Investigations of Azimuthal and Flight Effects on Noise from Realistic Turbofan Exhaust Geometries

Abstract

For airplane noise certification, noise measurements directly under the flight path, as well as at a lateral location, are required. Both azimuthal and flight effects are evaluated, with the aid of axisymmetric and realistic dual-stream nozzle geometries, in a controlled anechoic facility. The sound field is axisymmetric for dual-stream nozzles, both for simple and complicated internal geometries, as long as the exit plane is axisymmetric. In general, two effects are anticipated due to the presence of the pylon. The first effect pertains to the introduction of azimuthal variations in the spectra. Surprisingly, the sound field in the azimuthal angular range from to is axisymmetric except at very high engine power and low for which there is an increase in level at large aft angles. The second effect of the pylon pertains to modifications to the spectra, relative to those from an axisymmetric nozzle system. There is a noise reduction between and for the pylon nozzle at large aft angles and an increase of at the lower inlet angles. In general, the pylon effect is most pronounced at high engine power and gradually diminishes as engine power is lowered progressively. The following effects of forward flight have been established: 1) at the lower inlet angles, and up to 110 deg, the magnitude of the noise reduction due to forward flight is nearly uniform at all frequencies, at all velocity ratios and area ratios; 2) more complicated effects of forward flight are observed at large aft angles, which are a reduction at the lower frequencies and from to at the higher frequencies; 3) the area ratio plays no role at high but becomes important at lower .