Fan proximity acoustic treatments for improved noise suppression in turbofan engines

Fan noise is one of the dominant sources of aircraft engine noise, both at approach and at take-off. Improved attenuation of fan noise with acoustic liners and the reduction of fan noise at source remain key technology challenges for the foreseeable future. Over- Tip-Rotor (OTR) acoustic treatments have been investigated experimentally during the last decade and significant fan noise reductions have been measured, most recently at NASA by using a rotor-alone rig and multiple lined circumferential grooves. This thesis aims at improving our understanding of the noise reduction mechanisms of OTR liners through the development of an analytical prediction model. The fan noise is modelled with point or distributed monopole and dipole sources based on Green's functions for infinite hard or lined cylindrical ducts containing uniform mean flow; these are combined with an anechoic or unflanged inlet termination and an embedded lined section of finite length representing the 'rotor-alone' source with its OTR liner. Key aspects of the propagation/ attenuation and liner impedance modelling are cross verified with reference FEM solutions. OTR predictions of liner insertion loss are obtained for comparison with the NASA experimental data, which entailed coupling of the propagation model with that of a partially non-locally reacting impedance model, and these yielded peak broadband in-duct noise reductions of up to 4 dB, in line with the measurements. To complement the analytical work, wind tunnel experiments have been conducted at ECL on a simplified over-tip liner configuration. The rotor and OTR liner were represented by a static aerofoil with its tip located over a flat plate containing a flush-mounted liner insert and separated from the airfoil tip by a small gap. The measured sound spectra exhibit peak broadband gap noise reductions of 5-10 dB due to the OTR liner and noise reduction in trailing edge noise of up to 5 dB in the absence of a gap.