Intermittent Nature of Subsonic Jet Noise

Abstract

Following on previous works showing that far-field 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 jet acoustic Mach numbers (), nozzle diameters (, 5.08, and 7.62 cm), and jet exit temperature ratios (0.84–2.70), these events are extracted from the far-field noise signals measured in the anechoic chamber of the NASA Glenn Aero-Acoustic Propulsion Laboratory. It is shown that a signal containing only these intermittent events retains all of the important aspects of the acoustic spectrum for jet noise radiating to aft angles, validating the assumption that intermittent events are the dominant feature of aft angle jet noise. Statistical analysis of the characteristics of these noise events reveals that these events can be 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 noise-radiation direction, nozzle diameter, and jet velocity and temperature are discussed. There is a strong correlation between the mean width of the intermittent events and the mean time between the events () in all the cases investigated in this work, implying a strong link between dynamics governing the two quantities.