Nonlinear Noise Propagation from a Fully Expanded Mach 3 Jet

Abstract

The high-intensity noise radiated by an unheated and fully expanded Mach 3 jet is investigated experimentally using arrays of microphones placed in the acoustic near and far far–field of the jet. Under these conditions Mach wave radiation is the most prominent component of turbulent mixing noise in this shock-free supersonic jet. Measurements of the pressure time series are acquired along a grid in the (x,r)-plane in order to quantify the degree of non-linearity in the pressure waveforms. The topography of the OASPL reveal a highly directive sound propagation path emanating from the post-potential core region at x/Dj = 20 and along 45 � from the jet axis; this coincides with the Mach wave radiation angle that is expected of this flow. Likewise, the spatial growth saturation and decay of the OASPL due to wave steepening is shown to peak around 127Dj from the source field identified in the post-potential core region. Various metrics for quantifying the degree of nonlinearity in the acoustic waveforms are computed and include skewness of the pressure derivative, wave steepening factor and the number of zero crossings per unit time. Each metric is shown to produce a slightly unique propagation path, albeit they all follow along similar paths as the OASPL. A second effort focuses on employing an augmented Burgers equation to numerically propagate the temporal waveform at 60Dj outward to a distance of 140Dj (along the same 45 � path coinciding with the maximum OASPL). Both linear and nonlinear forms of the algorithm are employed and include effects of absorption, dispersion and geometrical spreading. Comparison of the predicted and measured waveforms and spectra reveal how the sound radiation is fairly linear over the spatial domain considered in this study.