Jun Yang

Downburst-induced wind shear poses a significant hazard to aircraft during takeoff and landing because of its spatial non-uniformity and transient evolution. A laboratory-scale downburst-like impinging-jet outflow was generated in a three-dimensional wind matrix under non-axisymmetric boundary conditions and measured using synchronized hot-wire anemometry and multi-point Pitot-tube surveys in two orthogonal sections. The measurements identified a quasi-steady interval for statistical analysis and revealed strong velocity fluctuations with an inertial-subrange-like spectral range. The mean flow was organized into inner impingement-dominated, transitional, and outer radial-flow regions, with the outer region exhibiting a nose-shaped wall-jet profile. Radial velocity profiles were better described by a generalized Gaussian function than by a conventional Gaussian profile, with the shape parameter decreasing with height. The vertical-flow fan array primarily controlled the outflow intensity, whereas the swirling-flow fan array mainly redistributed near-ground momentum. Although the two sections exhibited different overall intensities, their peak velocities became comparable in the outer radial-flow region, indicating that directional non-uniformity was mainly associated with the vertical velocity-profile shape. Standardized velocity distributions approximately collapsed onto a common Gaussian-like form, while large-amplitude events concentrated around z/D ≈ 0.25–0.45. These results quantify the spatial and directional characteristics of downburst-like outflows and provide experimental data for modeling aircraft encounters with downbursts and assessing low-altitude flight safety.

Beryllium oxide (BeO), as an optically stimulated luminescence (OSL) material, possesses unique application advantages in the field of radiation dosimetry due to its effective atomic number close to that of human tissues. To break through the limitation that the research and development of conventional BeO‐based OSL materials relies on extensive experimental exploration, the density functional theory calculation method is introduced into the study of doped BeO systems in this work. The optical properties of Li‐, Na‐, K‐, and Cu‐doped BeO systems were systematically investigated. The obtained results show a generally good agreement with publicly reported experimental observation data, which to a certain extent validates the rationality of the calculation method adopted in this work for dopant performance prediction and is expected to provide a feasible theoretical reference for the subsequent screening of doping components in BeO‐based OSL materials. Based on this foundation, this study further conducted predictive investigations on the optical properties of Cu─Dy and Cu─Er, as well as Cu─Dy─Er‐doped BeO systems. The preliminary results indicate that multielement doped BeO systems exhibit potential advantages over single‐element doped systems at the theoretical prediction level.

Unlike conventional nonreciprocal acoustic systems that rely on nonlinear effects, moving components, or active modulation, this study proposes a passive approach for achieving nonreciprocal acoustic transmission using Tesla-valve-based structure under background flow. The proposed design exploits the flow-rectification capability of Tesla valves to generate asymmetric flow fields and direction-dependent acoustic responses. A multistage Tesla valve and a hexagonal annular configuration are numerically investigated using coupled fluid–acoustic simulations. The results show the geometric asymmetry of the Tesla valve alone cannot break acoustic reciprocity. Instead, nonreciprocal transmission arises from the interaction between the asymmetric background flow and the valve geometry. Increasing inlet Mach number enhances flow rectification, resulting in larger differences in acoustic energy convection, phase accumulation, and dissipation along opposite propagation paths. Acoustic pressure analyses further reveal that local and coupled resonance modes, together with flow-induced velocity asymmetry, govern the redistribution of acoustic energy and the emergence of frequency-selective nonreciprocity.

In this study, we present a rational donor‐acceptor‐donor (D‐A‐D) molecular design strategy to develop high‐performance organic cathodes by enhancing the intramolecular charge transfer (ICT) effect. We designed and synthesized two organic molecules: 2,6‐bis(10H‐phenothiazin‐10‐yl)benzo[1,2‐d:4,5‐d’]diimidazole‐4,8‐dione (PTZBQ), featuring a strong quinone‐type acceptor core, and the control compound 2,6‐bis(10H‐phenothiazin‐10‐yl)benzo[1,2‐d:4,5‐d’]diimidazole (PTZTAB), without a quinone core. The strong push‐pull electronic structure of PTZBQ not only results in a significantly narrowed bandgap and improved electrode kinetics, but also allows the quinone core to contribute extra n‐type capacity, thereby remarkably boosting the electrode's specific capacity and reaction dynamics. Moreover, the extended π‐conjugation and D‐A‐D configuration‐induced polarity endow both small molecules with exceptional electrolyte dissolution resistance. Consequently, PTZBQ exhibits bipolar redox activity, delivering a high discharge potential of 3.05 V, a high specific capacity of 163.3 mAh g ⁻¹ at 0.1 A g ⁻¹ , an excellent rate capability (77.6% retention at 5 A g ⁻¹ ), and exceptional long‐term cycling stability with 89.4% retention after 5000 cycles at 1 A g ⁻¹ . DFT calculations and ex situ spectroscopy confirm that the unique D‐A‐D architecture possesses spatially separated n‐type and p‐type redox centers, facilitating the redox process. Our findings highlight that quinone‐core engineering enhances intramolecular charge transfer and represents a powerful approach for developing high‐performance cathode materials.

In this study, we present a rational donor‐acceptor‐donor (D‐A‐D) molecular design strategy to develop high‐performance organic cathodes by enhancing the intramolecular charge transfer (ICT) effect. We designed and synthesized two organic molecules: 2,6‐bis(10H‐phenothiazin‐10‐yl)benzo[1,2‐d:4,5‐d’]diimidazole‐4,8‐dione (PTZBQ), featuring a strong quinone‐type acceptor core, and the control compound 2,6‐bis(10H‐phenothiazin‐10‐yl)benzo[1,2‐d:4,5‐d’]diimidazole (PTZTAB), without a quinone core. The strong push‐pull electronic structure of PTZBQ not only results in a significantly narrowed bandgap and improved electrode kinetics, but also allows the quinone core to contribute extra n‐type capacity, thereby remarkably boosting the electrode's specific capacity and reaction dynamics. Moreover, the extended π‐conjugation and D‐A‐D configuration‐induced polarity endow both small molecules with exceptional electrolyte dissolution resistance. Consequently, PTZBQ exhibits bipolar redox activity, delivering a high discharge potential of 3.05 V, a high specific capacity of 163.3 mAh g ⁻¹ at 0.1 A g ⁻¹ , an excellent rate capability (77.6% retention at 5 A g ⁻¹ ), and exceptional long‐term cycling stability with 89.4% retention after 5000 cycles at 1 A g ⁻¹ . DFT calculations and ex situ spectroscopy confirm that the unique D‐A‐D architecture possesses spatially separated n‐type and p‐type redox centers, facilitating the redox process. Our findings highlight that quinone‐core engineering enhances intramolecular charge transfer and represents a powerful approach for developing high‐performance cathode materials.

The rapid expansion of the low-altitude economy has intensified the need for experimental facilities capable of simulating the complex, unsteady, and non-uniform wind fields characteristic of the atmospheric boundary layer. Traditional wind tunnels often struggle to replicate the low-frequency fluctuations and spatial gradients encountered by small-scale aircraft in urban environments. We propose a three-dimensional wind matrix to generate the unsteady non-uniform incoming flow. Through systematic experimental characterization, the facility’s ability to generate three distinct flow regimes—uniform inflow, shear flow, and sinusoidal gusts—is demonstrated. Results indicate that the generated flow achieves a quasi-homogeneous state for uniform wind at a downstream distance of 𝑥∕𝐻 ≥ 1.2, with high turbulence intensity and mean velocities scaling linearly with fan rotational speeds. For shear flow, the study shows that vertical velocity gradients can be precisely regulated by adjusting fan speed increments across the array. A simplified one-dimensional momentum diffusion model, based on the Prandtl mixing-length hypothesis, is developed and successfully alidated against experimental data to predict the downstream evolution of these shear flows. Furthermore, the investigation into unsteady gust flows reveals that the wind generator acts as a low-pass filter. Higher-frequency modulations experience significant amplitude attenuation and phase lag, whereas low-frequency gusts maintain high fidelity. A predictive transfer function model is proposed to quantify these dynamic responses. This research provides a robust theoretical and experimental framework for utilizing fan-array systems to evaluate the aerodynamic performance and flight stability of low altitude vehicles.

In recent years, micro-electromechanical systems (MEMS) actuators have emerged as innovative solutions for enhancing the dynamic control of terahertz devices, leveraging their advantages of miniaturization, low power consumption, and high integration. This paper provides a comprehensive review of the fundamental technological advancements in terahertz MEMS actuators, with a particular emphasis on the analysis of the performance characteristics of various driving mechanisms and the integration strategies. Furthermore, it systematically presents the diverse forms of terahertz MEMS actuators utilized in terahertz switches and tunable resonators, highlighting the significant advancements they have made in applications including sensing, frequency and polarization tuning, beamforming, and logical operations. By leveraging cutting-edge microfabrication techniques and functional materials, terahertz MEMS actuators are capable of achieving wideband tuning, high-sensitivity sensing, and the modulation of intricate electromagnetic responses. Additionally, the review examines prospective development trajectories, offering theoretical insights and technical strategies to support the transition of terahertz technology from laboratory settings to practical applications in domains such as 6 G communication, high-resolution imaging, and intelligent sensing.

This paper proposes a novel 3D printing technology for fabricating high-performance volumetric electronic circuits. It introduces a particle-free conductive system based on a nontoxic reductant-functionalized polymer coating, offering a microscopic localized enhancement technique. By in-situ reducing silver onto custom-designed substrates, highly conductive and controllable coatings are formed, allowing frequency selective surfaces (FSS) to be extended from planar to spatial volumes (3DFSS). As a result, more desirable FSS characteristics can be achieved, such as significantly stable filtering performance for different incident angles and polarization modes of electromagnetic waves regardless of obliquity, compared to corresponding 2DFSS. The proposed FSS achieves a bandwidth of 4.4–7.8 GHz (fractional bandwidth of 54.8%) with a reflection coefficient below −10 dB. Design guidelines based on the equivalent circuit model (ECM) illustrate the transition from this 3DSS to a filter response. The designed array FSS boasts excellent structural compactness and mechanical robustness, enabling seamless integration with building materials. It provides electromagnetic signal modulation across scales, offering a technological solution for electromagnetic compatibility and signal transmission quality issues in miniaturized communication systems.