Search Results (343)

Changes in aviation turbulence at cruise altitudes have important implications for aviation safety under global warming scenarios in the future. Using projections from the NorESM2-MM model within the CMIP6 framework, this study evaluates changes in clear-air turbulence (CAT) at 250 hPa over Southwest China during the twenty-first century based on an ensemble of 15 diagnostic indices. The results show: (1) Historical moderate-or-greater (MOG) CAT peaks in a zonal belt near 30–35° N, with annual frequencies up to 1.6% over the Hengduan and Karakoram Mountains. Future increases remain focused in this belt, are stronger and more extensive under SSP5-8.5, peak in winter and spring, and weaken over much of the Plateau interior in summer. (2) Future changes are intensity-dependent: stronger categories show larger relative increases, and PDF changes are concentrated in the right tail, indicating amplified extreme turbulence. The 19-year moving-average time series shows that MOG-CAT increases by 28.3% and 36.5% under SSP2-4.5 and SSP5-8.5, respectively, by the mid-twenty-first century, and by 26.0% and 69.4% by the late twenty-first century. (3) Along the Chengdu–Lhasa corridor, winter MOG-CAT increases in all three segments. Under SSP5-8.5, median increases are about 50% in the Basin and Plateau segments and about 85% in the Transition segment, with most diagnostics ranging from 50% to 180%. (4) High-emission scenarios are more likely to cause turbulence and instability in the southwestern region, potentially posing greater challenges for aviation turbulence warning and safety assurance. Full article

The traditional Yikeyin dwellings in central Yunnan exhibit a distinctive spatial layout and skywell design that passively adapt to the mild plateau monsoon climate through natural ventilation. Although their courtyard-based configuration and skylight design are widely recognized for climatic adaptability, the quantitative relationship between courtyard orientation and ventilation performance remains insufficiently explored. This study integrates on-site environmental monitoring with validated Computational Fluid Dynamics (CFD) simulations to investigate how different courtyard orientations influence airflow organization and the indoor thermal environment. Based on detailed field surveys and measured data, three representative orientation schemes were established. The RNG k-ε turbulence model was adopted, and one-way coupled simulations using OpenFOAM and EnergyPlus were conducted to evaluate seasonal ventilation behavior and indoor thermal comfort. The findings reveal synergistic design principles between building orientation and courtyard spatial configuration, as well as spatial differentiation patterns contributing to thermal environment stability. Three orientation types—leeward, windward, and transitional—were identified, each demonstrating distinct advantages and limitations. The study quantitatively confirms the effectiveness of Yikeyin dwellings in utilizing natural ventilation for environmental regulation during both summer and winter seasons. These results provide scientific evidence and design support for modern buildings seeking to achieve enhanced ventilation performance and climatic adaptability. Full article

Dam-break flows over erodible beds represent a complex fluid–solid interaction problem characterized by extreme turbulence and rapid morphological changes. This study investigates the dynamics of such flows over inclined granular beds by integrating an advanced Moving Particle Semi-implicit (MPS) method. To accurately resolve the transition between static and kinetic granular regimes, I introduce a state-dependent tangential friction model that explicitly distinguishes between sticking and sliding conditions based on local force balance. Furthermore, the momentum exchange between the fluid and solid phases is rigorously modeled using the porosity-dependent drag formulation. The numerical results demonstrate a distinct regime shift in energy dissipation: while low-inclination beds (0–$4\%$) promote distributed sediment transport, steep-inclination beds (8–$12\%$) trigger a localized “Shielding Effect”. In this regime, the surge’s horizontal kinetic energy is rapidly converted into vertical potential energy and frictional work, forming a deep sacrificial scour hole that acts as a topographical energy sink. This mechanism effectively mitigates the destructive potential of the surge in downstream areas. The proposed method provides a robust tool for predicting morphological feedback and designing topographical countermeasures for disaster mitigation in hydraulic and coastal environments. Full article

Hydropower development in high-altitude regions increasingly confronts a challenging “trilemma”: high hydraulic heads, large unit discharges, and spatially constrained narrow valleys. Under such conditions, conventional energy dissipation measures frequently fail to prevent downstream riverbed scour, thereby threatening both ecological integrity and infrastructure safety. This study aims to propose, parametrically optimize, and physically validate a novel composite energy dissipation structure designed to resolve this specific trilemma based on a pressure-dividing transition mechanism. Using the Louli Hydropower Project as a case study (Q_max = 6944 m³/s, unit discharge q = 119 m³/(s·m), available basin length L = 78 m), we conducted systematic 1:100 scale physical model tests. The results demonstrate that conventional optimizations, such as secondary stilling basins and dentated sills, are ineffective under these boundary conditions, leading to incomplete hydraulic jumps and extended high-velocity zones. In contrast, the proposed composite structure, which integrates a deepened stilling basin (depth = 9 m), asymmetric sidewall widening (20 m offset), and a gentle slope transition (1:20 gradient), achieved superior performance. Under the 50-year design flood with controlled discharge operation, the energy dissipation rate increased significantly from 32.11% (baseline) to 63.49% (composite) at the end sill. Furthermore, the structure reduced comprehensive turbulence intensity by 17.8% and floor slab impact stress by 23.4%. These findings validate the composite system as a sustainable solution for high-head dams in constrained settings, offering benefits for riverbed protection and structural durability. Full article

Background: Assessing aortic dissection (AD) in its early stages is crucial for cardiovascular surgeons to improve patient outcomes and avoid complications associated with surgical intervention for type A aortic dissection. Initial evaluations rely on patient referrals for computed tomography (CT) scans, which involve measuring the maximum aortic diameter. Objective: This study aimed to improve current diagnostic thresholds for type A aortic dissection by using computational fluid dynamics (CFD) modeling to correlate hemodynamic factors related to the wall shear stress with maximum aortic diameter growth rate, offering insights into predicting AD progression and reassessing current diameter-based diagnostic criteria. Methods: The pre- and post-AD scan data, with an average duration of three and a half years for the 15 patients, were converted into 3D geometries. These geometries were analyzed using the transitional-turbulent CFD model. Wall shear stress (WSS), its derivatives, and the pressure gradient from the pre-AD CT scans were compared across 15 patients, grouped according to the aortic diameter growth per year. Results: For patients in group 1 (nine patients with normal diagnosis), pre-AD time-average wall shear stress (TAWSS) was mostly 2–4 Pa, above physiologic levels. Post-AD, values dropped below 1.5 Pa (stagnant, thrombus-prone), with oscillatory shear index (OSI) elevated (0.24–0.32). In group 2 (n = 6, abnormal diagnosis), post-AD TAWSS was <3 Pa (thrombosis risk), with OSI 0.1–0.31 near tear sites. These findings confirm a dual-risk profile: low TAWSS promotes thrombosis, while high TAWSS drives dissection progression. Conclusions: WSS parameters, such as TAWSS and OSI, can be utilized to assess the development of a dilated ascending aorta, especially for extreme maximum aortic diameter. Pre-AD analysis for some patients revealed a strong negative correlation, indicating that high shear stress in the true lumen (TL) results in a drop in diastolic pressure post-AD at the upward-going section of the aorta. Full article

Energy dissipation in stepped weirs depends on the complex interaction between geometry, flow regime, and surface aeration. The research proposes a dimensionless empirical model (RE3T) to predict the overall energy dissipation in three-section stepped weirs with variable slopes. The formulation integrates dimensional analysis based on the Vaschy–Buckingham theorem, controlled physical experimentation, and three-dimensional numerical simulations using CFD employing the RANS–SST turbulence model implemented in ANSYS CFX. Eighteen numerical simulations were performed covering seven geometric configurations and four hydraulic inlet conditions, covering slug, transitional, and skimming flow regimes. The CFD model was previously validated by comparison with a physical scale model, obtaining a discrepancy of only 0.38% in relative energy dissipation. The validated dataset was then used to calibrate an empirical multiplicative correlation composed of eight dimensionless groups associated with sectional slopes, number of steps, overall geometric ratio, and upstream Froude number. The proposed model achieved a coefficient of determination R² = 0.81, with relative errors generally less than 1% and a maximum deviation of 2.34%. The statistical indicators (RMSE, MAE, and bias) confirm the absence of significant systematic trends within the defined domain of validity. The results show that the Froude number and the slopes of the sections are the variables with the greatest influence on overall dissipation. The RE3T formulation is a physically consistent and computationally efficient predictive tool for the design and analysis of stepped weirs with variable slopes, extending the scope of traditional correlations developed for uniform slopes. Full article

This study evaluated the hydraulic performance of regular and irregular stepped spillways experimentally to reduce the hydraulic leap length and enhance energy dissipation. The study tested fourteen physical models with 40° and 45° slopes and step numbers of 5 and 10, analyzing the effect of a single barrier block and its horizontal position through 98 rectangular flume experiments to evaluate energy dissipation and hydraulic jump length. The results showed that when the nappe flow transitioned to the skimming flow, energy dissipation decreased as discharge increased. Irregular stepped spillways achieved higher energy dissipation than regular ones by about 10–25%, with five-step models outperforming ten-step models due to increased turbulence. A strong positive relationship between discharge and hydraulic jump length was also observed, with jump length increasing by approximately 30–45% at 40° and 45° slopes. Five-degree irregular stepped spillways produced the shortest hydraulic jump lengths, confirming that step irregularity reduces downstream residual energy. Adding a single barrier block improved performance by shortening the hydraulic jump by about 20–35%, especially at higher discharges, with the optimal position at B/2. Overall, an irregular stepped spillway with a barrier block at B/2 was identified as the most effective configuration, enabling shorter hydraulic jumps, smaller stilling basins, and more efficient and economical spillway designs. Full article

While twisted tape inserts are widely used for heat transfer enhancement, the specific impact of tape thickness remains under-explored. This study provides a systematic numerical investigation into the thermo-hydraulic performance of a double-pipe heat exchanger equipped with twisted perforated tape (TPT) inserts of varying thicknesses (1, 1.5, and 2 mm). Using a validated 3D SST k−ω model across Re = 1000–12,000, the research establishes a mechanistic distinction between flow regimes. The results indicate that the 2 mm TPT yields the highest enhancement, achieving a 78.6% increase in the average Nusselt number (Nu_avg) and a 67.8% improvement in the overall heat transfer coefficient at Re = 12,000. Quantitative analysis of secondary flow intensity and turbulence kinetic energy confirms a transition from geometry-induced swirl at low Re to turbulence-driven shear at high Re. Despite a pressure drop penalty of up to 3.26 times the plain tube, the thermal performance factor remained above unity for all cases, peaking at 1.17 at Re ≈ 4000. These findings establish tape thickness as a first-order design variable for optimizing high-performance thermal systems. Full article

To support the increasing complexity of innovation, design, and performance evaluation in the maritime industry, a ship-specific computational fluid dynamics (CFD) software suite tailored to incompressible viscous flow is required. This study utilizes the MarineFlow marine fluid dynamics code to explore numerical simulation schemes for cylindrical flow problems across a broad range of Reynolds numbers (1–10⁷) that are applicable to self-developed codes. Additionally, an analysis of the flow around a cylinder is conducted from the perspective of code developers. Various grid types and turbulence model schemes are employed to analyze and compare the drag coefficient, separation points, and pressure distribution characteristics of the cylinder. The results obtained from these simulations are then contrasted with those derived from commercial CFD software to assess their accuracy. Despite the presence of certain numerical artifacts, within the Reynolds number range of 1–10⁵, the unstructured grids combined with the laminar flow models effectively capture experimental data. Further exploration of the transitional Reynolds number range ($Re$ = $2\times {10}^5$–$6\times {10}^5$) shows a consistent decreasing trend in the mean drag coefficient, although significant deviations from theoretical predictions are evident. From the perspective of code developers, this study aims to reveal the limitations of current computational schemes and code architecture in accurately capturing flow dynamics within the transitional Reynolds number range. This provides a crucial basis for future optimization of turbulence models and algorithmic improvements, which are essential for the continued development of self-developed CFD codes and their engineering applications. Full article

Financial markets often display nonlinear and turbulent dynamics during periods of stress, and crude-oil and global equity systems frequently demonstrate closely connected forms of instability. Earlier studies report multifractality, chaotic features and regime-dependent spillovers across commodities and equities, yet existing approaches rarely succeed in capturing both the intrinsic complexity of oil-market behavior and the changing structure of cross-asset dependence. This limitation reduces the ability to distinguish calm from turbulent regimes and weakens short-horizon risk assessment. The present study introduces a unified framework that quantifies and predicts systemic instability within the coupled oil–equity system. The analysis constructs a crude-oil complexity index based on multifractal fluctuation analysis, permutation and approximate entropy, and Lyapunov-based indicators of chaotic dynamics. At the same time, it develops an information-theoretic network of global equity and energy-sector returns and summarizes its instability through measures of edge turnover, spectral radius, degree entropy and strength dispersion. These components are combined to form the Coupled Multiscale Chaos Index (CMCI), a scalar state variable that distinguishes calm, transitional and chaotic market regimes. Empirical results indicate that Brent and WTI exhibit pronounced multifractality, elevated entropy and positive Lyapunov exponents, while the dependence network becomes more centralized, more clustered and more capable of shock amplification during high-CMCI states. The CMCI moves closely with realized volatility and provides significant predictive content for five-day variance across major global equity benchmarks, with performance superior to models that rely only on macro-financial controls. Out-of-sample evaluation shows that forecasts incorporating measures of complexity record substantially lower MSE and QLIKE losses. The findings indicate that systemic instability reflects the interaction between local chaotic dynamics in crude-oil markets and turbulence in the global dependence network. The CMCI offers a practical early-warning indicator that supports risk management, forecasting and macroprudential supervision. Full article

High-speed, low-pressure turbines in geared turbofans operate at transonic exit Mach numbers and low Reynolds numbers. Engine-relevant data remain scarce. The SPLEEN C1 linear cascade was investigated at $M_{\mathrm{out}}=0.70–0.95$ and $R{e}_{\mathrm{out}}=\mathrm{65,000}–\mathrm{120,000}$ under steady inlet flow. Experiments were combined with 2D RANS and MISES, including transition modeling and inlet-turbulence decay calibrated to measurements. Results are consistent with conventional LPT behavior: loss decreased with increasing Mach and Reynolds numbers, except when shocks interacted with the blade boundary layer ($M\approx 0.95$). Profile loss dropped by $23\%$ from $M=0.70$ to $0.95$ at $Re=\mathrm{70,000}$, as well as by $19\%$ at $M=0.80$ when open separation is suppressed. Secondary loss decreased by up to $25\%$ at $Re=\mathrm{70,000}$ and showed weak sensitivity to the Reynolds number. A coupled loss model predicted profile loss with a root-mean square error of 4.7%. Secondary-loss modeling reproduced global trends: separating endwall dissipation from mixing kept errors within $\pm 10\%$ for most cases, but accuracy degraded near the shock–boundary layer interaction case and at the highest Reynolds number. Mixing dominated endwall loss (∼75%), with the passage vortex contributing ∼50% (±10%) of the mixing component. Full article

Laminar separation bubbles (LSBs) on low-Reynolds-number airfoils are sustained by intrinsic unsteadiness driven by Kelvin–Helmholtz (K-H) growth in the separated shear layer. Using incompressible 2D URANS with the SA-$\gamma$ transition model for a NACA 0012 airfoil at $Re=5.3\times {10}^4$, we reveal that numerical dissipation behaves as a critical bifurcation parameter. Validated against the recent Jardin (2025) experimental benchmark, the physical state correctly resolves the LSB-induced pressure plateau ($C_p$) and local negative skin friction ($C_f<0$). However, when numerical dissipation exceeds the K-H instability growth rate, the physical limit-cycle oscillation collapses into a spurious fixed-point attractor—a phenomenon defined as numerical quenching. This pseudo-convergence triggers a catastrophic ∼30% deficit in mean lift ($C_l$). Furthermore, at $\alpha =6^{\circ }$, a drag-mechanism inversion is identified: while the physical branch is dominated by LSB-induced pressure (form) drag, the quenched branch exhibits a non-physical drag surge that exceeds the fully turbulent baseline. Phase portraits and power spectral densities ($St\approx 0.2$) provide objective diagnostics, demonstrating that standard residual convergence is a deceptive indicator of physical fidelity in transitional separated aerodynamics. Full article

To address the challenge of coordinating economic and environmental objectives for Multi-energy Virtual Power Plants (MEVPPs), particularly under ambitious decarbonization policies such as China’s “dual carbon” goals, this paper proposes a novel two-stage scheduling framework that integrates Deep Reinforcement Learning (DRL) with Model Predictive Control (MPC). The core innovations include the following: (1) high-fidelity physical models capturing wind turbulence correction, photovoltaic temperature-irradiation coupling, and state-of-charge-dependent energy storage efficiency, improving equipment dynamic characterization accuracy by 12.7% compared to conventional models; (2) an enhanced Multi-Agent Deep Deterministic Policy Gradient (MADDPG) algorithm incorporating priority experience replay and adaptive noise exploration, which accelerates convergence by 15.6%; (3) a pioneering coordination architecture of “Day-Ahead MADDPG—Real-Time MPC” that manages uncertainties through bidirectional feedback, where real-time deviations refine the long-term policy via experience replay. Simulation results using historical data from a North China industrial park demonstrate that the framework reduces operating costs by 13.3% and carbon emissions by 17.7% compared to particle swarm optimization, outperforms standard DDPG with 3.2% lower operating costs, 5.8% lower carbon emissions, and a 3.3% higher renewable utilization rate (88.6%), and achieves 55% renewable penetration with only 4.1% curtailment. These results validate the framework’s scalability for high-renewable penetration grids and its real-time feasibility, as confirmed by edge computing deployment with latency below 50 ms. This study offers a technically viable and scalable solution for the operation of low-carbon virtual power plants (VPPs), supporting the transition towards sustainable power systems. Full article

The global transition of offshore wind energy into deep-water environments necessitates precise modeling of the complex, nonlinear dynamic responses of floating offshore wind turbines (FOWTs) to stochastic loads. Traditional industry-standard simulation tools often rely on potential flow theory, which neglects critical viscous effects and requires manual, empirical tuning of damping coefficients, reducing model reliability, while CFD modeling demands large computational resources. This paper introduces an application of advanced neural network techniques to model the coupled dynamic response of FOWTs under varied ocean conditions, reducing the simulation time required for training high-fidelity models. The architecture was trained using experimental data from the OC5 semi-submersible platform under the LC4.1 load case and further validated across a matrix of heterogeneous conditions, encompassing steady, turbulent, and irregular wind and wave environments. Results demonstrate exceptional predictive accuracy across coupled degrees of freedom (Heave, Pitch, and Surge), with the model achieving a coefficient of determination (${\mathrm{R}}^2>0.9$) and maintaining superior phase coherence without discernible time lag. Power spectral density analysis confirms the model’s robust ability to capture resonant frequencies and hydrodynamic restoration across varied sea states. This data-driven framework provides a robust, near-instantaneous alternative for simulating FOWTs global dynamics. By successfully capturing complex nonlinear interactions and inertial effects, the methodology enables rapid decision-making in preliminary design, real-time digital twinning, and accelerated long-term fatigue analysis for safety-critical offshore applications. Full article

Vertical takeoff and landing (VTOL) aircraft must balance the conflicting demands of hover and cruise performance. To address the lack of integrated design methodologies in the existing literature, a unified design-optimization framework is presented, coupling high-fidelity CFD simulations with a genetic algorithm to refine a gas-driven thrust fan (GDTF) VTOL nacelle. Key geometric parameters—fan pressure ratio pressure ratio, fan tilt, nozzle angle, tail inclination, and tip shape—were varied in a comprehensive parametric study to maximize lift-to-drag ratio and maintain constant mass flow. The optimization reveals that a nearly horizontal fan axis maximizes cruise efficiency (${\displaystyle \frac{L}{D}}\approx 2.98$), a nozzle angle of about $22°$ offers the best lift-vs-drag compromise during transition, and refining the tip geometry yields a $10$–$20\%$ performance boost. To validate the numerical predictions, a 1:1.05 scale VTOL nacelle model (fan diameter $D = 0.42 \mathrm{m}$) was fabricated and tested in a low-speed wind tunnel at $52 {\displaystyle \frac{\mathrm{m}}{\mathrm{s}}}$ ($Re \approx 5 \times {10}^6$, turbulence intensity ≈ 2%). Total-pressure probes at the intake exit plane and static taps along the inner cowl wall provided detailed pressure distributions, from which exit Mach number, velocity and the equivalent flow coefficient φ (≈0.68 under test conditions) were derived. Oil-flow visualization on the external cowl surface confirmed smooth, attached streamlines with no large separation bubbles. This dual validation combining surface-flow visualization and pressure-recovery mapping demonstrates the accuracy and reliability of the proposed simulation methodology. By successfully bridging detailed CFD with genetic-algorithm-driven design and validating against comprehensive wind-tunnel measurements, this integrated approach paves the way for next-generation VTOL configurations with longer range and lower fuel consumption. Full article