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The generation of sound by vorticity waves in swirling duct flows

Published online by Cambridge University Press:  12 April 2006

M. S. Howe  and
J. T. C. Liu
M. S. Howe
Affiliation:
Division of Engineering, Brown University, Providence, Rhode Island 02912 Permanent address: Engineering Department, University of Cambridge.
J. T. C. Liu
Affiliation:
Division of Engineering, Brown University, Providence, Rhode Island 02912
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Abstract

Swirling flow in an axisymmetric duct can support vorticity waves propagating parallel to the axis of the duct. When the cross-sectional area of the duct changes a portion of the wave energy is scattered into secondary vorticity and sound waves. Thus the swirling flow in the jet pipe of an aeroengine provides a mechanism whereby disturbances produced by unsteady combustion or turbine blading can be propagated along the pipe and subsequently scattered into aerodynamic sound. In this paper a linearized model of this process is examined for low Mach number swirling flow in a duct of infinite extent. It is shown that the amplitude of the scattered acoustic pressure waves is proportional to the product of the characteristic swirl velocity and the perturbation velocity of the vorticity wave. The sound produced in this way may therefore be of more significance than that generated by vorticity fluctuations in the absence of swirl, for which the acoustic pressure is proportional to the square of the perturbation velocity. The results of the analysis are discussed in relation to the problem of excess jet noise.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 81 , Issue 2 , 24 June 1977 , pp. 369 - 383
Copyright
© 1977 Cambridge University Press

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