Experiments on Shielding of Jet Noise by Airframe Surfaces

We present parametric subscale experimental studies of jet noise shielding in two basic configurations. The first configuration used a single-stream Mach 0.9 cold air jet with a rectangular shield. The second configuration was composed of a dual-stream bypass ratio 10 nozzle that operated at realistic cycle conditions and a hybrid-wing body (HWB) planform shield. For both configurations, the shield was adjustable relative to the nozzle. Tabs, chevrons, and wedge fan flow deflectors were added to the nozzles to compact the noise source region. Aeroacoustic surveys were conducted inside an anechoic chamber using an eight-microphone polar array. The experiments with the single-stream jet and rectangular shield showed an excess of noise at low frequencies, yet reduced noise at high frequencies. Since the excess noise occurs at low frequencies, it has a minor impact on the perceived noise level of a full-scale engine. Significant EPNL reductions, up to 7.4 dB cumulative (downward plus sideline), were measured with an aft shield length of 5 jet diameters. The addition of mixer tabs increased this benefit to 10.7 dB, even though the jet itself became noisier. The HWB shielding effectiveness with the plain nozzle is marginal, even with the engine translated two fan diameters upstream of its nominal location. However, applying fan flow deflectors or chevrons yielded a significant EPNL benefit, about 6.5 dB cumulative, with the engine positioned at its nominal location. Thus, the compaction and/or redistribution of the noise source are essential for effective jet noise shielding on the HWB. A proposed correlator for the noise reduction due to shielding is the average illumination angle. The experimental data show a trend of increasing EPNL reduction with decreasing average illumination angle. For average illumination angles less than 45 deg, the EPNL reduction in a particular azimuthal direction exceeds 3 dB.