Jittering wave-packet models for subsonic jet noise

Abstract

Three simplified models for wave-packets representing large-scale structures in subsonic jets are presented. These models consist of temporal changes of a basic wave-packet shape comprising a convected wave whose amplitude is modulated by a Gaussian function. We consider the temporal variations of the amplitude and spatial extent of the spatial modulation, separately for the first two model problems; and for the third model both effects are considered together. Analytical expressions for the radiated far field acoustic pressure are obtained for the first and third models. We show that the temporal, intermittent changes of such wave-packet shapes can keep the highly directional behaviour and produce high-amplitude bursts in the acoustic field; such intermittency, which is observed in subsonic jets at the end of the potential core, may help explain the higher noise levels and the noise intermittency of high subsonic jets at lower emission-angles. With the definition of an efficiency ratio, relating the radiated acoustic power to the fluctuation energy of the source, we show that the source becomes more powerful as its temporal localisation is increased. The utilisation of LES data of a Mach 0.9 jet to input a temporally-changing wave-packet leads to predictions of the radiated sound field within 1.5dB of the LES acoustic pressure for the low axial angles. © 2010 by Peter Jordan.