Aeroacoustics of Swirling Exhaust Flows in High Bypass Ratio Turbofan Nozzles for Drag Management Applications

Abstract

The aeroacoustics of swirling exhaust flows in high bypass ratio turbofan nozzles are assessed for aircraft approach drag management applications. Aerodynamic designs of swirl vane hardware were tested in an anechoic jet test facility to quantify their thrust reduction (or equivalent drag generation) capability and noise. Simple vaned-disk (visk) hardware using a row of periodically spaced swirl vanes was used to study flow interactions and noise generation mechanisms associated with swirl generated in the bypass duct. The swirling bypass exhaust stream creates suction on the core stream, which elevates the core flow and decreases the bypass ratio by up to 40 percent for the geometries tested. Swirling the bypass exhaust enhances mixing, and phased array measurements suggest that the apparent noise source moves upstream and radially inward toward the inner shear layer, and radiates preferentially in the direction of swirl. The presence of a pylon with a deflected trailing edge and asymmetric swirl vanes is found to produce a coherent swirling outflow with slightly greater noise generation than its idealized counterpart. The relationship between gross thrust reduction and noise generation is quantified and applied to a simple steep approach noise simulation at fixed aircraft speed. Results suggest a noise reduction potential of up to 3.1 dB, tonecorrected peak perceived noise level (PNLT) and 1.8 dB effective perceived noise level (EPNL) for a single-aisle, twin engine aircraft. NomenclatureRoman a ∞ = Freestream sound speed A ref = Reference (total fan circular) area C d,eq = Equivalent drag coefficient CNPR = Core nozzle pressure ratio D b = Fan (bypass) nozzle exit diameter FNPR = Fan (bypass) nozzle pressure ratio F gross = Gross thrust F net = Net thrust M B = Ideal bypass-stream Mach number M C = Ideal core-stream Mach number M MA = Ideal mass-averaged Mach number M ∞ = Freestream (approach) Mach number V app = Freestream approach velocity Greek ρ = Density θ app = Approach glideslope angle

Full-Scale Turbofan Demonstration of a Deployable Engine Air-Brake for Drag Management Applications1

“…An alternative scenario using only measured static noise deltas at all polar angles to capture the directivity (without the effect of forward flight) suggests a 3.1 EPNdB noise reduction. A more realistic forward flight effect penalty, based on the separate flow nozzle data and the measured gross thrust reduction, would be about +2.5 dB based on static-to-flight deltas measured in previous nozzle tests [22]. The resulting noise reduction is 2.8 EPNdB.…”

“…Table 3 presents the results of an ANOPP comparison study between the two approach trajectories under various assumptions for jet noise penalty. Ahead of the present engine testing, a more severe 9.3 dB jet noise penalty was estimated using previously measured noise data on separate flow nozzles [21], [22] that included the effect of forward flight. Under this scenario, the EPNL metric still showed a 1.1 dB noise benefit.…”

“…In quantifying the relationship between swirl, flow, drag, and noise, aircraft-onapproach noise simulations were used to demonstrate that an appropriately designed EAB could enable a steep approach trajectory (from a baseline 3.2 degree glideslope to 4.4 degrees) for a 737-800-class aircraft at a fixed speed. A peak tonecorrected perceived noise (PNLT) reduction of up to 3.1 dB was predicted at the ground observer location, with a potential 1.8 dB effective perceived noise level (EPNL) reduction [21], [22].…”