Aft fan noise reduction with a lined afterbody

Aft fan noise is becoming a more dominant source as engine bypass ratio is increased and improved methods are for required for its control. Acoustic linings are an effective means of reducing internally generated noise and bypass liners are especially effective in attenuating aft fan noise. Previous calculations have shown that as the fan noise emerges from the bypass exhaust nozzle, a significant part of the sound field radiates directly out through the shear layer but there is also another propagation path, along which the sound field is first reflected from the hard ‘afterbody’ before it propagates out to the far field. It has been suggested that if the afterbody were acoustically lined, it would reduce the strength of that reflected field and hence the far field noise level. Here we describe results from some preliminary tests on a rig, using scaled, locally reacting, linear SDOF acoustic linings to simulate the conventional, internal bypass liners and also the new external Afterbody Liner (AL). The measured data from this ‘No-flow’ rig indicate that up to 3 dB PWL insertion loss can be achieved with an AL in addition to that achieved with conventional bypass liners. Calculations agree well with the measured PWL insertion loss spectrum although the measured SPL reductions are not so well predicted at some angles. This novel application of conventional liner technology to the external afterbody of an engine nacelle may provide some significant reductions in aft broadband fan noise levels although it should be emphasised that the results obtained so far are without flow.