Aeroacoustics of a dual-stream rectangular supersonic jet with three-sided fluid shield

This paper describes the results of aeroacoustic experiments on a dual-stream rectangular high-speed jet. The experiments were conducted with a new nozzle which 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 adjacent to the bottom, or top and bottom of an aspect ratio 2 rectangular supersonic jet. Such a design proved to have noise reductions in the far-field in the directions perpendicular to the major axis of both the nozzle and the one or two fluid shields. The noise benefits in this direction were shown to be substantial, depending on the operating conditions of the jets. The new nozzle described in this paper was designed to explore the potential noise benefit in the major axis (sideline) directions. The new nozzle has fluid shields on three sides of the nozzle in a U-shaped configuration. The concept was an attempt to reduce the noise radiating to the sides of a nozzle flow, in the sideline direction of an aircraft equipped with rectangular jet engine exhausts. Experiments were conducted on the newly fabricated nozzle operating with supersonic core velocities and a range of bypass flow velocities from high subsonic to supersonic velocities. 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. These were spaced every 10 degrees from 20 to 130 degrees measured from the downstream direction. For acoustic measurements, the jet conditions were used to estimate a single-stream equivalent jet for purposes of comparisons to experiments conducted previously with single-stream rectangular supersonic jets. The single-stream equivalent jet designation (SSEJ) specified jets of comparable thrust and total mass flow rate (of the dual-flow jets). Non-dimensional acoustic spectra are compared at a range scaled to 100 equivalent jet diameters for the dual-flow nozzle and the single-stream nozzle jets. Integration of the spectra produce overall sound pressure levels (OASPLs) whose emission directivities are shown. A major outcome was that there was a large broadband shock-associated noise reduction (5 dB) on the nozzle sideline (0 and 180 deg. azimuthal angles) in the forward arc. The bottom bypass (270 deg.) gave the greatest noise reduction (3 dB) in the direction of maximum noise emission, at the 30 deg. microphone position downstream of the jet exhaust. It was found that there is not a simple correlation between bypass stream thickness and noise reduction. Past experiments have concluded that noise reductions in supersonic jets produce their best results when the exhaust jets are heated, successfully undertaken with simulated hot jets produced from helium-air mixture jets. Experiments, interrupted by the closure of the university, will soon be undertaken, with the inclusion of cases with heat simulated jets.