Aeroacoustic Assessment of Performance of Overtip Liners in Reducing Airfoil Noise

Abstract

The reduction of fan broadband noise in the next generation of Ultra-High By-pass-Ratio (UHBR) engines remains a key technology challenge for the foreseeable future. The Over-Tip-Rotor (OTR) 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 Over-Tip-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 post-processed spiral array data have shown broadband gap noise reductions of 5-10 dB, even in the absence of a tip gap. Over-tip liners are found to suppress noise sources when located in the immediate vicinity, irrespective of the generation mechanism, mainly due to back-reaction effects on the source. An analytical prediction model for the over-tip 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 back-reaction effects and provides qualitative agreement with the measured data, including the variation of noise reduction with increasing gap size.

Modelling of ducted noise sources in the proximity of acoustic liners

“…In contrast, the power radiated to the 'farfield' is finite in all cases and generally reduces when the source is located closer to the lined wall. This corresponds to experimental observations in [28,29,30].…”

Development of the SmartAnswer Demonstrator: a Didactic Wind Tunnel for Aeroacoustic Applications

“…Over-the-rotor liners An important noise component in the turbofan engines is that which is generated by the interaction of the flow with the rotor blades and stator vanes. In particular, the tip clearance is a relevant source of the self-noise because the pressure difference between the two sides of a blade starts a tip-leakage vortex, which interacts with the blade tip edge [22,23]. To this end, the over-the-tip liners (OTLs) have been investigated during the past decades as a technology with the potential to attenuate the fan noise in turbofan engines [24].…”

“…The set-up of the OTL in the demonstrator is based on the wind tunnel experiments conducted by Palleja-Cabre et al [22], in which the rotor and OTL are represented by a static airfoil with its tip located over a flat plate containing a flush-mounted liner insert and separated from the airfoil tip by a small gap of 5 mm. On the exterior wall of the test section, two hinges allow easy swapping of two different panels, i.e., a hard wall or a liner in the over-tip position with a surface of 0.21 m × 0.12 m. An elastic O-ring is used to seal the panels to the test section.…”

“…On the exterior wall of the test section, two hinges allow easy swapping of two different panels, i.e., a hard wall or a liner in the over-tip position with a surface of 0.21 m × 0.12 m. An elastic O-ring is used to seal the panels to the test section. The geometric design of the liners is based on the point source prediction model by Palleja-Cabre et al [22], adequately tuned for the airfoil chord and flow speed of the demonstrator. The mitigation technology is a 10 mm thick, one degree of freedom (passive) liner, consisting of a cubic aluminum honeycomb structure, with a wire mesh on the flow side.…”

Modeling of Over-Tip-Rotor Liners for the Suppression of Fan Noise

“…Although that work does not involve OTR liners, it shows that the coupling of the liner and nearby sound sources should be considered when targeting the liner optimum impedance. A recent study on the reduction of tip noise sources with an over-tip liner has been reported in [7] using a static aerofoil. It shows that the noise reduction benefits of over-tip liners are greater when the noise sources are close to the liner surface.…”