Book Description

A continued desire for high-performing tactical military aircraft motivates the advancement of low-bypass ratio turbofan engine technologies. The high-speed, high-temperature jets exhausting from these engines create dangerously high noise levels in the vicinity of the aircraft and are a source of noise pollution for communities surrounding military bases. Methods to reduce noise without negatively impacting aircraft performance are difficult. Low-bypass ratio, multi-stream exhaust engine architectures for tactical aircraft are currently under development. The concept aims to enhance fuel efficiency while still maintaining the high specific thrusts required to minimize engine size. It is anticipated that the engine nozzles will have low-aspect ratio, rectangular shapes.The primary goal of this study seeks to gain insight into the aeroacoustic and mean-flow characteristics of low-bypass ratio, rectangular, dual-stream, supersonic jets. Accurate comparisons of the noise radiated from these military-style, dual-stream jets with single-stream jets are lacking. Emphasis is placed on studying the effects of dual-stream nozzle configuration and comparing the noise of dual-stream jets with single-stream jets. Comparisons between jets are made on an equal estimated thrust and mass flow rate basis.Pitot pressure data acquired at the exit planes of the dual-stream nozzles are used to assess the accuracy of thrust and mass flow rate estimates. It is found that the experimentally estimated thrust values agree to within 6\% of the theoretical estimates, while the experimentally estimated mass flow rates agree to within 1\% of the theoretical estimates. Stream-wise Pitot pressure measurements and schlieren flow visualizations are used to characterize the dual-stream jet flow fields. Centroid line Mach numbers and shear layer thickness are compared with single-jet data. Fully-developed, dual-stream jet axial velocity profiles collapse to a self-similar shape.Far-field acoustic measurements aimed at isolating the effects of jet operating conditions are presented. The effects of bypass-to-core jet velocity ratio, and net thrust, on the far-field noise are also studied. Increases in core jet velocity result in an increase in peak noise levels as well as an upstream shift in the peak noise direction. For subsonic core jet velocities, the thin bypass jet is found to make a 5dB OASPL contribution to the turbulent mixing noise. Turbulent mixing noise decreases with decreasing bypass-to-core jet velocity ratio.The effects of bypass ratio on noise is are presented for dual-stream jet with a single fluid shield and two, symmetric fluid shield. As bypass ratio is increased, the mixing noise of the symmetric jet is reduced but the mixing noise is invariant for the fluid shield jet. At a bypass ratio of 0.50, the broadband shock-associated noise of the single fluid shield jet is eliminated.Flow field measurements show that the shock noise reduction is due to a weakening of the semi-regular shock-cell system in the jet. This trend is not observed for the symmetric fluid shield jet. The results indicate that nozzle configuration, i.e., the placement of the bypass jet(s) relative to the core jet, affects which noise components are sensitive to changes in operating conditions. The effects of an aft deck installed downstream of the dual-stream nozzle exit plane are also studied.The noise radiated from the single fluid shield and symmetric fluid shield jets is compared with equivalent single-stream round and rectangular jets. In the minor axis direction, the peak noise levels of dual-stream jets are within 1dB OASPL of the equivalent round jet, but up to 4dB OASPL louder than the equivalent rectangular jet. A second series of comparisons are presented for the single fluid shield jet operating at elevated bypass ratios, up to 0.91. In the minor axis direction, the dual-stream jets reduce overall sound pressure level by 4dB compared with the equivalent round and rectangular jets.