Helmholtz Resonator Liner with Flexible Walls

Abstract

Liners are part of every modern commercial aero-engine. Usually, they are installed in the engine intake; but also in the bypass duct or in the outlet some liners can be found. Despite the decrease of overall engine noise due to the increase of bypass-ratio (BPR), cut-off design for rotor-stator combinations, and various other means, there is an increasing demand for efficient broad-band noise absorption with the final goal of further overall noise reduction. This demand is mainly caused through the reduction of the dominating tonal components, but might be also connected to an increase in broadband noise itself. In addition, the increase in BPR requires shorter nacelles in order to reduce associated drag and weight penalties. This leads not necessarily to a smaller area for liner installation e.g. in the intake of the engine, but to a shorter length of the intake and thereby to a shorter propagation distance of emitted noise over a lined surface in axial direction. State of the art for inlet liners are singleand double-degree of freedom (SDOF and DDOF) liners consisting of cells of fixed size (for DDOF for instance with a septum dividing the individual cells) covered with a perforated face sheet, and a rigid back plate. The whole liner structure must be very robust, but at the same time of light weight, withstand various fluids and environmental conditions etc. Current liners are