Prediction of jet noise shielding with forward flight effects

Aircraft noise continues to be a major concern among airport-neighboring communities. A strong component of aircraft noise is the jet noise that is generated from the turbulent mixing between the jet exhaust and ambient medium. The hybrid wing body aircraft suppresses jet noise by mounting the engines over-the-wing so that the airframe may shield ground observers from jet noise sources. Subscale jet noise shielding measurements of a scaled-down turbofan nozzle and a model of the hybrid wing body planform are taken with two 12-microphone polar arrays. Chevrons and wedge-type fan flow deflectors are integrated into the baseline bypass ratio 10 (BPR10) nozzle to modify the mean flow and alter the noise source behavior. Acoustic results indicate that the baseline BPR10 nozzle produces a long noise source region that the airframe has difficulty shielding, even when the nozzle is translated two fan diameters upstream of its nominal position. The integration of either chevrons or fan flow deflectors into the nozzle is essential for jet noise shielding because they translate peak intensities upstream, closer to the fan exit plane. The numerical counterpart of this study transforms the system of equations governing the acoustic diffraction with forward flight into the wave equation. Two forward flight formulations are considered: uniform flow over slender body; and non-uniform potential flow at low Mach number. The wave equation is solved numerically in the frequency domain using the boundary element method. The equivalent jet noise source is modeled using the combination of a wavepacket and a monopole. The wavepacket is parameterized using the experimental far-field acoustic autospectra of the BPR10 jets and knowledge of their peak noise locations. It is shown that the noise source compacts with increasing Mach number and consequently there is an increase in shielding. An assessment of the error associated with the non-uniform formulation for forward flight shows that the error is low for Mach numbers less than or equal to 0.2, but can be on the same scale as the acoustic scatter field when the Mach number is 0.6.