in commercial aircraft a topic of growing interest in the aerospace industry. To meet airworthiness requirements new noise reduction technologies have to be developed and numerical methods for correct assessment of these technologies are desirable. This thesis deals with predictions of near-field flow and far-field acoustic signature of subsonic turbulent single-stream and dual-stream jets at isothermal and heated conditions. The flowfield predictions are obtained using large-eddy simulation (LES), and Kirchhoff's surface integration technique is used to extend the acoustic domain to far-field locations. In all cases studied, the nozzle geometry is included in the calculation domain. For the single-stream jet, predicted near-field flow statistics and far-field sound pressure levels (SPL) are both in good agreement with experiments. Predicted SPL for all observer locations, where evaluated, are within a deviation of 3.0 dB from measured levels and for most locations within a deviation of 1.0 dB. For the specific cases studied, Mach 0.75 jets, only small differences in radiated sound could be identified between an isothermal jet and a jet with temperature twice that of the surrounding fluid. The effects of changes in inflow conditions, Reynolds number and subgrid-scale (SGS) model on the flowfield and acoustic signature were investigated. Only minor changes could be identified in the predictions of flow statistics and radiated sound. For the dual-stream jet, changing the subgrid-scale filter width and introducing a TVD limiter gave significant changes in the shear layer flow. Sound radiated in the upstream direction was shown to depend appreciably on the initial shear layer development. Vortex generators placed on the outside wall of the inner nozzle were found to effectively break up ring-shaped vortical structures in the initial inner shear layer region and speed up the mixing between the core and bypass streams.