Noise generation by turbulence-propeller interaction in asymmetric flow

This paper is concerned with a particular source of both broadband and tonal aeroengine noise, termed unsteady distortion noise. This noise arises from the interaction between turbulent eddies, which occur naturally in the atmosphere or are shed from the fuselage, and the rotor. This interaction produces broadband noise across a broad frequency spectrum. In cases in which there is strong streamtube contraction, which is especially true for open rotors at low-speed conditions (such as at take-off or for static testing), tonal noise at frequencies equal to multiples of the blade passing frequency are also produced, owing to the enhanced axial coherence caused by eddy stretching. In a previous paper (Majumdar & Peake, J. Fluid Mech., vol. 359, 1998, pp. 181-216), a model for unsteady distortion noise was developed in axisymmetric flow. However, asymmetric situations are also of much interest, and in this paper we consider two cases of asymmetric distortion: firstly that induced by the proximity of a second rotor, and secondly that caused by non-zero inclination to the flight direction, as found at take-off. This requires significant extension of the previous axisymmetric analysis. We find that the introduction of asymmetric flow features can have a significant decibel effect on the radiated sound power. For instance, in low-speed conditions we find that the tonal level is reduced significantly by the proximity of a second rotor, compared to the axisymmetric case, while the effect on the broadband levels is rather modest.