REU at the Bonneville Salt Flats

Broadband shock-associated noise (BSN) is a major source of high-frequency noise in imperfectly expanded supersonic jets. While BSN has been extensively studied, source characterization from full-scale engines remains limited. This paper investigates BSN source and radiation characteristics from a full-scale, installed GE F404 engine on the T-7A trainer aircraft using acoustic holography. Apparent BSN sources are identified along the nozzle lipline and corroborated with in-situ imaging. The observed shock spacing aligns with similar jets in the literature but deviates significantly from traditional analytical models. Likewise, BSN peak frequencies at forward angles match trends from other full-scale jets but differ from simulations and lab-scale data, likely due to temperature and scale-related differences. A widely used BSN frequency model underperforms when relying on historical analytic shock spacing predictions but yields excellent agreement when corrected with measured spacing. Coherence analysis reveals connections between upstream-directed BSN and downstream Mach wave radiation, and shows elevated coherence between shock cells, indicating a partially coherent, distributed BSN source.

Noise sources in heated, supersonic jets are challenging to measure directly due to the extreme environment. Inverse methods, such as acoustic beamforming, using data collected near these sources can be used to construct equivalent source models that accurately describe the acoustic radiation. However, generating these models for a complex source can require thousands of parameters for a complete description. To address this challenge, a multiple wavepacket decomposition is introduced, reducing the source into a set of analytic wavepackets that recreate the full-rank model. This paper provides an overview of the decomposition procedure and applies it to acoustic data collected near a T-7A-installed GE F404 engine. The decomposition is validated for a peak frequency at military power and physical implications are discussed. It is shown that the downstream radiation at MIL can be accurately reconstructed with as few as five wavepackets with minimal error. The number of wavepackets required to capture the primary radiation region tends to increase with frequency and engine power. Beyond the dominant frequencies, the number of wavepackets per wavelength increases drastically, indicating a rapid decrease in source coherence. Finally, a single wavepacket model is fit to the data at MIL, which captures primary radiation features.

The Carpet Determination In Entirety Measurement (CarpetDIEM) III campaign provided insights into the variability of sonic boom metrics due to atmospheric turbulence. ARray Instrumentation for Sonic Thump Observations in TurbuLEnce (ARISTOTLE) Jr., a 2D array consisting of 23 microphones, recorded 17 sonic booms during the measurement campaign. On average, the Perceived Level (PL) across the array had a range of 7.3 dB and a standard deviation of 1.7 dB. While high wind speeds coincided with the largest PL range, significant variability also occurred during lower wind conditions, suggesting ambient noise and atmospheric turbulence play a role in the variability of metrics. The data further indicated directional dependency in metric variability, affirming the necessity of a 2D measurement approach. Building on these results, a full-scale ARISTOTLE array, consisting of 61 microphones over an area of 1,000 ft x 1,000 ft (305 m x 305 m) is under development and will be used in a future low-boom measurement campaign. Ultimately, ARISTOTLE will support the characterization of signatures generated by the X-59 aircraft, enabling improved understanding of turbulence effects on sonic boom metrics.

In the age of commercial spaceflight, many organizations are designing rockets for reuse. Most designs employ some form of propulsive landing either on land or at sea. The foremost among these organizations is Space Exploration Technologies Corporation (SpaceX) with their Falcon-9 rocket. As such rockets return, they produce audible sonic booms over the surrounding areas. The Falcon-9 booster's sonic boom signature is unique, consisting of three primary shocks instead of the two associated with traditional N-waves. This provides an opportunity to study sonic boom formation from a unique geometry and to see whether the triple boom can be physically explained. This paper considers F-function and computational fluid dynamics methods to model the booster's sonic boom under conditions ranging from Mach 1.5 to 2.5. Results support the conclusion of Anderson and Gee (2025) [JASA Express Lett. 5, 023601 (2025)] that the central shock is the result of a rearward-migrating rarefaction wave produced by the lower portions of the booster merging with a forward-migrating compression wave produced by the grid fins. Although it is clear that both the grid fins and the lower portions of the booster contribute to the central shock, the different models disagree on their relative importance in producing the final shock.

Prior work [e.g., McInerny (1992). Noise Control Eng. J. 38(1), 5–16; McInerny (1996). J. Aircraft 33(3), 511–517; Franken (1958). Noise Control 4(3), 8–16] has resulted in models for estimating overall sound power levels (OAPWLs) and maximum overall sound pressure levels (OASPL_max) from jet and rocket engines. Based on fundamental flow properties, this paper builds on previous results and presents simple methods for predicting OAPWL and OASPL_max from heated supersonic jets and rockets. A method for estimating ground effects on OASPL_max is also presented. The model's performance is evaluated for launched Atlas V and Vulcan Centaur rockets and an installed F404 jet engine at engine conditions ranging from 38% thrust through afterburner. The results show good agreement for OASPL_max ⁠, where the root mean square error is confined to less than 2 dB for the rockets and jet engine conditions considered.

The Firefly Alpha launch, featuring an unexpected engine shutdown, offered a unique opportunity to study the acoustic effects of clustered nozzles on rocket noise. Measurements revealed a 0.75 dB drop in overall sound pressure levels (OASPL) and a 30% frequency shift, compared to predictions of 1.2 dB and 20%, respectively. While direct comparisons are limited by the dataset’s uniqueness, the results generally align with existing rocket noise models, highlighting areas for refinement. This study provides valuable data for improving noise prediction methods and deepening the understanding of launch vehicle acoustics.