Broadband sound radiated from subsonic rotors

Recent theoretical work on sound sources in subsonic turbomachinery is applied to the correlation of axial-flow fan and compressor noise measurements. Correlations are presented for directivity and sound-power spectra associated with broadband noise. A simple explanation is given to account for the relative amounts of sound power transmitted upstream and downstream from a single-stage fan

Acoustic Energy and Momentum in a Moving Medium

By exploiting the mathematical analogy between the propagation of sound in a non-homogeneous potential flow and the propagation of a scalar field in a background gravitational field, various wave ``energy'' and wave ``momentum'' conservation laws are established in a systematic manner. In particular the acoustic energy conservation law due to Blokhintsev appears as the result of the conservation of a mixed co- and contravariant energy-momentum tensor, while the exchange of relative energy between the wave and the mean flow mediated by the radiation stress tensor, first noted by Longuet-Higgins and Stewart in the context of ocean waves, appears as the covariant conservation of the doubly contravariant form of the same energy-momentum tensor.Comment: 25 Pages, Late

The effects of viscosity on sound radiation near solid surfaces

Although the acoustic analogy developed by Lighthill, Curle, and Ffowcs Williams and Hawkings for sound generation by unsteady flow past solid surfaces is formally exact, it has become accepted practice in aeroacoustics to use an approximate version in which viscous quadrupoles are neglected. Here we show that, when sound is radiated by non-rigid surfaces, and the smallest dimension is comparable to or less than the viscous penetration depth, neglect of the viscous-quadrupole term can cause large errors in the sound field. In addition, the interpretation of the viscous quadrupoles as contributing only to sound absorption is shown to be inaccurate. Comparisons are made with the scalar wave equation for linear waves in a viscous fluid, which is extended using generalized functions to describe the effects of solid surfaces. Results are also presented for two model problems, one in a half-space and one with simple cylindrical geometry, for which analytical solutions are availabl

Corrigendum/Erratum to: Aerodynamic sound production in low speed flow ducts [79 (2) (1981) 263-289]

An error has occurred in the presentation of experimental data in Figs. 7, 9, and 10. The error can be corrected by subtracting 6 dB from the vertical scales; equivalently, the values of K2(St) presented in these figures should be divided by 4. The correction applies to all data presented in Figs. 7, 9, and 10

Active control of source sound power radiation in uniform flow

Many suggested applications of active noise control involve the presence of a background air flow such as in ventilation ducts, exhaust stacks and the radiation from turbofan engines. This paper is a theoretical study aimed at assessing the effect of a uniform flow on the ability of simple sources actively to control free field sound power radiation. In particular, the minimum sound power radiated by two point volume velocity sources situated in an unbounded uniform fluid which moves with a uniform steady velocity is determined. Also the maximum sound power that can be absorbed by a point volume velocity source from an incident plane wave is determined.</p

Active control of source sound power in uniform flow

This paper is mainly a theoretical study of the minimum sound power radiated by two point volume velocity sources situated in an unbounded uniform fluid which moves with a uniform steady velocity. The paper also calculates the maximum sound power that can be absorbed by a point volume velocity source from an incident plane wave in a uniform flow field. The study is in response to the growing interest in applying active control to reduce the sound radiated from aircraft turbofan intakes on approach. For the idealised volume velocity source distribution used in this paper convection effects, to leading term, are found to be of order M2, where M is the Mach number. Mean flows with high subsonic Mach numbers are therefore found to cause a significant deterioration in the radiated sound power that can be reduced at a fixed source separation distance.</p