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β-Ga2O3 with an ultra-wide bandgap demonstrates great promise in applications of space missions as power electronics and solar-blind photodetector. Unraveling the radiation damage effects on its material properties is of crucial importance, especially for improving the radiation tolerance of Ga2O3-based devices. Herein, we evaluate the formation energy of gallium and oxygen vacancy defects and comprehensively investigate their influence on the electronic and optical properties of β-Ga2O3 using first-principles calculations. Ga vacancies act as deep acceptors and produce p-type defects in β-Ga2O3, while the defective Ga2O3 with O vacancies exhibits the n-type characteristics. A semimetal characteristic is observed in the defective Ga2O3 with Ga vacancies, and an apparent optical absorption peak in the infrared spectral range emerges. Moreover, the self-compensation effect emerges when β-Ga2O3 contains both Ga vacancies and O vacancies, leading to the reduced absorption peak. The doping effect on the defect formation energy of β-Ga2O3 is also investigated, and Ga vacancies are found to be easily formed in the case of In doped β-Ga2O3 (InGa2O3) compared to the undoped β-Ga2O3, while O vacancies are much harder to form. This work provides insights into how gallium and oxygen vacancy defects alter electronic and optical properties of β-Ga2O3, seeking to strengthen its radiation tolerance. [ABSTRACT FROM AUTHOR]

The propagation of the weak shock wave (WSW) to the tunnel exits and their radiation as micro-pressure waves (MPWs) may cause sonic booms or structural resonance of buildings, posing potential hazards to humans, animals, and buildings in the exit's environment. The characteristics of the WSW and sonic booms of a maglev train/tube coupling model were studied based on the two-dimensional axisymmetric unsteady Reynolds average Navier–Stokes turbulence model. In the later stage of a MPW, the formation mechanism, geometry, and kinematic characteristics of compressible vortex rings (CVRs) were systematically analyzed. The inertial effect causes the initial wavefront to gradually transition from a Gaussian-shape waveform to a triangular waveform during its propagation, eventually coalescing into a WSW. The overpressure, density jump, and shock Mach number at the WSW location all increase with the increasing train speed, while the WSW thickness decreases accordingly. The formation distance of the WSW is inversely proportional to the amplitude of the initial wavefront gradient, and the WSW directly causes the occurrence of the exit sonic boom. The MPW amplitude has significant directionality with a largest value in the axial direction. Within the speed range of 450–700 km/h, the sound pressure level of the MPW exceeds the hearing threshold and even reaches the feeling threshold. The evolution of CVRs includes primary CVR, secondary CVR, and Kelvin–Helmholtz vortices. Primary CVR has the greatest impact on the axial MPW among them. The occurrence of CVRs will cause a second small noise level other than the sonic boom. [ABSTRACT FROM AUTHOR]

β-Ga2O3 is a new generation of semiconductor material with a wide bandgap of 4.9 eV. However, the β-Ga2O3 devices inevitably produce defects within them after irradiation, leading to changes in their thermal conductivities. At present, the effect of radiation-damage-induced defects on thermal conductivity of β-Ga2O3 has not been carried out. Herein, we have employed molecular dynamics simulations to investigate the impact of defects on the thermal transport of β-Ga2O3, and the obtained thermal conductivity of non-defect β-Ga2O3 is in good agreement with recent reports. Our findings indicate that the thermal conductivity of β-Ga2O3 at room temperature exhibits a consistent decrease with an increase in the concentration of Ga vacancies, but shows a decreasing and then increasing trend as the number of O vacancies increases. In addition, doping/alloying is found to improve the irradiation resistance of β-Ga2O3 based on reported defect formation energy calculations, so the mechanism of alloying effect on the thermal conductivity is deeply analyzed through first-principles calculations. Moreover, the lattice thermal conductivities of ordered InGaO3 and BGaO3 alloys are predicted by solving the phonon Boltzmann transport equation. The obtained results that κ(Ga2O3) = κ(BGaO3) > κ(InGaO3) are attributed to the combined effect of volume, specific heat capacity, group velocity, and phonon lifetime of the three materials. This work can help to disclose the radiation damage influence on thermal properties of β-Ga2O3 semiconductors. [ABSTRACT FROM AUTHOR]

The effect of a free-stream turbulence intensity level on the wake dynamics of a square-back Ahmed body is modeled using the improved delayed detached eddy simulation at Re = 9.6 × 10 4 . The center of pressure, pressure gradient on the base surface, and the barycenter of the momentum deficit on the wake plane are analyzed to characterize the wake bi-modality dynamics. Given that different flow dynamics have different dominant frequencies, the spectral proper orthogonal decomposition is utilized to separate the wake bi-stability, pumping motion of the whole recirculation region, the Von Kármán vortex shedding and the shear layer instability. The results show that entrainment of the oncoming flow into the wake is enhanced, the vorticity thickness is thickened and the length of the wake recirculation region is decreased with the increasing free-stream turbulence, resulting in a lower base suction pressure and a higher level of shear stress. The frequency of the pumping motion is increased with the increase in the oncoming turbulence intensity, while the frequency of Von Kármán vortex shedding is irrespective of the level of the background turbulence. Though the correlation between the switching rate and the oncoming turbulence intensity cannot be put forward due to the relatively short numerical simulation time compared with the wind tunnel experiment, it is still known that the turbulence intensity has a positive effect on the wake bi-stability switching. [ABSTRACT FROM AUTHOR]

Interpenetrating phase composites (IPCs) are a new type of materials with designable mechanical properties. In this work, to explore a simple, applicable, and accurate method for property designability of interpenetrated phase composites, the interaction between the matrix phase and the reinforcing phase of IPC under compressive stress is discussed, and a microstructured representative volume element is proposed for interpenetrating metal/polymer composites under compressive loading by considering the interaction of constituent phases. The stress–strain relationship of the representative volume element is established based on the iso-stress and iso-strain assumptions. Compared with the experimental results with those of interpenetrated metal/polymer composites, interpenetrated metal/metal composites, and interpenetrated metal/ceramic composites, it is shown that the analytical solutions can accurately predict the elasto-plastic compressive stress–strain curves. [ABSTRACT FROM AUTHOR]

This paper investigates the flow dynamics around a square finite wall-mounted square cylinder of two different aspect ratios (A R = H / D = 6 and 10, H and D are the height and width, respectively) under six boundary layer thickness ( δ / D = 0.5 – 4.0 , δ is the boundary layer thickness) using the improved delayed detached eddy simulation turbulence model. The Reynolds number based on the square cylinder width and incoming velocity is Re = 1.4 × 10 4 . The aerodynamic force, surface mean and fluctuating pressure, wake closure length, Reynolds stresses, turbulence production, and the anisotropy of turbulence are analyzed in detail to investigate the effect of A R and δ / D combinations. The Q-criterion vortex identification method is adopted to identify and classify the vortex structures of different A R and δ / D combinations. The parametric diagram based on A R and δ / D to predict the dipole/quadrupole wake structure proposed in Yauwenas et al. ["The effect of aspect ratio on the wake structure of finite wall-mounted square cylinders," J. Fluid Mech. 875, 929–960 (2019)] is refined based on results of the present study (quadrupole wake for A R = 6 with δ / D ≥ 1.5 and A R = 10 with δ / D ≥ 1.0). The instantaneous space–time pressure distribution along the whole span proves the existence of cellular shedding (A R = 10 for δ / D = 0.5 – 2.0), and the spectral proper orthogonal decomposition technique is adopted to investigate the cellular shedding phenomenon and its distribution. With the increase in δ / D from 2.0 to 4.0, the cellular shedding process is absent, implying the inhibitory effect of increasing boundary layer thickness on the cellular shedding. [ABSTRACT FROM AUTHOR]

Coherent structures in surrounding flows around ground vehicles play an important role in characterizing their aerodynamic features. However, due to restrictions of traditional reduced-order models, extracting physically meaningful coherent structures from turbulent flows with massive separation still remains a challenging issue. The spectral proper orthogonal decomposition (SPOD), which extracts modes optimally representing the space-time flow statistics, enables the feasibility of further modeling and control of vehicle aerodynamic features. This study intends to investigate the influence of spectral estimation parameters on SPOD results, so as to serve as fundamentals for future works into this topic. The time-resolved pressure field obtained from a large-eddy simulation considering a generic high-speed train is used as the snapshot database. Spectral estimation parameters including block number, frequency resolution, and cutoff frequency are, respectively, discussed to quantify their impacts on both SPOD spectra and modes. The results reveal that, with the increasing of block number, higher reliability and accuracy of SPOD prediction can be achieved, with the block number of 20–30 that leads both requirements of efficiency and precision. The frequency axis with finer resolution reproduces more detailed spectral information, with the eigenvalue distribution and spatial distribution of mode energy under acceptable accuracy when dimensionless frequency resolution reaches 0.025. Moreover, the reducing of cutoff frequency results in increasing unresolved energy content, which will be distributed mostly near the corresponding cutoff frequency, and more minor scaled spatial structures in SPOD modes. The findings and approaches could also work as references for wider application field of SPOD approach. [ABSTRACT FROM AUTHOR]

Ocean wave energy is a promising energy source for large-scale exploitation owing to its abundant reserve and renewability. An effort is underway to develop a generator that can adapt to the ultra-low frequency of ocean wave vibration. Here, we designed a cylindrical triboelectric nanogenerator with an internal swing structure for effective water wave energy harvesting. Based on the supporting effect of the bearing component, the rotary dielectric films can be suspended over the stator electrodes, rather than coming into direct contact with them. Benefiting from the largely reduced resistance and continuous swing of an internal rotary component, the fabricated triboelectric nanogenerator can operate for about 85 s and produce over one thousand current pulses after one water wave excitation. Being agitated by a 0.033 Hz water wave (during a period of 30 s), the optimized cylindrical triboelectric nanogenerator can produce a peak power density of 231.6 mW·m−3 and an average one of 39.8 mW·m−3. Successful demonstrations of powering portable electronic devices and iron material corrosion inhibition under such simulated excitation indicate the robust capability for ultra-low-frequency water wave energy conversion. This triboelectric nanogenerator offers a new vision for efficient ultra-low-frequency wave energy harvesting with the possibility of large-scale blue energy. [ABSTRACT FROM AUTHOR]