On Modelling the Acoustic Forcing Functions of the Near-Field Noise of High-Speed Jets

Abstract

As part of a research programme into acoustic fatigue for the UK Ministry of Defence, QinetiQ is developing a suite of prediction tools for the jet near-field acoustic loading on the aircraft structure. For the mathematical modelling and prediction of the jet noise sources, a pragmatic approach is being used, which combines classical theory with model scale noise measurements. This paper describes progress in the mathematical synthesis of the pressure time history of the jet noise impinging on the airframe. This is needed for a new nonlinear structural analysis tool developed by QinetiQ that requires the actual pressure fluctuation history as an input parameter, when predicting the structural response under high acoustic loading. For the synthesis of the input pressure fluctuation history, a technique, involving Finite Impulse Response (FIR) theory, has been devised that uses the mathematical models of the jet noise sources and their spatial distribution. The technique ensures that the relevant statistical properties (spectrum, level and skewness) of the random jet noise signal are replicated at each point, while ensuring that the spatial coherence is correct between different points in the noise field. Noise measurements made on modern combat aircraft are used in the validation process.