Time-Domain Analysis of Excited Subsonic Jet Noise

Abstract

Building on previous work showing that far-field jet noise has significant intermittent aspects and the statistical analysis of those intermittent aspects, the present work applies the same analysis techniques to an excited jet. Using an experimental database covering several operating conditions [jet Mach number ( Mj = 0.9), nozzle diameter ( D = 2.54 cm), and jet stagnation temperature ratios (TTR = 1.0 – 2.5)] and a wide array of excitation parameters (azimuthal mode mF = 0, 1, & 3 and Strouhal number StDF = 0.09 – 3.0), these events are extracted from the far-field noise signals measured in an anechoic chamber. This database is analyzed to determine how the noise event characteristics are altered by excitation. The relationship between the noise events and the flow-field dynamics of the excited jet are discussed. Analysis of the excited jet reveals the existence of a resonance condition. When excited at the resonance condition, large noise amplification occurs – this is associated with nearly every large-scale structure producing a noise event. Conversely, noise reduction occurs when only one noise event occurs per several large-scale structures. The impact of the azimuthal extent of large-scale structures is explored using a wave-packet model to provide support to the discussion of azimuthal mode radiation efficiency. The results indicate that there is a competition for flow energy among neighboring structures that dictates if and how their dynamics will produce noise that radiates to the far-field. Furthermore, the azimuthal model supports the conclusion that higher modes are less efficient radiators. These various factors provide support for the conclusion that the mechanism of noise reduction involves inducing the jet into a condition where the naturally occurring structures are suppressed by excited structures which are less efficient radiators (i.e. higher azimuthal modes at frequencies where neighboring structures destructively compete for flow energy).