Development of a dual-stream rectangular supersonic jet with a three-sided fluid shield for acoustic benefits

This paper describes the results of aeroacoustic experiments on a dual-stream rectangular high-speed jet. The experiments were conducted with a new nozzle that represents a second-generation design following experiments reported during the past two years on first generation multi-flow rectangular jets. In the first-generation nozzles, the bypass flow consisted of one or two fluid shields on the bottom, or top and bottom of an aspect ratio 2 rectangular supersonic jet. Noise reductions were observed in the far-field in the directions perpendicular to the major axis of both the nozzle and the one or two fluid shields. A new nozzle has been designed with fluid shields on three sides of the nozzle in a U-shaped configuration to explore potential noise benefits in the major axis (sideline) directions. The nozzle is referred to as a Dual Stream U shaped Bypass-stream nozzle (DSUB). Experiments were conducted on the new nozzle operating with supersonic core velocities and a range of bypass flow velocities from high subsonic to supersonic. Far-field noise measurements were performed with an array of microphones mounted on an arc with the microphones approximately 2 meters from the nozzle exit. The microphones were spaced every 10 degrees from 20 to 130 degrees measured from the downstream jet axis. For the acoustic measurements, the jet conditions are used to estimate a single-stream equivalent jet for purposes of comparisons to experiments conducted previously with a single-stream rectangular supersonic jet. The Single-Stream Equivalent Jet designation (SSEJ) specifies jets of comparable thrust per unit exit area (of the dual-flow jets). Non-dimensional acoustic spectra are compared at a polar distance scaled to 100 equivalent jet diameters for the dual-flow nozzle jet to measurements previously performed with a the single-stream rectangular nozzle jet, designated SSRJ. Integration of the spectra produce overall sound pressure levels (OASPLs) whose emission directivities are shown and compared. Noise benefits in the maximum noise emission directions of approximately 3 dB are achieved with jets operating with helium-air gas mixtures that simulate hot jets.