Optimization of Annular and Cylindrical Liners for Mixed Exhaust Aeroengines

The increase in bypass ratios for modern day turbofan engines has led to fan rearward broadband noise emerging as one of the principal aircraft noise sources. The most widespread method for mitigating this noise source is the installation of acoustic liners in the ducting downstream of the fan. Mixed exhaust designs have the potential to be quieter than a three quarter cowl aeroengine due to the additional surface available for liner application. As a result, the Silent Aircraft concept design contains an embedded distributed propulsion system to exploit this opportunity. This paper outlines a suitable method for optimizing the annular and cylindrical liners that make up the exhaust system. To gain maximum benefit, liners should be designed to absorb the modal disturbances that are the most significant for an observer on the ground. The optimization uses an advanced noise footprint cost function to achieve this. Consequently, the liners and the length of the installation have been chosen to meet the aggressive noise target set for the aircraft. In addition, we explore the effect that modal scattering between axial liner junctions can have on the acoustic performance of the system. We find that energy interchange between radial modes will often have a detrimental effect on the overall attenuation, although this can be addressed by selecting the order of the liners in a more considered way. The assumption that scattering between liners can be neglected during optimization to allow design of the individual components therefore appears to be valid in this case. However, subsequent attention has to be directed to the order in which they are placed. Current and next generation mixed exhaust aeroengines will be restricted to smaller liner segment lengths than the Silent Aircraft. This analysis suggests that scattering and hence liner order is likely to more important for these designs.