Propeller tone scattering

Because of recent uncertainty in the price of fuel and also because of concerns over carbon emissions from aircraft, aeronautical engine manufacturers are exploring various alternative propulsors to the turbofan engine. One such propulsor is the advanced open rotor - which consists of two coaxial, counter-rotating propellers, and which promises a significant fuel burn reduction relative to current generation turbofan engines. However, in order for the open rotor to become a viable propulsor it must meet stringent noise emission targets. Therefore, having noise prediction tools which allow the quick and accurate assessment of the noise produced by a large number of advanced open rotor designs are a necessary part of the design process. Frequency domain formulae are often used for this purpose. These formulae contain a Fourier integral in the coordinate coaxial with the propeller axis, which in the near-field must be evaluated numerically at a high computational cost. For far-field noise predictions, and if it is assumed that the propeller operates in an environment with no scattering surfaces, then the Fourier integral can be evaluated in a relatively straight-forward manner, and at low computational cost, using the method of stationary phase. Consequently, most of the published literature regarding propeller and advanced open rotor noise ignores scattering from surrounding aircraft structures. In this paper methods for including the effect of scattering from structures such as the fuselage and the open rotor hub are presented. Some results showing the effect of scattering on the radiated sound field are also presented.