Jet Noise Suppression with Fan Flow Deflectors in Realistic-Shaped Nozzle

Abstract

This is an exploratory study of jet noise reduction using fan flow deflectors in a nozzle shaped like the exhaust of a separate-flow turbofan engine with bypass ratio 4.8. The deflectors directed the bypass plume downward and/or sideward relative to the core plume. Vaneand wedge-type deflectors were tested. The noise reduction achieved by the vane deflector was strong in the downward direction and moderate in the sideline direction. Wedge-type deflectors generated significant attenuation in both directions. Combination of vanes and wedge yielded the best results. The effective perceived noise level was suppressed by 5.5 dB in flyover mode and 3.8 dB in sideline mode. IntroductionThe increase in bypass ratio over the last three decades has resulted in a dramatic suppression in the jet noise of turbofan engines. Modern engines are so quiet that further reduction in noise becomes very challenging. The success of the high-bypass engine is offset by the increasing volume of aircraft operations. This creates more environmental and political pressures for quieter aircraft. Today the most successful technique for reducing jet noise from high-bypass engines involves the installation of chevron mixers on the exhaust nozzles [1]. However, the ever increasing need for quieter engines requires exploration of alternative techniques that could be used by themselves or in conjunction with existing methods.The Fan Flow Deflection (FFD) technology is tailored for suppression of "large-scale" turbulent mixing noise from aircraft engines. Large-scale mixing noise is the dominant noise source in turbulent jets. The overarching principle of the FFD technology is reduction of the convective Mach number of turbulent eddies that generate intense downward and sideward sound radiation. In a coaxial separateflow turbofan engine this is achieved by tilting in * Professor, Associate Fellow AIAA † Graduate student, Member AIAA the general downward direction, by a few degrees, the bypass (secondary) plume relative to the core (primary) plume. Mean flow surveys show that the misalignment of the two flows causes a thick, lowspeed secondary core on the underside of the highspeed primary flow, especially in the region near the end of the primary potential core which contains the strongest noise sources. 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 [2]. Tilting of the bypass stream is possible by means of fixed or variable vanes installed near the exit of the bypass duct. Figure 1 depicts the general concept.Earlier works on FFD used simple coaxial nozzles for the investigation of noise emission and mean flow field [2,3]. This paper extends the previous work to nozzles shaped like the actual nozzle of a separate-flow turbofan engine. Also, it includes deflector configurations not covered by earlier studies. The treatment is exploratory. We report only on acoustic data and some preliminary flow visualizations. Aerodynamics and f...

Computational Analyses of Offset Stream Nozzles for Noise Reduction

“…Figure 10 and 11 show contours of velocity and turbulence at stations in the jet plume for each of the S-duct offset stream nozzles. Similar to what Papamoschou demonstrated for offset stream nozzles using vanes [10][11][12][13] , the S-duct offset stream nozzles also significantly change the structure of the jet plume. Whereas the plumes of Papamoschou's vane nozzles resemble an upside-down pear -fat on the top and thin on the bottom --the S-duct nozzle plumes resemble a right-side-up pear -fat on the bottom and thin on the top.…”

“…The plume of the vane nozzle was less full on the sides than the plume of the S-duct nozzle. Furthermore, the jet plumes observed by Papamoschou [10][11][12][13] in his experiments with vane nozzles show more similarities with the S-duct nozzles analyzed here than the vane nozzles. This is because the S-duct nozzles and Papamoschou's vane nozzles are full and round on the lower side.…”

“…Papamoschou investigated offsetting the fan stream using vanes and wedges [10][11][12][13] . The idea was to direct the fan stream to the lower side of the jet plume and to shield the observer on the ground from jet noise.…”

Aerodynamics of Fan Flow Deflectors for Jet Noise Suppression

“…Subscale experiments at U.C. Irvine have demonstrated reductions as much as 5 decibels in effective perceived noise level [2]. It is important, however, to also assess the aerodynamic efficiency of this scheme.…”

RANS Analyses of Turbofan Nozzles with Wedge Deflectors for Noise Reduction

“…1- 3 The idea is to provide a thicker layer of fan flow on the lower (observer) side of the nozzle. This thick layer of low speed flow reduces the convective Mach number of the turbulent eddies generated by the high speed core flow.…”