Sound from Explosive Chemical Reactions

The lowermost portion of an explosive volcanic eruption column is considered a momentum-driven jet. Understanding volcanic jets is critical for determining eruption column dynamics and mitigating volcanic hazards; however, volcanic jets are inherently difficult to observe due to their violence and opacity. Infrasound from the 2011 eruption of Nabro Volcano, Eritrea has waveform features highly similar to the crackle phenomenon uniquely produced by man-made supersonic jet engines and rockets and is characterized by repeated asymmetric compressions followed by weaker, gradual rarefactions. This infrasonic crackle indicates that infrasound source mechanisms in sustained volcanic eruptions are strikingly similar to jet noise sources from heated, supersonic jet engines and rockets, suggesting that volcanologists can utilize the modeling and physical understandings of man-made jets to understand volcanic jets. The unique, distinctive infrasonic crackle from Nabro highlights the use of infrasound to remotely detect and characterize hazardous eruptions and its potential to determine volcanic jet parameters.

Characterization of the large-scale turbulent structures of jet noise sources can lead to improved noise diagnosis and reduction methods. Array-based inverse methods, in conjunction with equivalent-wave models of the field, are useful for imaging source and radiation properties. In this work, acoustical holography methods are used to reconstruct the three-dimensional sound field of a jet from the installed engine on a military fighter aircraft. The jet was measured in the presence of a rigid reflecting plane (a concrete run-up pad), and the equivalent wave model incorporates this effect with an image source representation of the field. Reconstructions are used to investigate jet source distributions and radiation properties.

Acoustical holography methods have been used to image the three-dimensional sound field of a jet from the installed engine on a military fighter aircraft. The characterization of the large-scale turbulent structures of jet-noise sources can lead to improved noise diagnosis and reduction methods. Jet noise sources are partially spatially coherent. In this work, proper orthogonal decomposition methods are employed to represent the jet sources as sets of equivalent, mutually incoherent wave packets (partial fields). The decomposition method, which is based on the multiple signal classification (MUSIC) algorithm, utilizes optimally located virtual references to generate physically meaningful partial fields. These source models demonstrate the number of independent sources in a jet, their relative distributions, and their spatial coherence. 

Microphone arrays used to measure acoustic intensity and other energy quantities traditionally have low-frequency bandwidth limitations (e.g., below 100 Hz) thereby excluding the lowest, and in some cases, the most energetic frequencies generated by large rocket motors. At these low frequencies, the phase and magnitude mismatch between microphones becomes greater and the acoustic phase separation between any two microphones becomes smaller, resulting in more error in estimating the pressure gradients. To investigate the low-frequency response of an acoustic intensity probe, a turntable is used to rotate a four-microphone probe in a low-frequency noise field. An experimental assessment of the bandwidth is given for both magnitude and directional response down to approximately 40 Hz. The effectiveness of a microphone interchange calibration technique to remove amplitude and phase mismatch and increase the usable bandwidth of the probe is also discussed.

Introducing the reader to some of the recent research regarding jet noise generation and propagation from high‐performance military aircraft and complementary research into noise source characterization and reduction using numerical simulations and laboratory‐scale models.

Broadband, partially correlated noise radiated from supersonic jets has characteristics that scale with nozzle size and flow properties. In particular, the spectral content of jet noise and variation with angle in many cases agree with empirically derived similarity spectra for large and fine-scale components of turbulent mixing noise [Tam et al., AIAA paper 96-1716]. In previous studies, measurements made near the F-22A Raptor agreed remarkably well with the similarity spectra, with two exceptions. First, the high-frequency slopes seen in the data were shallower than the similarity spectra at many angles. Second, the data exhibit a double frequency peak, which is absent from the similarity spectra [Neilsen et al., J. Acoust. Soc. Am. 132, 1993 (2012)]. These observations are explored further by examining the spectral characteristics of noise from a different military jet and a laboratory-scale, unheated jet. In both cases, there is evidence that for supersonic cases the measured spectra are shallower than the similarity spectra due to nonlinear propagation effects. In addition, the military data support the observation that the double spectral peak is a feature of full-scale jet noise. Recommendations are made for applying the similarity spectra to predict spectral levels for full-scale jets. [Work supported by ONR.]