Reduced-order Jet Noise Modelling for Chevrons

Abstract

Jet noise has been a major community concern since the 1940s. Among noise-suppression devices, chevron nozzles are most effective as they significantly reduce low-frequency noise without appreciable thrust loss. The objective is to understand the noise-suppression mechanisms of chevron nozzles by identifying the noise sources and predicting the far-field noise for both chevron and round jets. To identify noise sources, the fourth-order space-time velocity cross-correlations are calculated based on an LES flow field. The Gaussian form fits axial, radial and azimuthal cross-correlations reasonably well. The axial length scales of the cross-correlations are 3-4 times the radial or azimuthal length scales for both chevron and round jets. For chevron jets, the cross-correlation lengths vary with azimuthal angle at axial positions within 6 jet diameters. Further downstream, both chevron and round jets behave similarly. Although for round and chevron nozzles R1111 the fourth-order cross-correlation of the axial velocity is the dominant component of the source, for chevron jets there is a significant contribution from other components such as R2222 and R3333. The cross-correlations decay rapidly with the axial distance for a chevron jet, whereas they remain constant for a round jet. Chevrons intensify R2222 and R3333 within 2 jet diameters downstream of the nozzle exit. The amplitude, length and time scales of the cross-correlations of the LES velocity field are investigated as functions of position and are found to be proportional to the turbulence amplitude, length and time scales determined from a RANS calculation. These proportionality constants are universal i.e. independent of nozzle geometry and position within the jet. The scales derived from RANS are used for source description and an acoustic analogy is used for sound propagation. At low-frequencies, chevrons drastically reduce noise by 5-6 dB at 30° to the jet axis and 2-3 dB at 90°. There is excellent agreement between far-field noise predictions and NASA measurements. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.