Mode scattering by hard strips in lined ducts

Abstract

When mounting acoustic liners inside the inlet duct of an aircraft jet engine, hard strips exist in-between liner sections in order to hold them in place. It has been generally acknowledged, from experiments, that the existence of hard strips inside lined ducts affects the attenuation behavior of the duct. The acoustic energy is scattered and rearranged among different modes so that it might be transferred into modes, which are less attenuated by the liner. Moreover, cut-off modes may scatter into cut-on modes. A previous paper by the authors was concerned with the calculation of the wavenumbers and modeshapes for all possible modes that can propagate inside an infinite duct lined with either locally or non-locally reacting liners with hard strips. The point-matching method was 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. In this paper, flow is included in the locally reacting liner case, which is more interesting for the aeronautics applications. Moreover, the duct is made finite and surrounded by two infinite hard inlet and outlet ducts. By using mode matching, it is possible to input any mode at the inlet side and to study the exiting modes on the outlet side. Comparisons are made between different hard strip cases and the splice-less liner case.