On the noise reduction mechanisms of over-tip liners

Ramesh Raja Subramanyam; Suresh Palani; Paruchuri Chaitanya; Sergi Palleja-Cabre

doi:10.1121/10.0023957

On the noise reduction mechanisms of over-tip liners

J Acoust Soc Am. 2024 Jan 1;155(1):29-43. doi: 10.1121/10.0023957.

Authors

Ramesh Raja Subramanyam¹, Suresh Palani¹, Paruchuri Chaitanya¹, Sergi Palleja-Cabre¹

Affiliation

¹ Institute of Sound and Vibration Research, University of Southampton, Southampton, Hampshire SO17 1BJ, United Kingdom.

PMID: 38169521
DOI: 10.1121/10.0023957

Abstract

The application of acoustic liners near/directly over a sound source has gained significant interest for the excess noise reduction achieved with Over-the-Rotor (OTR) liners compared to the conventional liner installations at the intake of an aero-engine. However, the mechanism of noise reduction achieved in the OTR liners is not clearly understood. This paper aims to explain this mechanism by considering a static monopole source placed over a finite liner insert with a zero background mean flow. This has been investigated numerically using COMSOL Multiphysics in a half-space domain and compared with reference analytical solutions for infinite lined walls. One of the key findings of the paper is the underlying physics of the source modification mechanism, which has been found to be the interference between the primary noise source and a secondary noise source forming on the liner surface. It is identified through an optimal impedance study that this back-reaction mechanism is dominant when the source is located for a normalised tip gap, e/λ<0.25, where e is the distance between the source and the liner surface and λ is the acoustic wavelength. Within this region, there exists an optimum normalised liner length, L / e providing a maximum insertion loss.