Graduate Student Savanah Turner Publishes Study of over 300 L and T dwarfs

We investigated the growth of carbon nanotubes (CNTs) directly on stainless steel substrates. The CNTs were grown using a two-step process: oxidation of the stainless steel surface and CNT growth. The samples were oxidized in an 800 °C furnace fed with a flow of air for 4 min. CNTs were grown by switching the flow to ethylene, which both reduces the oxide and initializes CNT growth. The time of CNT growth was varied to understand how the samples evolved over time. To better understand the growth mechanisms, we isolated cross-sections of the CNT-substrate interface using a focused ion beam. These cross-sections were investigated with transmission electron microscopy and energy dispersive X-ray spectroscopy. CNTs were seen to grow from iron-rich nanoparticles embedded in the oxide layer. The oxide layer was also seen to lose iron over time, suggesting that these iron nanoparticles were reduced out of the oxide. The base particles were embedded in the oxide layer, leaving cavities when the CNTs were removed. The diameters of the nanotubes were also seen to grow over time as a result of carbon infiltration. The effects of the embedded particle and infiltration quickly isolate the catalyst, leading to short CNTs (1–10 µm).

The hexagonal antiferromagnet MnTe has attracted enormous interest as a prototypical example of a spin-compensated magnet in which the combination of crystal and spin symmetries lifts the spin degeneracy of the electron bands without the need for spin-orbit coupling, a phenomenon called nonrelativistic spin splitting (NRSS). Subgroups of NRSS are determined by the specific spin-interconverting symmetry that connects the two opposite-spin sublattices. In MnTe, this symmetry is rotation, leading to the subgroup with spin splitting away from the Brillouin zone center, often called altermagnetism. MnTe also has the largest spontaneous magnetovolume effect of any known antiferromagnet, implying strong coupling between the magnetic moment and volume. This magnetostructural coupling offers a potential knob for tuning the spin-splitting properties of MnTe. Here, we use neutron diffraction with in situ applied pressure to determine the effects of pressure on the magnetic properties of MnTe and further explore this magnetostructural coupling. We find that applying pressure significantly increases the Néel temperature, but decreases the ordered magnetic moment. We explain this as a consequence of strengthened magnetic exchange interactions under pressure, resulting in higher 𝑇N, with a simultaneous reduction of the local moment of individual Mn atoms, described here via density functional theory. This reflects the increased orbital hybridization and electron delocalization with pressure. These results shed light on the competition between magnetic exchange interactions and the strength of individual magnetic moments and show that the magnetic properties of MnTe can be controlled by pressure, opening the door to improved properties for spintronic applications through tuning via physical or chemical pressure.

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.