In-duct measurement techniques for the characterisation of broadband aeroengine noise

Abstract

The focus of this thesis is the measurement of broadband fan noise, which due to reductions in tonal and jet noise, is now the dominant source at approach, and a major contributor at takeoff. This thesis proposes three new in-duct measurement techniques for the characterisation and measurement of broadband fan noise. A complete characterisation of broadband noise involves determining the sources of sound, their location,and the sound field they generate.The first new in-duct measurement technique uses inverse methods to determine the noise source strengths on a ducted fan. The novel aspect of this technique is that it allows source strengths to be determined both in the rotating (or rotor-bound) reference frame and stationary (stator-bound) reference frame.The second technique is the development of an in-duct, rotating focus beamformer that allows beam forming in both rotating and stationary reference frames using an in-duct microphone array. The ability of the beam former to determine the strengths of the rotor-based and stator-based sources is demonstrated using numerical simulations, with particular emphasis on the determination of the relative contributions of the rotor and the stator to overall broadband noise. The third measurement technique is designed to allow radiated directivity patterns to be predicted from in-duct measurements. This technique allows the prediction of radiated directivity from hollow no-flow ducts, fan inlets and annular exhaust ducts from in-duct measurements. The technique is also validated using experimental data acquired from a no-flow duct rig. Finally, an in-situ phase calibration method for in-duct axial arrays is proposed. This technique allows the rapid calibration of microphone arrays used in the three measurement techniques presented in the thesis. The technique is validated using experimental data.

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