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

Abstract

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.

Inclusion of Broadband Shock-Associated Noise in Spectral Decomposition of Noise from High-performance Military Aircraft

“…Evidence for cumulative nonlinear propagation effects for the tied-down F-35 is presented in Refs. [48,49]. The differing high-frequency slope confirm what was reported for a different aircraft engine's noise in Ref.…”

“…This combination region extends to approximately 105°, after which the addition of FSS spectrum no longer improves the agreement, and the LSS spectrum matches all except the high frequencies ( Figure 5(b)), which have elevated levels due to nonlinear propagation [48,49]. In the maximum radiation region (inlet angles approximately 110°-140° at 75% ETR), the LSS spectrum captures the overall shape of the peak region of the PSD (Figure 6(a)), but misses several important features: 1) multiple spectral peaks [43,44,45], 2) shallower high-frequency slope (of 1/ K ) resulting from nonlinear propagation [36,37,38,39,48,49], and 3) steeper low-frequency slope. All three features were noted in prior spectral decompositions of high-performance military aircraft [14,15,33], and are discussed in more detail in Sec.…”

“…5 of Ref. [49]-a significant value as analytical work has shown that a threshold of five indicates the presence of acoustic shocks [51].…”

“…Nonlinear propagation effects across near to far-field arrays were analyzed in Refs. [48] and [49]. In addition to the far and mid-field arrays, a ground-based array was deployed closer to the aircraft.…”

Study of the generation of shock waves by high-speed jets using conditional averaging

“…Notably, it is well established that the sound waves they generate are strong enough to be affected by nonlinear distorsions. 6,7 These nonlinear effects cumulate during propagation, leading to the progressive formation of shocks, as observed in measurements performed in the acoustic far fields of military jet aircrafts, 8 as well as in numerical simulations. 9 However, estimations of the propagation distances required for shock formation have shown that, in some cases, steepened waveforms were found too close from the jet to be only the result of nonlinear propagation effects.…”