A 3D printed acoustic metamaterial liner to reduce low-frequency aeroengine noise

Abstract

As the level of air travel continues to increase, so too does the demand for better noise-reduction technologies for aircraft. The environmental noise produced by aircraft has been shown to have a significant adverse effect on the health and well-being of communities living near airports and along flight paths. Engine noise is one of the two main sources of aircraft noise. The current trend in engine design to increase the nacelle radius and reduce the nacelle length adds constraints to the size and weight of novel acoustic treatments. The use of additive manufacturing allows novel acoustic materials to be developed which would not be feasible using traditional manufacturing techniques. This holds significant promise for the development of novel acoustic liners for aircraft engine nacelles, which are able to meet the growing size and weight concerns.

This works follows the development of an acoustic metamaterial from the initial design optimisation up to an assessment of the liner?s performance in NASA's Advanced Noise Control Fan (ANCF) rig, now located at the University of Notre Dame, which has a Technology Readiness Level (TRL) of 3. The design optimisation utilised numerical and analytical techniques to maximise the absorption of the liner under normal incidence. The design was made possible by the use of additive manufacturing which allowed geometries to be explored and implemented that would not have been possible using traditional manufacturing techniques. The optimised geometry of the liner was assessed in grazing incidence in Trinity College Dublin (TCD) and the Laboratoire d'Acoustique de l'Universit? du Mans (LAUM) at a range of SPL and flow conditions. These preliminary assessments identified the frequencies and flow conditions at which the liner performed well.

The acoustic metamaterial was printed in 237 parts which were installed in the ANCF rig. The liner was tested as a function of RPM and reduced both the broadband noise and tonal noise amplitudes, particularly in the region of 1000 Hz. In this region, the first harmonic of the blade passing frequency was reduced by 18.5 dB when the ANCF was tested at its nominal operating speed of 1800 RPM. At this speed, an overall PWL reduction of 2.7 dB was achieved across all harmonic bands.