An experimental investigation of overexpanded jets with chevrons

Results are presented for experiments investigating the impact of chevrons on overexpanded and subsonic jet plumes and associated acoustic radiation. Faceted, bi-conic convergent-divergent nozzles with design Mach numbers equal to 1.51 and 1.65 are used in the experiments. A design space of nine chevrons with a range of penetrations, lengths and widths are investigated. Low-penetration chevrons are shown to have limited impact on broadband and shock-associated noise for all jet Mach numbers investigated. High-penetration chevrons produce significant peak-noise reduction (relative to the baseline nozzle) in the peak-jet-noise direction with greater noise reduction for subsonic than for supersonic exhausts. High-penetration chevrons are found to increase broadband-shock-associated noise for the highest Mach number jet investigated. While particle image velocimetry results indicate chevrons introduce axial vorticity at all jet conditions, the associated mixing is limited for supersonic exhausts relative to that for subsonic exhausts due to the strong and periodic radial velocity components associated with the jet-shock-cell structure. The introduction of chevrons modifies the near-nozzle shock-cell structure and the impact of internally (within the nozzle) generated shocks on the shock-cell structure generated in the jet plume. The modifications in the near-nozzle shock-cell structure can lead to increased broadband-shock-associated-noise levels for some operating conditions.