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

An understanding of jet noise source mechanisms can facilitate targeted noise reduction efforts. This understanding has been enhanced with acoustic imaging technologies, such as near-field acoustical holography (NAH). In this study, multisource statistically optimized NAH (M-SONAH) was used to image the sound field near a tethered F-35 aircraft at multiple frequencies. A linear microphone array, placed along the ground, spanned the length of the jet exhaust plume. A multisource model of the sound field was included in the algorithm to incorporate the effects of the ground reflection on the measurement. Narrowband reconstructions elucidated fine details of the radiation patterns, such as multilobe radiation patterns (which may supersede “dual-lobe” patterns shown in previous studies), and broadband shock-associated noise. [Work supported by F-35 JPO.]

A new method for the calculation of vector acoustic intensity from pressure microphone measurements has been applied to the aeroacoustic source characterization of an unheated, Mach 1.8 laboratory-scale jet. Because of the ability to unwrap the phase of the transfer functions between microphone pairs in the measurement of a radiating, broadband source, physically meaningful near-field intensity vectors are calculated up to the maximum analysis frequency of 32 kHz. The new intensity method is used to obtain a detailed description of the sound energy flow near the jet. The resulting intensity vectors have been used with a ray-tracing technique to identify the dominant source region over a broad range of frequencies. Additional aeroacoustics analyses provide insight into the frequency-dependent characteristics of jet noise radiation, including the nature of the hydrodynamic field and the transition between the principal lobe and sideline radiation.

The search for an equivalent acoustic source model for high-speed jet noise has recently focused on wavepacket representations. A wavepacket is defined as a spatially extended source with an axial amplitude distribution that grows, saturates and decays, an axial phase relationship that produces directional noise, and correlation lengths longer than the integral length scales of the turbulent structures. This definition of a wavepacket has the same characteristics as the large-scale turbulent mixing noise; if the turbulent mixing noise can be isolated, the associate equivalent acoustic wavepacket—defined as a pressure fluctuation on a cylinder around the jet nozzle—can be found. An estimate of the frequencydependent, spatial variation in the large-scale turbulent mixing noise comes from a similarity spectra decomposition of the measured autospectral density, which in turn leads to data-educed wavenumber axial spectra associated with the frequency-dependent equivalent wavepackets. This wavepacket eduction technique has been applied to acoustical measurements of an unheated, Mach 1.8 jet in the near and far fields. At both locations, the resulting frequency-dependent, data-educed wavenumber spectra exhibit different types of self-similarity for low and high frequency regimes that become apparent when the axial wavenumber is scaled by the acoustic wavenumber, with a transition band between the two regimes. As expected, the data-educed wavenumber spectra can be used to predict field levels in the dominant radiation lobe. Addition of an uncorrelated source distribution, derived from the similarity spectra decomposition associated with the fine-scale turbulent mixing noise, creates a model that accounts for the sideline levels. This field-prediction ability of the wavepacket-plus-uncorrelated-distribution model is tested using the near and far field measurements. When predicting the field at the other location, the model’s average error is less than 2 dB for St = 0.04-0.25 but increases for larger St because the apparent directivity changes from near to far field, likely due to the frequency dependence of the extended source region.

Jet noise consists of extended, partially correlated sources such that a single-wavepacket source representation is inadequate. A multiple-wavepacket (MWP) model provides an analytical framework for jet-noise-like radiation to simulate jet noise field levels as well as the corresponding spatial coherence properties within the field. Here, a beamforming method with regularization is applied to noise measured by a linear array near a high-performance military aircraft. Beamforming results are decomposed into a reduced-order MWP model and the predicted radiation is validated in terms of level and coherence properties using benchmark measurements. 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. The MWP model is shown to predict full-scale specific features such as multilobe directivity patterns, and the addition of an uncorrelated distribution (UD) model adequately predicts the sideline radiation that is otherwise difficult to reproduce from wavepacket radiation. The MWP model predicted radiation characteristics are an improvement over single-wavepacket models, which do not incorporate spatiotemporal features of the radiation.

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.

The primary source of supersonic jet noise originates from the interaction of the turbulent flow with the ambient air. Tam et al. [AIAA Paper 96-1716 (1996)], proposed similarity spectra for a two-source model corresponding to omnidirectional fine-scale turbulence structures (FSS) and directional large-scale turbulent structures (LSS). These empirical similarity spectra agree reasonably with angular variation in mid and far-field spectra of both military and laboratory-scale jets. Near-field measurements of an ideally expanded, Mach 1.8 laboratory-scale jet from the Hypersonic and High-Enthalpy Wind Tunnel at Kashiwa Campus of the University of Tokyo were analyzed. Similarity spectra decompositions adequately describe the turbulent mixing noise as close as 10 jet diameters. Neglecting the effect of the hydrodynamic field, the LSS spectrum provides consistent fits at 15°-40° from the jet axis. A combination of LSS and FSS spectra match the measured spectra at 45°-55°. FSS spectrum matches the spectra at angles greater than 60°, except very close to the nozzle exit plane where there is an overprediction at high frequencies. Comparison of near and mid-field locations may provide insights into propagation radials.