Jet noise shielding from airframe surfaces

This work investigates the potential suppression of jet noise by airframe surface shielding with application to the Hybrid Wing Body aircraft (HWB). Subscale experiments utilized a bypass-ratio-10 (BPR10) turbofan nozzle that operated at realistic cycle conditions using helium-air mixture jets. The shielding surface was a flat plate shaped in the form of a generic HWB and featured vertical fins and an adjustable elevator. Chevrons and porous wedge fan flow deflectors were incorporated into the baseline nozzle to restructure and alter the jet noise source distribution.;Acoustic measurements of the unshielded and shielded jets were conducted inside an anechoic chamber using a 24-microphone apparatus composed of two polar arrays, each consisting of twelve condenser microphones. One polar array was mounted at an azimuth angle &phis; = 0° (downward) while the second array was mounted at &phis; = 60° (sideline). Noise source distribution measurements were performed by densely grouping twenty-four microphones on a linear array in the downward direction. Surveys of the jet mean flow were conducted with a fully automated three-dimensional Pitot rake traverse. The Pitot rake consists of five high-resolution Pitot probes that measure the local jet total pressure.;The potential for noise suppression was quantified in terms of the estimated Effective Perceived Noise Level (EPNL), a noise metric used in aircraft certification. Acoustic results of the baseline BPR10 nozzle with the HWB shield yielded only marginal EPNL reductions, even when the engine was translated two fan diameters upstream of its nominal location. Using the estimated cumulative (downward plus sideline) EPNL reduction as a figure of merit, shielding of the baseline nozzle with the nominal shield configuration yielded a 2.5 dB reduction. Acoustic imaging of the baseline jet noise source revealed that the large fan diameter, inherent to the high bypass ratio, created a long noise source region. The integration of nozzle devices significantly improved EPNL reduction with the engine positioned at its nominal location. Application of the aggressive chevrons increased the reduction to 6.5 dB, while the best wedge configuration improved this value to 6.9 dB. Combination of wedge and aggressive chevrons yielded a benefit of 7.6 dB. Examination of high-definition noise source maps showed a direct link between insertion loss and axial location of the peak noise. Ultimately, the enhanced shielding effect from the nozzle devices resulted from translation of peak intensities upstream closer to the nozzle plug. Thus the compaction and/or redistribution of the noise source via nozzle devices 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 average illumination angle is defined as the average angular sector between noise source and observer not intercepted by the shield. For average illumination angles less than 45 deg, the EPNL reduction exceeds 3 dB. A correlation between the acoustic and mean flow results reveal that the mean velocity field by itself cannot provide useful information for inferring the noise source location.