Improvement of one- and two-layer liner design with bias flow

Abstract

The coupling of the resistance and reactance in acoustic liners has in the past limited aircraft engine liner design performance. This is due to the parametric dependence of the impedance on the perforated sheet geometry and cavity depths both having a significant effect on both parts of the impedance. For normal incidence liner impedance, designing a liner with its reactance approaching zero and resistance approaching unity is essential to the goal of producing maximum broadband absorption. Introducing a mean flow through the liner has shown that the liner resistance can be changed with minimal effect on the reactance. Liner geometry can be designed to achieve a broadband reactance that is as close to zero as possible. The resistance can then be increased to near unity by allowing a mean bias flow through the liner to produce the maximum broadband absorption. With sufficient modeling of liner impedance, an optimization routine can be used to find the liner characteristics that produce maximum absorption over a broad range of frequencies and sound-pressure levels. It will be shown that using bias flow can improve normal incidence liner absorption by 17% for single layer liners and 7% for double layer liners.