Time-Domain Analysis of Subsonic Jet Noise

Abstract

Following on previous works showing that jet noise has significant intermittent aspects, the present work assumes that these intermittent events are the dominant feature of jet noise. A definition and method of detection for intermittent noise events is devised and implemented. Using a large experimental database of acoustically subsonic jets with different acoustic Mach numbers (Ma = 0.5 – 0.9), diameters (D = 2.54, 5.08, & 7.62 cm), and exit temperature ratios (ETR = 0.84 – 2.70), these events are extracted from the noise signals measured in the anechoic chamber of the NASA Glenn AeroAcoustic Propulsion Laboratory. It is shown that a signal containing only these events retains all of the important aspects of the acoustic spectrum for jet noise radiating to shallow angles relative to the jet downstream axis, validating the assumption that intermittent events are the essential feature of low angle jet noise. The characteristics of these noise events are analyzed showing that these events can be statistically described in terms of three parameters (the variance of the original signal, the mean width of the events, and the mean time between events) and two universal statistical distribution curves. The variation of these parameters with radiation direction, diameter, velocity, and temperature are discussed. A simple model for this kind of signal is formulated and used to derive a relationship between the characteristics of the noise events and the fluctuations in the integrated noise source volume.