High Bypass Ratio Jet Noise Reduction and Installation Effects Including Shielding Effectiveness

Abstract

An experimental investigation was performed to study the propulsion airframe aeroacoustic installation effects of a separate flow jet nozzle with a Hybrid Wing Body aircraft configuration where the engine is installed above the wing. Prior understanding of the jet noise shielding effectiveness was extended to a bypass ratio ten application as a function of nozzle configuration, chevron type, axial spacing, and installation effects from additional airframe components. Chevron types included fan chevrons that are uniform circumferentially around the fan nozzle and T-fan type chevrons that are asymmetrical circumferentially. In isolated testing without a pylon, uniform chevrons compared to T-fan chevrons showed slightly more low frequency reduction offset by more high frequency increase. Phased array localization shows that at this bypass ratio chevrons still move peak jet noise source locations upstream but not to nearly the extent, as a function of frequency, as for lower bypass ratio jets. For baseline nozzles without chevrons, the basic pylon effect has been greatly reduced compared to that seen for lower bypass ratio jets. Compared to Tfan chevrons without a pylon, the combination with a standard pylon results in more high frequency noise increase and an overall higher noise level. Shielded by an airframe surface 2.17 fan diameters from nozzle to airframe trailing edge, the T-fan chevron nozzle can produce reductions in jet noise of as much as 8 dB at high frequencies and upstream angles. Noise reduction from shielding decreases with decreasing frequency and with increasing angle from the jet inlet. Beyond an angle of 130 degrees there is almost no noise reduction from shielding. Increasing chevron immersion more than what is already an aggressive design is not advantageous for noise reduction. The addition of airframe control surfaces, including vertical stabilizers and elevon deflection, showed only a small overall impact. Based on the test results, the best overall nozzle configuration design was selected for application to the N2A Hybrid Wing Body concept that will be the subject of the NASA Langley 14 by 22 Foot Subsonic Tunnel high fidelity aeroacoustic characterization experiment. The best overall nozzle selected includes T-fan type chevrons, uniform chevrons on the core nozzle, and no additional pylon of the type that created a strong acoustic effect at lower bypass ratios. The T-fan chevrons are oriented azimuthally away from the ground observer locations. This best overall nozzle compared to the baseline nozzle was assessed, at equal thrust, to produce sufficient installed noise reduction of the jet noise component to enable the N2A HWB to meet NASA's noise goal of 42 dB cumulative below Stage 4. = total temperature ratio of the secondary fan stream θ = polar directivity angle, degrees, jet axis at 180 degrees ψ = azimuthal directivity angle, degrees

The Effects of Surfaces on the Aerodynamics and Acoustics of Jet Flows

“…This is a highly complicated problem that has received significant attention. 7,8,18 It is characterized by a large number of parameters that are highly interdependent. A nondimensional number is formed, which has arguments involving the flow-conditions and jet position, that can be used as a basic guide to ascertain whether the aerodynamic source is affected by the airframe relative to the equivalent isolated jet aerodynamic source.…”

“…Recent studies have investigated the effects of liners on propulsion airframe surfaces. 18 The effectiveness of airframe liners is associated with the location of the airframe relative to the jet flow. The implications of Γ could be useful in future PAA liner studies.…”

Acoustic Characterization of Compact Jet Engine Simulator Units

“…In this case, the one-third octave equivalent data of Configuration E in Fig. 13 are compared to the same setpoint run at Boeing's Low Speed Aeroacoustic Facility (LSAF) during the experimental investigation described in Thomas et al 12 The efforts to reduce rig noise at the larger scale LSAF are noteworthy 1,7 , thus offering a rigorous comparison for the CJES evaluation. Spectra in the mid-frequency range match very well, particularly in the upstream arc.…”

“…Two sets of convergent core and fan nozzles accompany each CJES unit -one set is axisymmetric while the other set includes an essentially uniform core chevron nozzle and an asymmetric fan chevron nozzle, designed based on extensive testing described by Thomas et al 12 A separate plug for the core stream also accompanies each set of nozzles. The core nozzle diameter is 3.28 inches and the fan nozzle diameter is 6.21 inches.…”

“…These nozzles serve as the baseline set for the HWB acoustic test. An additional low-noise nozzle set was downselected from extensive testing described in Thomas et al 12 The main objective of that work was to design an optimum low-noise bypass ratio 10 nozzle set for the HWB acoustic test. The best low-noise candidate from that test was an essentially uniform core chevron nozzle coupled with a circumferentially varying fan chevron design that built upon prior efforts that included pylon effects studies 8 .…”