It's not...wait, it IS rocket science!

Crackle, the impulsive quality sometimes present in supersonic jet noise, has traditionally been defined in terms of the pressure waveform skewness. However, recent work has shown that the pressure waveform time derivative is a better quantifier of the acoustic shocks believed to be responsible for crackle perception. This paper discusses two definitions of crackle: waveform asymmetry versus shock content and crackle as a source or propagation-related phenomenon. Data from two static military jet aircraft tests are used to demonstrate that the skewed waveforms radiated from the jet undergo significant nonlinear steepening and shock formation, as evidenced by the skewness of the time derivative of the pressure waveforms. To the extent that crackle is caused by the presence of shock-like features in the waveform, crackle's perceived quality is likely to be heavily influenced by propagation through the geometric near field and into the far field. 

Because jet noise consists of extended, partially correlated sources, a multi-wavepacket model is appropriate to simulate jet noise field levels as well as the appropriate spatial coherence properties within the field. Here, the Hybrid method is used to reconstruct the levels and coherence properties of the source region for multi-wavepacket numerical models. The beamforming source results are then propagated to the acoustic field, where they show good agreement with benchmark levels and coherence lengths. The Hybrid method is further applied to a planar measurement of noise near a high-performance military aircraft, and the beamforming results are propagated to a ground-based microphone array. Sound levels and coherence lengths generated by the beamforming results show good agreement with benchmark measurements over a range of frequencies that contribute significantly to the overall radiation. Results indicate that the multi-wavepacket representation is an improvement over single-wavepacket models, which do not incorporate spatiotemporal features of the radiation.

An analytical wavepacket-based jet noise model has been applied for the first time to highperformance, military aircraft noise. Ground-based acoustical measurements were made of a tethered high-performance military aircraft with one engine cycling through four engine conditions. The resulting spectra have been decomposed into fine and large-scale similarity spectral components.  The spatial distribution of the  large-scale similarity spectrum decomposition provides the ability to extract level-based, data-educed wavenumber spectra and an estimation of the convective speed as a function of frequency and engine condition. The data-educed wavenumber spectra are compared with the axial wavenumber amplitude spectra associated with an analytical wavepacket ansatz. A simulated annealing algorithm is employed to minimize the difference between the analytical wavenumber amplitude spectra and the data-educed wavenumber spectra. The frequency-dependent wavepacket shapes obtained from the optimizations follow expected trends as a function of distance of contracting is length as frequency increases but appear to extend for approximately the same number of wavelengths. The level-based, data-educed wavenumber spectra model the Mach wave radiation associated with the large-scale turbulent mixing noise. This wavepacket model is a step towards producing an equivalent source representation of noise from tactical gas turbine engines to guide future noise environment modeling efforts.

The high noise levels associated with full-scale military aircraft result in nonlinear propagation, which results in acoustic shock formation and can alter noise perception. This propagation has been modeled for other aircraft but previous studies have been limited in scope, showing results for only select engine conditions and angles. Recent data measured near an F-35B allow for a more complete analysis of nonlinear propagation. Visual inspection of waveforms shows shock formation and persistence out to distances of up to 1220 m. Using an algorithm based on the Burgers equation, modified to include weak shock theory and an empirical correction for meteorological and ground effects, nonlinear and linear predictions are compared to measurements over a broad range of angles at 305 m. These analyses show that nonlinear effects become important in the maximum radiation direction at 75% thrust and increase with engine condition. At high engine powers, evidence of nonlinear propagation is found in the forward direction. 

Mach stem formation during outdoor acoustic shock propagation is investigated using spherical oxyacetylene balloons exploded above pavement. The location of the transition point from regular to irregular reflection and the path of the triple point are experimentally resolved using microphone arrays and a high-speed camera. The transition point falls between recent analytical work for weak irregular reflections and an empirical relationship derived from large explosions.

Vector acoustic intensity provides both the direction and magnitude of energy flow at the probe location and is, hence, more informative than acoustic pressure measurements. However, this important quantity has seen little application previously in aeroacoustics. In the present work, an intensity probe, consisting of four microphones, captured the radiated field to the sideline and aft of a tethered, full-scale military jet aircraft as one engine was operated at multiple engine conditions. Data from each probe location provide a frequency-dependent map of the sound flow near the aircraft. The vector acoustic intensity is estimated using a recently developed processing technique that extends the upper frequency limit of the traditional cross-spectrum-based calculations. The dominant intensity vectors are traced back to the jet centerline as a method of approximating the extent and location of the source region as a function of frequency. As expected for jet mixing noise sources, the resulting source region estimates contract and move upstream with increasing frequency. A comparison of estimated source regions and intensity directionalities between military and afterburner engine conditions reveals important distinctions in the sound fields.