Search Results (689)

Owing to its inherent flight safety, low takeoff and landing requirements, and favorable economic efficiency, the autogyro UAV, especially its electric and hybrid-electric variants, has become a core platform for low-altitude aviation missions such as transportation, inspection, and surveillance in plateau and offshore regions. However, the low air density and low Reynolds number conditions encountered in plateau regions can induce aerodynamic issues such as premature laminar flow separation, dynamic stall, and increased induced drag, which directly reduce payload capacity and endurance of small electric autogyro UAVs. In offshore environments, strong winds, turbulence, and gust disturbances intensify rotor–wake interactions, cause abrupt variations in aerodynamic loads, and reduce control margins, which severely restricts the mission reliability and flight safety of low-altitude unmanned platforms. These environmental effects collectively degrade rotor performance, including reduced aerodynamic efficiency and insufficient lift generation, and further amplify the energy constraint of electric/hybrid-electric propulsion systems. In response to bottlenecks that restrict the practical application of autogyro UAVs in extreme environments, this paper systematically reviews research progress on lift vector control for autogyro UAV rotors operating under such conditions. First, the typical aerodynamic problems encountered by autogyro UAVs in plateau and offshore environments are summarized, and their underlying physical mechanisms are analyzed from both system-level and local-flow perspectives, with a focus on how environmental factors affect the autorotation stability of unmanned platforms. Subsequently, the development of passive lift vector control technologies is reviewed, with an emphasis on the aerodynamic benefits of passive pitch mechanisms, vortex generators, and blade-tip winglets, as well as their engineering feasibility for small autogyro UAV blades. Active lift vector control technologies are then examined, including air-jet flow control, synthetic jets, and trailing-edge flaps, with discussions of their potential to delay flow separation and stall, enhance rotor aerodynamic efficiency, and an assessment of their adaptability to the energy and structural constraints of unmanned platforms. Finally, a lift vector control strategy suitable for autorotating rotors of autogyro UAVs is proposed, based on careful consideration of energy consumption, structural constraints, and control effectiveness. It provides a reference for aerodynamic optimization and flight control research on electric and hybrid-electric autogyro UAVs operating in extremely low-altitude environments. Full article

Photon-counting constitute a promising technology for ultra-weak signal satellite communications. Considering the Poisson shot noise impairment, atmospheric turbulence fading, and multi-user interference, in this paper, a meta-learning-driven photon-counting multi-user single-input multiple-output (MU-SIMO) scheme is developed and analyzed. Referred to as meta-learning-driven signal detection (Meta-SD), this scheme can achieve rapid convergence with limited samples and significantly improve system detection performance. Simulation results demonstrate that the proposed meta-learning scheme outperforms the mean square error based signal detection (MSE-SD) baseline, in terms of detection accuracy, robustness to signal-dependent Poisson shot noise, convergence speed, and generalization to few-shot detection tasks with previously untrained signal classes. Specifically, Meta-SD achieves nearly a tenfold reduction in BER, compared with the derived MSE-SD benchmark, in a $4\times 8$ MU-SIMO scenario at $E_{\mathrm{s}}=-140$ dBJ. Full article

To addressthe challenge of attitude control in quad-ducted fan unmanned aerial vehicles (UAVs) under coupled disturbances comprising thrust lag, ground effect and a composite wind field during near-ground flight and to mitigate the inherent trade-off between disturbance rejection and noise suppression in fixed-bandwidth extended state observers (ESOs), this paper proposes a robust attitude control method based on a Sigmoid law adaptive-bandwidth extended state observer (AB-ESO). An attitude dynamic model covering the above multi-source disturbances is established, with all uncertainties uniformly treated as lumped disturbances. An adaptive-bandwidth mechanism with filtering and rate-limiting modules is designed for smooth continuous bandwidth tuning. A composite control framework integrating disturbance feedforward, lag compensation and attitude feedback is constructed, and the uniform ultimate boundedness of the closed-loop system is proved. Comparative simulations are conducted against six baseline controllers, including a cascade proportional–integral–derivative (PID) controller, fixed-bandwidth ESOs, incremental nonlinear dynamic inversion (INDI), fast terminal sliding mode control (FTSMC) and a time-varying bandwidth ESO, in a near-ground composite wind scenario. Results show that the proposed method achieves improved comprehensive performance: the three-axis average tracking root mean square error (RMSE) is approximately 72% lower than of the PID controller and 15.8% lower than that of the high-bandwidth ESO, and the control output total variation is reduced by about 27.8%. Monte Carlo verification with 100 random turbulence groups further validates the strong statistical robustness of the proposed method. All validations in this work are based on numerical simulations. This study provides a technical reference for high-precision control of ducted fan UAVs in near-ground environments. Full article

The Magdalena River Estuary hosts the access channel to the Port of Barranquilla, where recurrent dredging is required to maintain navigable depths of up to approximately 12 m. Chronic siltation in this channel is closely linked to the dynamics of the Turbidity Maximum Zone (TMZ), which remain poorly understood in tropical, microtidal systems with extreme sediment loads. This study investigates the spatiotemporal variability of the TMZ in the Magdalena River Estuary (MRE), Colombia, using a previously calibrated and validated MOHID 3D numerical model coupled with sediment transport. Sixteen scenarios covering river discharges from 2000 to 5500 m³ s⁻¹ under neap and spring tidal conditions were analyzed. Results show that the TMZ core position follows a nonlinear inverse relationship with discharge (R² = 0.976), migrating from km 13–15 under extreme low-flow conditions (Q = 2000 m³ s⁻¹) to the estuary mouth for discharges above 5000 m³ s⁻¹. Within the simulated discharge range of 2000–5500 m³ s⁻¹ and under the modeled neap and spring tidal conditions, the position where ε = 0.005 tracks the TMZ core location (R² = 0.96, RMSE ≈ 1 km), suggesting that this threshold can be used as a first-order spatial indicator of maximum sedimentation under the conditions evaluated in this study. Contrary to macrotidal estuaries, the MRE exhibits higher suspended-sediment concentrations during neap tides than during spring tides, with SSC up to 77 percent greater for Q = 2000 m³ s⁻¹. This reversal is driven by the suppression of turbulent mixing (R_i > 20) during neap conditions, which preserves the salt-wedge structure and enhances stratification-controlled sediment trapping. These results provide two process-based criteria for predicting turbidity-maximum behavior in the MRE: the ε = 0.005 stratification isoline and the discharge–TMZ polynomial. More broadly, the methodological framework may support the development of site-specific predictors for other highly stratified, microtidal estuaries subject to strong discharge variability. Full article

Pressure losses in heating, ventilation, and air-conditioning (HVAC) duct elbows significantly increase fan power requirements and reduce overall system efficiency. This study presents a comprehensive computational fluid dynamics (CFD) investigation aimed at identifying effective strategies for reducing pressure losses in 90° HVAC duct elbows. The numerical methodology was first validated against published experimental measurements, demonstrating excellent agreement and providing confidence in the predictive capability of the CFD model. The validated model was then employed to evaluate the influence of duct geometry, inlet velocity, guide vane configuration, inter-vane spacing, perforated guide vanes, and duct material roughness on aerodynamic performance using the SST k–ω turbulence model. The results show that round elbows reduce pressure losses by approximately 50% compared with hydraulically equivalent rectangular elbows, highlighting the strong influence of duct geometry on flow separation. Among the flow-control strategies investigated, curved guide vanes produced the greatest improvement, with an optimized three-vane arrangement and a non-dimensional spacing of $\mathrm{s}/{\mathrm{D}}_{\mathrm{h}}\approx 0.15$ (corresponding to 150 mm for the specific geometry tested) reducing pressure losses by approximately 31% relative to the baseline elbow without guide vanes. In contrast, the investigated perforated guide vane provided only marginal improvement, indicating that its geometry requires further optimization to minimize blockage and mixing losses. The material roughness study showed that smooth, rigid duct materials produced only minor differences in pressure loss, whereas flexible ducts generated noticeably higher losses because of their increased surface roughness. These findings demonstrate that optimizing elbow geometry and guide vane design is considerably more effective than modifying duct material or using the investigated perforated vane configuration. The study provides practical design recommendations for improving the aerodynamic performance and energy efficiency of HVAC duct systems. Full article

Accurate prediction of NO emissions is important for the development of low-emission gas turbine combustors. This study evaluates several NO formation models for a 78 MW F-class gas turbine using a decoupled post-processing framework. Steady RANS simulations were performed with a partially premixed flamelet/PDF combustion model. Thermal NO, prompt NO, the N₂O intermediate pathway, and turbulence–chemistry interaction were assessed at 50% and 100% load. Thermal NO was the dominant pathway and showed strong load dependence. Using partial equilibrium for O radicals increased outlet NO by 16.46% at 50% load and 43.69% at 100% load. Including partial-equilibrium OH further increased NO by 8.67% at 50% load but had little effect at full load. Prompt NO remained on the order of 10⁻³ ppm. The N₂O pathway and turbulence–chemistry interaction also affected the prediction, especially at full load. The selected model was further compared with field measurements during load ramping and pilot-ratio variation. Most load-ramping predictions agreed with measurements within 18%. The results demonstrate the applicability of the proposed framework for engineering NO emission prediction while also identifying limitations under transitional operating conditions. Full article

Can the distress of a cryptocurrency be predicted from its market behavior, and is that predictability reliable when it matters most? Using daily data for 609 USD-quoted coins traded on Kraken between 2013 and 2025, we built a survivorship-inclusive coin-quarter panel and model the onset of severe, sustained price distress—a deep, non-recovering drawdown relative to a trailing peak. A panel logit confirmed that realized volatility, illiquidity, weak momentum, and asset youth predict distress, with a coin-stratified cross-validated out-of-sample AUC of about 0.68. Our central contribution was to show that this predictability is regime-dependent. Interactions between coin-level signals and contemporaneous market-wide volatility are jointly significant (likelihood-ratio p < 0.001), and a rolling-origin evaluation reveals prospective accuracy swinging from no better than chance (AUC 0.43) to strong (0.79) across years. This regime-dependence is robust across alternative distress thresholds, regime proxies, data frequencies, cluster-bootstrap inference, and replication on a second exchange (Binance), though the individual signal channels are not. Testing the most natural mechanism—rising cross-asset co-movement in turbulent markets—we find no support. Microstructure-based early-warning signals for crypto distress are thus conditionally reliable: informative in calm markets but unreliable in the turbulent conditions where warning is most valuable. Full article

Physical control technology as an alternative to chemical pesticides is of great significance for modern sustainable agriculture. However, small pests such as tea green leafhoppers and tea thrips possess sensitive aerodynamic receptors that easily perceive disturbances generated by suction airflow and initiate stress-induced escape behavior. The current bottleneck in the state of the art is that conventional straight tube capture devices generate severe airflow pulsations and turbulence near the intake which act as alarm signals causing pests to escape before entering the effective capture zone. To resolve this trade-off, this study developed and validated a variable cross-section wind-suction channel based on the principle of biological behavioral suppression. This design incorporates a three-stage functional structure consisting of a flow-stabilizing intake section a guided acceleration section and a high negative-pressure throat to achieve a synergistic balance between low disturbance induction and strong aerodynamic confinement. Numerical simulations and prototype experiments show that this design establishes a stable rectified environment at the intake with relative wind speed deviations within 5%. Under controlled laboratory conditions, the average capture rate for target pests exceeded 85% within an effective operating radius of 20 cm. Comparative biological validation further confirms that the capture efficiency of this design is improved by more than 32 percentage points compared with a conventional straight-tube benchmark device. Furthermore, the critical stress-response distance was reduced by approximately 50%, significantly enhancing the concealment of the capture process. These findings elucidate the critical role of channel geometry in resolving the conflict between suction intensity and environmental disturbance providing a theoretical foundation and technical support for the development of high efficiency and precision plant protection equipment. Full article

Entrepreneurial marketing (EM) has emerged as a critical strategic orientation for small and medium-sized enterprises (SMEs) navigating volatile markets, yet the mechanisms through which EM is associated with environmental performance of SMEs remain theoretically underdeveloped. Drawing on entrepreneurial marketing theory, dynamic capabilities theory, the resource-based view, the natural resource-based view, and contingency theory, this study proposes and tests a two-stage capability model in which EM is linked to market agility and customer agility as parallel dynamic mechanisms that subsequently build marketing capability, which in turn is associated with environmental performance of SMEs. Market turbulence is examined as a boundary condition moderating the agility-to-capability pathways. Data were collected from 402 SME owners and managers across manufacturing and service sectors in the United Arab Emirates and analyzed using partial least squares structural equation modeling (PLS-SEM) in SmartPLS 4. Results confirm that EM is positively associated with market agility, customer agility, and marketing capability, and that both agility constructs partially mediate the EM-to-marketing capability relationship. Marketing capability shows a strong positive association with environmental performance of SMEs. Market turbulence significantly strengthens the market agility-to-marketing capability and customer agility-to-marketing capability relationships, while its moderating role in the direct EM-to-marketing capability path is not significant. These findings contribute to the entrepreneurial marketing and dynamic capability literature by specifying the organizational mechanisms linking EM to environmental performance and by identifying market turbulence as a selective boundary condition that strengthens agility-driven, but not orientation-driven, capability development. Practical implications for SME managers in emerging market contexts are discussed. Full article

Ammonia is increasingly regarded as a carbon-free energy carrier for hard-to-electrify power sectors, including marine propulsion, heavy-duty transport, and distributed generation. Its direct use in internal combustion engines, however, is constrained by high ignition energy, low laminar flame speed, narrow flammability limits, slow low-temperature chemistry, and strong trade-offs among efficiency, nitrogen-containing emissions, and unburned ammonia slip. Hydrogen enrichment is one of the most effective routes for improving ammonia combustion reactivity, but it also introduces a multivariable control problem: hydrogen fraction, ammonia injection timing, injection mode, air-path dilution, ignition strategy, and aftertreatment operation are tightly coupled and strongly condition-dependent. This review synthesizes recent progress in ammonia–hydrogen and ammonia-based dual-fuel engine control from a control-oriented perspective. The discussion first summarizes application scenarios, nonlinear combustion-mode transitions, emission-formation pathways, and control-relevant metrics. It then compares actuator-level strategies, including ammonia injection timing and staging, port and direct injection, hydrogen energy-fraction scheduling, excess-air-ratio and EGR control, high-energy ignition, and turbulent jet ignition. Advanced optimization methods are further reviewed, with emphasis on model predictive control, control-oriented combustion and emission models, artificial-intelligence-based virtual sensors, and reinforcement-learning control. The analysis shows that the central challenge is no longer whether ammonia can burn in an engine, but how a controller can keep the system inside a narrow moving window bounded by misfire, knock, NO_x, N₂O, and NH₃ slip. Finally, future research priorities are proposed, including engine–aftertreatment co-optimization, physics-informed virtual sensing, digital-twin-assisted calibration, lightweight deployment on electronic control units, and robust control under fuel and aging uncertainty. Full article

Construction dust is a major contributor to urban inhalable particulate matter (PM₁₀) pollution, posing severe respiratory and cardiovascular health risks to construction workers and nearby residents, severely undermining urban environmental sustainability. Construction fences are widely adopted as a primary dust mitigation measure, yet their underlying dispersion mechanisms and comprehensive impacts on vertical air quality remain poorly understood due to the limitations of traditional field monitoring and empirical models, creating critical barriers to site-level pollution control and long-term urban sustainability. In this study, a reliable computational fluid dynamics (CFD) method was developed to investigate the spatial distribution of construction dust and quantify the dust suppression performance of fences with heights ranging from 0 to 3 m. Three mainstream k-ε turbulence models (Standard, RNG, and Realizable) were evaluated using on-site measurement data, and the RNG k-ε model was found to provide the best agreement with field observations, with statistical metrics of q = 1, FB = 0.052, and NMSE = 0.028. The results show that construction fences effectively reduce dust dispersion into the surrounding environment, particularly in the pedestrian breathing zone (z < 1.5 m). Increasing the fence height from 1.5 m to 3 m improves the breathing-zone dust reduction rate from 39% to 55%, with the most significant mitigation effect observed within 50 m downwind of the fence. However, a critical dual effect was identified: while fences suppress near-ground pollution, they induce strong upward airflow and turbulence, leading to elevated dust concentrations in the upper part of the near-ground region (z = 1.5–9 m), a phenomenon absent in the no-fence scenario. These findings provide practical implications for urban construction site management, suggesting that fence height and configuration should be carefully designed not only to reduce pedestrian-level exposure but also to avoid unintended pollutant accumulation aloft, thereby improving overall air quality control strategies and delivering balanced, long-term environmental sustainability at construction sites. Full article

Small unmanned aerial vehicles (UAVs) are widely used in civil and military fields, and their noise problem has always been the industry’s focus. Compared with a traditional propeller fan, a ducted fan offers higher aerodynamic efficiency, lower aerodynamic noise, and greater safety. It has become the key power component of small UAVs. However, due to the rigid restriction on tip clearance, the traditional integral ducted fan is prone to generating a tip leakage vortex, which produces high-intensity aerodynamic noise and significantly reduces propulsion efficiency. To address the above key problem restricting the quiet flight of small UAVs, this paper designs a segmented ducted fan (SDF). It preliminarily explores the influence of the segmented clearance on the fan’s flow field structure and acoustic radiation characteristics. Specifically, the k-ω SST (shear stress transport) turbulence model and the broadband noise source model were used to establish a computational fluid dynamics model, and the effects of fan speed (20,000–40,000 rpm) and duct spacing (0–20 mm) on its aeroacoustic characteristics were systematically studied. The results showed that the SDF’s acoustic power level maximum (APL_max) was significantly higher than that of the traditional integral structure, especially at high speed. At 40,000 rpm, increasing the duct spacing to 20 mm resulted in a sudden increase in APL_max to 194.5 dB, 61.3 dB higher than that of the integral type. Its essence was derived from the three-stage chain amplification mechanism: (1) strong tip leakage vortex induced by geometric clearance; (2) broadband noise caused by vortex impacting the duct wall; (3) resonant coupling of leakage vortex harmonic frequency and duct cavity standing wave. Based on this, a collaborative noise reduction path was proposed: compressing the spacing to ≤10 mm to suppress the intensity of leakage vortex, designing the periodicity of failure vortex combined with the serrated blade tip/inner wall rubber strip, and blocking the acoustic cavity resonance with non-uniform wall stiffness or 8–10 kHz Helmholtz resonator, providing a solution for the low-noise design of UAV propulsion system. Unfortunately, our study cannot currently resolve transient characteristics; only time-averaged velocity/pressure flow-field contours and total acoustic power distribution are obtained for qualitative analysis of macroscopic noise variation laws and flow-sound correlation. Full article

Addressing unstable stratified oceanic turbulence, this paper develops a composite stratified oceanic turbulent phase screen model using power spectrum inversion, which fully accounts for the coupled effects of turbulence diffusion, absorption, and scattering. We investigate the intensity and phase evolution of Perfect Vortex Beams (PVBs) after propagation, comprehensively analyzing scintillation index variations across different topological charges, propagation distances, and turbulence parameters, alongside the Bit Error Rate (BER) of OOK-modulated underwater wireless optical communication (UWOC) systems. To mitigate turbulence-induced fading, multi-aperture reception is introduced, with performance gains evaluated as a function of aperture diameter D and number N. Results show that at propagation distances exceeding 55 m, higher-order PVBs exhibit significantly lower scintillation indices than lower-order ones due to their superior topological stability. Scintillation and BER intensify with decreasing kinetic energy dissipation or increasing mean-square temperature dissipation and temperature–salinity balance parameters, with temperature dissipation being the dominant factor. Multi-aperture reception effectively smooths channel fading by leveraging intensity fluctuation decorrelation. The equivalent scintillation index decreases significantly with increasing N and D, though marginal gains diminish as N grows. In weak turbulence, increasing D from 0.02 m to 0.06 m for a single aperture reduces the scintillation index by 46.3%; when the aperture number increases from N = 1 to 2, the equivalent scintillation index drops by an average of approximately 42%, confirming that N = 4~6 provides an optimal trade-off between complexity and performance. In strong turbulence, multi-aperture reception efficiency is higher; the first three apertures contribute approximately 65% of the total gain, and the marginal gain inflection point shifts from N ≈ 7 to N ≈ 5. This study provides a theoretical basis for designing robust UWOC systems. Full article

The lubrication performance of high-speed transmission systems directly affects mechanical power consumption and operational reliability. During high-speed oil-stirring lubrication, strong gas–liquid interfacial shear, liquid-film deformation, droplet splashing, and oil-mist transport generate an unsteady multiphase turbulent flow field. Conventional continuum-based numerical methods often face difficulties in resolving interface breakup and transient turbulent dissipation under high-speed rotational excitation. To address this problem, this study develops a coupled Lattice Boltzmann–Large Eddy Simulation (LBM–LES) method for oil–air two-phase flow in a high-speed oil-stirring lubrication system. The D3Q27 discrete velocity model, cumulant collision operator, WALE subgrid-scale model, free-surface tracking, and local grid refinement are integrated to analyze free-surface deformation, oil-mist evolution, and power-loss characteristics. Taking a notched toothless oil-stirring disk as the reference configuration, the effects of oil immersion depth and disk topology on gas–liquid phase distribution, oil-mist coverage, power consumption, and vortex-induced energy dissipation are investigated. The results indicate that oil immersion depth has a nonlinear influence on lubrication performance and power loss. Among the investigated cases, an immersion depth of 20 mm provides a favorable balance between upper-region oil-mist coverage and lower-region oil-pool stability. At this depth, the notched disk exhibits directional oil delivery and relatively low power consumption, whereas the double-rhombus structure expands the oil-mist coverage but increases the average power consumption to approximately 175 W. These findings provide numerical support for balancing oil-mist coverage, mechanical power consumption, and disk topology design in high-speed transmission lubrication systems. Full article

Understanding which structural parameters govern flow stability and particle separation is essential for turbo air classifier design. In this study, the Y160L-6 turbo air classifier was used to examine whether different categories of spatial structural parameters influence classification performance through the same flow mechanism or play distinct roles in regulating the internal flow field. Two representative parameters, namely the spacing between the secondary air inlet and the rotor cage and the spacing between the secondary air inlet and the feed inlet, were analyzed using computational fluid dynamics (CFD) coupled with the RNG k–ε turbulence model and the discrete phase model (DPM). The results show that the two parameters affect the classifier through different mechanisms. Increasing the secondary air inlet–rotor cage spacing causes a non-monotonic variation in wall pressure and tangential velocity, indicating a strong influence on the global swirling structure. At a spacing of 1490 mm, the pressure distribution in the classification zone becomes more uniform, the tangential velocity reaches a relatively high level, and the intensity of the precessing vortex core (PVC) is reduced. Under this condition, the cumulative proportion of 2–5 μm particles at the fine powder outlet increases by 34.1% compared with the initial configuration. In contrast, variations in the secondary air inlet–feed inlet spacing exert only a limited influence on the overall flow structure and classification characteristics under relatively low feed inlet velocity conditions, indicating that this parameter mainly affects local flow disturbance rather than global flow stability. These findings demonstrate that structural parameters associated with the coupling between secondary airflow and rotor rotation dominate classifier performance, whereas parameters related to feed–air interaction exert only a secondary effect under low feed momentum conditions. These findings provide design guidance for the investigated Y160L-6 turbo air classifier and may serve as a reference for similar classifier structures under comparable operating conditions. Full article