Acoustic modelling of Over-the-Rotor liners

Abstract

Due to the additional noise reduction obtained with Over-the-Rotor (OTR) liners compared to the conventional liners at the intake of an aero-engine, the application of acoustic liners close to or immediately over a sound source has attracted substantial interest. This enhanced noise reduction in OTR liners is attributed primarily to the source modification effect (caused by the back reaction). The exact mechanism of source modification is not entirely understood, though. This paper seeks to explain this mechanism, by separating and measuring the source modification effect from the overall noise reduction obtained by the OTR liner configuration. The problem is investigated numerically in COMSOL Multiphysics by assuming a static monopole point source over a finite liner insert in the half space domain. Background mean-flow is considered to be zero in this study, and the insertion loss is predicted for varying source locations and liner lengths. This paper has determined that the back reaction effect is significant only in the region within the e/$\lambda = 0.25$, where $e$ is the distance between the source and the liner wall and $\lambda$ is the acoustic wavelength. An optimal liner length ($L/e\approx15$) that provides the maximum insertion loss as a result of source modification can be predicted within this region. It has also been discovered that the liner length, source location, and frequency have an impact on the liners' ideal impedance.