Hard strips in lined ducts

Abstract

Acoustic liners are widely used to attenuate sound waves inside aircraft jet engines. In most cases, the existence of hard strips in the lined ducts is inevitable. Previous research has proved that segmenting the liner and the positioning of the liner segments affect the attenuation characteristics of the liner. The aim of this work is to investigate these effects, and to compare the properties of circumferentially segmented duct liners with those of uniform liners, in order to identify any potential benefits of circumferentially segmented liners. The point-matching method is used to analyze the problem. It is a straightforward numerical method based on a closed form ansatz, which fulfils the governing equations and is matched to the boundary conditions point-wise. A computer code is used to obtain the wave numbers of the different modes, from which the transmission loss for each mode can be calculated at the desired range of frequencies. An earlier paper by the authors was published on this subject. It was a step on the road towards the solution. It was intended to test the method in solving simple cases of circumferential variation of only nonlocally reacting liners. The present study includes both locally reacting and non-locally reacting liners. More realistic cases are investigated. It is found that non-locally reacting liners are less affected by the presence of hard strips than locally reacting liners, where the behavior of each mode depends on the number and width of hard splices in the duct. © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.