Mode detection in turbofan inlets from acoustic pressure measurements in the radiated field

Castres, F., Joseph, P.F. and Astley, R.J. (2004) Mode detection in turbofan inlets from acoustic pressure measurements in the radiated field. 10th AAIA/CEAS Aeroacoustics Conference, Manchester, UK. 09 - 11 May 2004. 16 pp .

Abstract

Knowledge of the modal content of the sound field radiated from a turbofan inlet is important for source characterisation and for helping to determine noise generation mechanisms in the engine. An inverse technique for determining the mode decomposition is proposed using pressure measurements from mode on the Turbulence Control Screen (TCS). The TCS offers a useful platform for locating microphones since they are often fitted to engines during ground testing to smooth the integrated flow. Array performance is tested on computer-generated data from modal radiation predictions using a model based on the Kirchhoff approximation for flanged ducts with no flow. An overdetermined system of linear equations that accounts for the radiation at the TCS due to all cut-on modes and nearly cut-on modes is constructed from this model and is inverted to determining mode amplitudes. The sensitivity of the reconstructed mode amplitudes to noise is determined by the condition number of the radiation matrix, containing modal directivity functions predicted at each sensor location. This paper discusses the number and configuration of microphones on the TCS needed for robust and accurate modal inversion. Finally, this paper discusses the use of constraining the solution by regularisation in order to improve inversion robustness to noise.

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