Acoustic Shock Formation in Noise Propagation During Ground Run-Up Operations of Military Aircraft

A distinctive feature of many propagating, high-amplitude jet noise waveforms is the presence of acoustic shocks. Metrics indicative of shock presence, specifically the skewness of the time derivative of the waveform, the average steepening factor, and a new wavelet-based metric called the shock energy fraction (SEF), are used to quantify the strength and prevalence of acoustic shocks within waveforms recorded 10-305 m from a tethered military aircraft. The derivative skewness is more sensitive to the presence of the largest and steepest shocks, while the ASF and SEF tend to emphasize aggregate behavior of the entire waveform. These metrics are applied at engine conditions ranging from 50% to 150% engine thrust request, over a wide range of angles and distances, to assess the growth and decay of shock waves. The responses of these metrics point to significant shock formation occurring through nonlinear propagation out to 76 m from the microphone array reference position. Although these strongest shocks decay, the metrics point to continued nonlinear propagation in the far-field, out to 305 m. Many of these features are accurately characterized using a nonlinear propagation scheme based on the Burgers equation, but this scheme fails to account for multipath interference and significant atmospheric effects over the long propagation distances, resulting in an overestimation of nonlinearity metrics.