Aeroacoustic Assessment of Performance of Overtip Liners in Reducing Airfoil Noise

Abstract

The reduction of fan broadband noise in the next generation of ultrahigh-bypass-ratio engines remains a key technology challenge for the foreseeable future. The overtip-rotor liner concept has been studied as a technology with the potential to further reduce fan noise, and significant noise reductions have been measured. This paper describes a fundamental experimental evaluation that represents the overtip-rotor liner as a static airfoil with its tip located over a flat plate containing a flush-mounted lined insert and separated from the airfoil tip by a small gap. Differences in measured far-field sound spectra and in source power estimates derived from postprocessed spiral array data have shown broadband gap noise reductions of 5–10 dB, even in the absence of a tip gap. Overtip liners are found to suppress noise sources when located in the immediate vicinity, irrespective of the generation mechanism, mainly due to backreaction effects on the source. An analytical prediction model for the overtip liner noise reduction, based on a point source located over an infinite lined plane, is evaluated and compared with the experimental data. The model captures the backreaction effects and provides qualitative agreement with the measured data, including the variation of noise reduction with increasing gap size.