Significance of cut-off modes in acoustic radiation from ducts

Baddour, Ben, Joseph, Phillip, Mcalpine, Alan and Leung, Ronnie (2021) Significance of cut-off modes in acoustic radiation from ducts. In Proceedings of the American Institute of Aeronautics and Astronautics (AIAA) 2021. American Institute of Aeronautics and Astronautics.. (doi:10.2514/6.2021-2299).

Abstract

This paper reports an investigation on the acoustic radiation from a source inside a hard-walled semi-infinite duct or waveguide, such as for ducted fans and rotors. In particular this work investigates the noise radiated from a rotor close to the duct open end. There is a wide range of published work in duct acoustics on propagating modes in ducts or waveguides. However, there is very little published work on the radiation of cut-off modes which may be significant if the source is located close to the duct open end. This work investigates the characteristics of cut-off modes to determine the influence of the mode radial order on the far-field directivity. The source weighting for a distribution of incoherent axial dipole sources is derived, valid above and below cut-off. The multi-modal acoustic radiation, from sources located close to the duct open end, including cut-on and cut-off modes, is computed to show the significance of cut-off modes, particularly in the radiation to the rear-arc.

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Contributors

Author: Alan Mcalpine

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