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A new method for reducing large-scale mixing noise from dual-stream jets is presented. The principle is reduction of the convective Mach number of turbulent eddies that produce intense downward sound radiation. In a jet representing the coaxial exhaust of a turbofan engine, this is achieved by tilting downward, by a few degrees, the bypass (secondary) plume relative to the core (primary) plume. The misalignment of the two flows creates a thick low-speed secondary core on the underside of the high-speed primary flow. The secondary core reduces the convective Mach number of primary eddies, thus hindering their ability to generate sound that travels to the downward acoustic far field. Tilting of the bypass stream is possible by means of fixed or variable vanes installed near the exit of the bypass duct. Subscale aeroacoustic experiments simulated the exhaust flow of a turbofan engine with bypass ratio 6.0. Deflection of the bypass stream resulted in suppression of the peak overall sound pressure level by 4.5 dB and the effective perceived noise level by 2.8 dB. For the nozzle configuration used, the thrust loss is estimated at around 0.5% with the vanes activated and 0.15% with the vanes deactivated. [ABSTRACT FROM AUTHOR]

Presents a study which characterized the acoustics of axisymmetric high-speed jets at a variety of Mach numbers and velocities and at pressure-matched, overexpanded, and underexpanded conditions. Components of supersonic jet noise; Noise measurements; Correlations of jet noise with engine operating parameters.

Presents a study which demonstrated directional suppression of noise from a high-speed jet using an asymmetric parallel secondary stream. Methodology; Results and discussion; Conclusions.

Presents information on a study that investigated the noise measurements for perfectly expanded coaxial jets composed of a supersonic primary stream at conditions designed to prevent formation of Mach waves. Description of the flow apparatus; Global features of the jet; Noise characteristics.