Selected Publications
The 15 January 2022 climactic eruption of Hunga volcano, Tonga, produced an explosion in the atmosphere of a size that has not been documented in the modern geophysical record. The event generated a broad range of atmospheric waves observed globally by various ground-based and spaceborne instrumentation networks. Most prominent is the surface-guided Lamb wave (≲0.01 Hz), which we observed propagating for four (+three antipodal) passages around the Earth over six days. Based on Lamb wave amplitudes, the climactic Hunga explosion was comparable in size to that of the 1883 Krakatau eruption. The Hunga eruption produced remarkable globally-detected infrasound (0.01–20 Hz), long-range (~10,000 km) audible sound, and ionospheric perturbations. Seismometers worldwide recorded pure seismic and air-to-ground coupled waves. Air-to-sea coupling likely contributed to fast-arriving tsunamis. We highlight exceptional observations of the atmospheric waves.
The phase and amplitude gradient estimator (PAGE) method [Thomas, Christensen, and Gee, J. Acoust. Soc. Am. 137, 3366–3376 (2015)] has been developed as an alternative to the traditional p-p method for calculating energy-based acoustic measures such as active acoustic intensity. While this method shows many marked improvements over the traditional method, such as a wider valid frequency bandwidth for broadband sources, contaminating noise can lead to inaccurate results. Contaminating noise degrades performance for both the traditional and PAGE methods and causes probe microphone pairs to exhibit low coherence. When coherence is low, better estimates of the pressure magnitude and gradient can be obtained by using a coherence-based approach, which yields a more accurate intensity estimate. This coherence-based approach to the PAGE method, known as the CPAGE method, employs two main coherence-based adjustments. The pressure magnitude adjustment mitigates the negative impact of uncorrelated contaminating noise and improves intensity magnitude calculation. The phase gradient adjustment uses coherence as a weighting to calculate the phase gradient for the probe and improves primarily the calculation of intensity direction. Though requiring a greater computation time than the PAGE method, the CPAGE method is shown to improve intensity calculations, both in magnitude and direction.
This paper presents an analysis of acoustic radiation characteristics of a T-7A-installed F404 engine, as derived from far-field measurements. Radiated directivity at different engine conditions is compared with contemporary investigations into Mach wave radiation. The peak directivity angles observed in the far-field are used to evaluate appropriate values for the convective Mach number. It is shown that velocity from the convective Mach number is approximately 70% of the centerline jet velocity, agreeing with contemporary supersonic jet noise literature. Spatiospectral maps from far-field data indicate the presence of spatiospectral lobes, like those observed in the near field. These spatiospectral maps also illustrate interference nulls caused by ground reflection interference. A ground reflection model is used to attempt to correct these errors. Using these corrected data, the overall sound power level is calculated and is used to find the acoustic radiation efficiency, a value rarely calculated for jet engines. The F404 OAPWL is proportional to Uₑ⁸ subsonically, and Uₑ³ supersonically. The efficiency at afterburner exists between 0.5% and 0.8%, exhibiting similar acoustic efficiency trends as those seen in launch vehicles 50 years ago.
Acoustic source characterization of full-scale supersonic jets remains a vital component of understanding jet noise. Identification of fundamental characteristics such as source location(s) and directivity will better inform physical understanding, noise models, engine design, and noise reduction technologies. This paper investigates an installed F404-GE-103 engine as an acoustic source using statistically optimized near-field acoustical holography (SONAH) for engine conditions ranging from 50% thrust to afterburner. Partial field decomposition is used to characterize the coherent nature of the source as a function of frequency. Spatiospectral reconstructions along the nozzle lipline show distinct behavior at different engine conditions. Lower engine conditions show two energetic spatial regions along the lipline from 200-300 Hz. Three distinct local maxima are observed at AB. These maxima are correlated with other studies about supersonic jet noise sources in the literature. Mach wave radiation is thought to be tied to the first two local maxima, which occur throughout the shear layer, ending ahead of the approximate supersonic core tip. The third, lower-frequency maximum is proposed to be correlated with noise from large-scale turbulence structures.
Spatiospectral lobes are still-unexplained phenomena seen in noise radiation from multiple high-performance aircraft. These lobes are observed as multiple peaks in noise spectra at a given field location or as multiple local maxima in noise directivity at a single frequency. Using hybrid beamforming with a 120-microphone near-field array, the lobe characteristics are studied for a GE F404 engine installed on a T-7A aircraft at different engine conditions. Both the measured and reconstructed fields show multiple spatiospectral lobes at different engine conditions, and the overall noise directivity is identified as being the superposition of multiple distinct lobes. The individual lobes appear, shift aft, and then disappear with increasing frequency, which is opposite the behavior of overall noise directivity. The lobes are ray-traced back to the jet centerline to determine an apparent acoustic source location and it is concluded that each lobe originates from a different source within the jet.
A distinctive feature of many 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, 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, whereas the average steepening factor and shock energy fraction tend to emphasize aggregate behavior of the entire waveform. These metrics are applied at various engine conditions, over a wide range of angles and distances, to assess the growth and decay of shock waves. This paper represents the first time that the development of these metrics is shown from the near field to the far field, out to 305 m. 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 past 35 m, continued shock formation and atmospheric absorption can make the steepened nature of the waveform more prominent out to 305 m.