In this review we describe current scientific and technological issues in the quest to reduce aeroengine noise, in the face of predicted rapid increases in the volume of air traffic, on the one hand, and increasingly strict environmental regulation, on the other. Alongside conventional ducted turbofan designs, new open-rotor contra-rotating power plants are currently under development, which present their own noise challenges. The key sources of tonal and broadband noise, and the way in which noise propagates away from the source, are surveyed in both cases. We also consider in detail two key aspects underpinning the flow physics that continue to receive considerable attention, namely the acoustics of swirling flow and unsteady flow-blade interactions. Finally, we describe possible innovations in open-rotor engine design for low noise.

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An analytical model is presented for the wake interaction tones produced by a contra-rotating propeller. We re-cast the usual far-field radiation formulae as a double integral over a nominal propeller source annulus. Assuming that the number of blades on both propellers is large, we evaluate the integral asymptotically in terms of its leading-order contributions from interior stationary or boundary critical points which represent the specific locations on the propeller annulus that dominate the sound radiation. The asymptotic approach is powerful producing results in the form of one-line algebraic formulae that contain no integrals or special functions yet remain accurate. The asymptotics show that sweep is not necessarily beneficial and can cause the blade design to become critical for particular tones and directions in terms of a continuum of interior points distributed along a line on the propeller source annulus producing a higher-order result and thus an enhanced radiated sound field. The paper also shows how the interior points are completely consistent with the sub- or super-critical gust response of a swept blade. Tones with low and zero azimuthal mode order are treated as special cases and the asymptotics show that, as the mode order reduces, the radiated sound becomes concentrated around the flight axis where even higher-order solutions are possible, including rings and annuli of stationary points around the propeller annulus. Full numerical calculations confirm the accuracy of the asymptotic approach.

This paper presents broadband noise predictions from uninstalled contra-rotating open rotors (CRORs). CRORs are being increasingly investigated as an alternative to single propellers and turbofan engines for power plants on aircraft, since they yield a significant reduction in fuel burn for short-haul flights. However, there is the need to develop schemes by which CROR noise can be predicted and reduced.Using semi-analytical prediction schemes, the principal broadband noise source mechanisms (rotor trailing edge noise and rotor-rotor interaction noise) are investigated, leading to strategies for their mitigation. For the configuration considered, trailing-edge broadband noise is predicted to be dominant at low rotor loading conditions (typical of cruise and approach) whereas rotorrotor interaction broadband noise is predicted to be dominant at high rotor loading conditions (typical of take-off). A systematic parameter study is also presented in which the dependence of CROR broadband noise on the rotor-rotor gap, rotor speeds and blade numbers are investigated at constant engine power, torque split and solidity.

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