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The absorption of sound by perforated linings

Published online by Cambridge University Press:  26 April 2006

I. J. Hughes  and
A. P. Dowling
I. J. Hughes
Affiliation:
Topexpress Ltd. Poseidon House, Castle Park, Cambridge CB3 0RD, UK
A. P. Dowling
Affiliation:
Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK
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Abstract

The efficiency of a perforated screen as a sound absorber can be greatly increased when a rigid surface is placed behind the screen, essentially because the sound can then interact many times with the perforations. We consider a practical application for a backed perforated screen with a bias flow through the perforations: the ‘screech liner’. This is a perforated lining which is inserted in the afterburner section of jet engines to suppress the acoustically driven combustion instability commonly known as screech. A pressure drop across the screen ensures that a bias flow of cool air is produced; this flow protects the liner from the intense heat in the afterburner.

Our analysis was developed in answer to a clear need for a theory which can predict the optimal geometry and bias flow to produce a highly absorptive liner. We show that it is theoretically possible to absorb all the sound at a particular frequency. Experimental results are presented which show encouraging agreement with the theoretical predictions.

Screech is thought to be the excitation of a transverse resonant oscillation in the jet pipe, but the insertion of a liner inevitably changes the frequency of such resonances because the boundary condition at the wall is altered. We examine the effect of a liner on the resonances which occur in a cylinder and show that a well-designed liner may suppress resonances over a range of frequencies.

The effect of the hot axial jet flow on the performance of a liner has not previously received attention. A simple model to account for this flow is included in our analysis.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 218 , September 1990 , pp. 299 - 335
Copyright
© 1990 Cambridge University Press

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