Modelling counter rotating open rotor installation noise sources

Abstract

Open rotors are an engine technology that could bring a step change in propulsive efficiency for the next generation of aircraft. As such, there is a large research effort that aims to accurately predict the noise radiated by open rotors. This thesis focuses on the noise radiated by an installed open rotor. In this configuration one of the major noise sources could be the ingestion of a turbulent boundary layer that has developed on the fuselage of the aircraft. This thesis presents an analytical model that predicts this noise source. The analytical model is based on the simplified rotor noise model of Amiet with blade-to-blade correlation modelled. The model is extended in three ways to include the boundary layer ingestion noise source. The first extension is to include a numerical switch to model the partial loading of a rotor ingesting a boundary-layer. The second extension is to include wall-effects using the method of images. The assumptions that are made in the method of images are tested by examining the simpler case of a two-dimensional aerofoil ingesting turbulence in proximity to a hard-wall. This is done by extending Amiet’s two dimensional leading-edge noise model and comparing the predictions from this model to two Computational Aeroacoustic (CAA) simulations. The first CAA simulation uses the method of images to model the wall and the second uses a physically representative hard-wall boundary condition to model the wall. By comparing the differences between the analytical predictions and the CAA simulations, it is shown that the method of images accurately predicts wall-effects except for a small range of observers that are in the shadow-zone of the aerofoil. The method of images is then implemented in the rotor noise model to predict wall-effects for a rotor. Finally, an axisymmetric anisotropic turbulence model is used to approximate boundary layer turbulence. The extended rotor noise model is validated by comparing it to experimental measurements and to a time-domain rotor noise model that does not model boundary layer turbulence.

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