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The study of unsteady aerodynamic phenomena in wind tunnels is supported by gust-generating devices capable of generating adjustable magnitude and periodicity velocity fluctuations in a flowfield. Gusts are typically generated actively by introducing moving vanes to direct the flow, or passively by tailoring the boundary layer growth and shape in the tunnel. The flow facility used here is a student-built closed-return low-speed wind tunnel, with a test section size of 750 mm × 750 mm and a maximum speed of 25 m/s. A two-vane gust generator utilizing NACA0018 airfoil sections of 150 mm chord length was designed and installed upstream of the test section. The flowfield was mapped with the installed vanes with and without gust actuation, utilizing a hot wire system. The tunnel with gust vanes exhibits a spatially uniform baseline turbulence intensity of 5%, with a steady state velocity deficit of 1 m/s in the vane–wake region. Upon introducing the gusting conditions at vane deflection angles of up to ±45°, velocity differences of up to 4 m/s were attained at 18 m/s freestream velocity at oscillation frequencies ranging between 1 Hz and 2 Hz.

Inconel alloy 718 is a high-strength and corrosion resistant alloy that is commonly used as a beamline vacuum window. The accumulation of irradiation-induced damage substantially decreases the window’s service lifetime, and replacing it engenders significant beamline downtime. With this application in mind, herein we examine whether post-irradiation annealing can alleviate irradiation-induced damage of Inconel alloy 718. Inconel alloy 718 was received in a solution annealed state. We then irradiated samples using two different modalities (1.5 MeV H+ and 5 MeV Ni2+) at three representative temperatures for beamline windows (room temperature, 100 °C, and 200 °C), followed by annealing at temperatures viable for in-situ annealing processes (no anneal, 300 °C, and 500 °C). Using nanoindentation, we determined that irradiation-induced hardening occurs but is largely mitigated by post-irradiation annealing. Overall, our results suggest that in-situ annealing of radiation damage in Inconel alloy 718 vacuum windows appears feasible, which could potentially decrease beam downtime and maintenance costs.

The role of cavities on deformation-induced martensitic phase transformations is studied in a laser-welded and neutron irradiated austenitic stainless steel. Orientation dependent nanoindentation experiments are performed in the base metal and the weld heat affected zone (HAZ) at room temperature. Transmission electron microscopy study of deformed microstructures indicates indentation-induced α’-martensite forms in the base metal, whereas α’- and e-martensite arise in the HAZ. The different pathway of martensite phase transformation is attributed to the laser weld-induced annealing of cavities. Our results suggest that deformation-induced martensitic phase transformation of austenitic stainless steel is correlated to neutron irradiated cavity structures.

This article investigates the integrity of laser welds on neutron irradiated, He-containing steels. Life extension of the current fleet of light water reactors could necessitate repair of cracks on irreplaceable internal components, but heat input of weld repairs exacerbates the problem by initiating He-induced cracking. Laser welding is a promising low-heat-input technology thought to limit the extent of He-induced cracking. In this study, we produce laser welds in a hot cell on AISI 304L stainless steel plates previously irradiated in the Experimental Breeder Reactor (EBR)-II. We select a systematic set of three specimens spanning fluences ~1–28 displacements per atom (dpa) at ~415–430 °C and He concentrations ~0.2–8 atomic parts per million (appm) amounting to ~0.2–2.8% swelling. He-induced cracking is observed only in specimens containing ≥3 appm He. Laser welding nearly eliminates all irradiation-induced cavities and reduces the dislocation loop number density, similar to conventional post-irradiation annealing. Microchemically, laser welding induces Cr-rich precipitation and suppresses grain boundary radiation-induced segregation. The mechanism of He-induced cracking is discussed in the context of these microchemical changes. The weld heat input is calculated and suggests that further refinement of laser welding parameters may improve the cracking resistance for higher dose and He conditions.

A surface coating using pure chromium has been proposed to increase the accident tolerance of Zircaloy cladding in pressurized water reactors. However, there is not much irradiation experience with Cr alloys and especially pure Cr. In the present study, pure chromium was irradiated with 5 MeV Fe ions to 50 peak dpa (displacements per atom) at temperatures of 450, 500, 550, 600 and 650 °C. Then irradiation at the peak swelling temperature of 550 °C was conducted to 50, 100, 150 peak dpa. Swelling at 50 dpa was observed over the entire temperature range studied, 450–650 °C, but appeared to be decreasing strongly at the temperature boundaries of the experiment. After an initial transient of rapid swelling, chromium was observed to swell at a rate of ~0.03–0.04%/dpa (up to 120 local dpa), which is much lower than pure Fe at 0.2%/dpa. This low swelling rate was found to be relatively insensitive to dpa rate, which varied by a factor of ~2 over the depth of data collection. Swelling was observed to begin quickly with an incubation period less than 10 dpa. Self-organization in the form of void ordering was observed to be developing at 50 dpa, becoming better defined with increasing dose. The void alignment direction is determined to be the 〈111〉 axial direction.

This work focuses on the full characterization of dislocation loops induced by proton irradiation in single-crystal ThO2. Irradiation was performed using 2 MeV H+ ions with sample temperature at 600oC and a dose of up to 0.47 displacements per atom (dpa). Transmission electron microscopy (TEM) characterization was performed on a large number of dislocation loops. Burgers vector ( b → ) analysis using standard g → . b → = 0 invisibility criterion revealed different variants of 1 / 3 〈 111 〉 type dislocation loops. TEM analysis of edge-on dislocation loops was used to determine habit planes as { 111 } type. The nature of dislocation loops was revealed using the inside-outside contrast method as interstitial type. Rel-rod dark field images were obtained by selecting streak at g → = 1 / 2 [ 31 1 ¯ ] in diffraction pattern to identify the Frank loops present in the microstructure. Subsequent analysis of a single dislocation loop using atomic resolution scanning transmission electron microscopy (STEM) confirmed the interstitial nature of the loop with { 111 } habit plane.

Thermal transport is a key performance metric for thorium dioxide in many applications where defect-generating radiation fields are present. An understanding of the effect of nanoscale lattice defects on thermal transport in this material is currently unavailable due to the lack of a single crystal material from which unit processes may be investigated. In this work, a series of high-quality thorium dioxide single crystals are exposed to 2 MeV proton irradiation at room temperature and 600 °C to create microscale regions with varying densities and types of point and extended defects. Defected regions are investigated using spatial domain thermoreflectance to quantify the change in thermal conductivity as a function of ion fluence as well as transmission electron microscopy and Raman spectroscopy to interrogate the structure of the generated defects. Together, this combination of methods provides important initial insight into defect formation, recombination, and clustering in thorium dioxide and the effect of those defects on thermal transport. These methods also provide a promising pathway for the quantification of the smallest-scale defects that cannot be captured using traditional microscopy techniques and play an outsized role in degrading thermal performance.

Deformation mechanism of a laser weld on neutron irradiated AISI 304L stainless steel was studied by in-situ microcompression test at room temperature. The deformation-induced austenite-to-martensite phase transformation occurs in {101}-oriented grains in the irradiated base metal, while deformation twinning prevails in {101}-oriented grains in the weld heat affected zone (HAZ). A high number density of irradiation-induced voids in the base metal provide sufficient nucleation sites for the austenite-to-martensite phase transformation under compression at room temperature. A deformation map is established to predict critical twinning stress for face-centered cubic (fcc) metals and alloys. Our study suggests that irradiation can tailor the deformation-induced phase transformation in austenitic stainless steels.

In this paper, a software-based simulator for the deployment of base station-equipped unmanned aerial vehicles (UAVs) in a cellular network is proposed. To this end, the Google Earth Engine platform and its included image processing functions are used to collect geospatial data and to identify obstacles that can disrupt the line-of-sight (LoS) communications between UAVs and ground users. Given such geographical information, three environment-aware optimal UAV deployment scenarios are investigated using the developed simulator. In the first scenario, the positions of UAVs are optimized such that the number of ground users covered by UAVs is maximized. In the second scenario, the minimum number of UAVs needed to provide full coverage for all ground users is determined. Finally, given the load requirements of the ground users, the total flight time (i.e., energy) that the UAVs need to completely serve the ground users is minimized. Simulation results using a real area of the Virginia Tech campus show that the proposed environment-aware drone deployment framework with Google Earth input significantly enhances the network performance in terms of coverage and energy consumption, compared to classical deployment approaches that do not exploit geographical information. In particular, the results show that the proposed approach yields a coverage enhancement by a factor of 2, and a 65% improvement in energy-efficiency. The results have also shown the existence of an optimal number of drones that leads to a maximum wireless coverage performance.