Duct effects on acoustic source radiation.

Abstract

This thesis details an investigation into the effect of a duct on acoustic source radiation. The duct is assumed to be hard-walled, hollow, have a constant cross-section and have no axial flow. This study was motivated by the increasing tendency for ducted propellers to be located close to the open end relative to the acoustic wavelength. Investigations are conducted using the widely accepted semi-infinite duct model, formed from the Wiener–Hopf technique, as well as a new finite length duct formulation. In this study, the conditions are established under which duct effects on source radiation are considerable, and the duct plays a governing role on the overall acoustic radiation of the source. The duct acoustic model assumes the acoustic pressure field can be reconstructed from a summation of propagating cut-on and evanescent cut-off modes, which have largely been unstudied. Systematic studies of the radiation characteristics of cut-off modes are documented in this thesis, detailing properties such as their modal directivity and radiation efficiency. It is found that cut-off modal radiation is predominantly directed towards the rear-arc with an effect of up to 5 dB at certain angles and a more general increase of 3 dB in the rear-arc far-field radiation. In many cases, sources close to the duct open end relative to the acoustic wavelength are shown to be largely unaffected by the duct, causing equal free-field and ducted radiation. The effect of source axial position and varying duct length are shown to have a diminishing effect on the ducted source radiation as frequency is increased. At high frequency the duct has a relatively small effect on the acoustic radiation. Dipole sources located near the duct rim are shown to have a significant increase in ducted radiation, causing an omni-directional directivity – which can largely be attributed to cut-off modal radiation. Dipole sources located exactly on the duct rim are shown to have directivity characteristics solely governed by modes which are cut-off and have almost all radiation directed towards the rear-arc.

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ORCID for Alan Mcalpine: orcid.org/0000-0003-4189-2167

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