Search Results (2,000)

Heating, ventilation and air conditioning (HVAC) accounts for an overwhelmingly large proportion of building energy consumption. Mass low-grade cold and heat energy dissipates during system operation, endowing pulsating heat pipes (PHPs) with promising application prospects in building energy systems. Combining experimental tests and the phase-space reconstruction method, this paper investigates the intrinsic correlation between the flow behavior of working fluids and chaotic characteristics under varied working fluids, heating powers and liquid filling ratios. The working fluid type dominates the oscillation characteristics of pulsating heat pipes. When distilled water serves as the working fluid, the attractor presents a scattered distribution. In contrast, the PHP charged with HFE-7100 achieves stable unidirectional circulation with high-frequency, small-amplitude pulsation, forming a densely distributed attractor. Increasing the flow velocity of the working fluid drives the attractor distribution to evolve from scattered to concentrated. Intermittent flow of the working fluid induces a multi-temperature-zone distribution on the tube wall, and the attractor takes on a multi-region spiral morphology. A low liquid filling ratio triggers working fluid dry-out, and the attractor trajectory maintains a continuous unidirectional upward trend; by comparison, the attractor shows a multi-region spiral distribution under high filling ratio conditions. Research on chaotic dynamic characteristic identification, evolutionary law analysis and stable domain regulation of pulsating heat pipes can lay a theoretical foundation for structural optimization and operating condition adjustment of high-performance pulsating heat pipe devices for building waste heat recovery. Full article

The start-up process of a siphon vertical axial-flow pump is accompanied by rapid internal-flow reconstruction, transient hydraulic loading and unsteady rotor response, which directly affect the operational stability of the pump system. In this study, the unsteady internal flow evolution and rotordynamic characteristics of a siphon vertical axial-flow pump during start-up were investigated using a transient numerical method with dynamic rotational-speed updating. The instantaneous impeller speed was solved based on a torque-balance equation considering motor driving torque, hydraulic resistance torque and rotor inertia, and the angular-velocity boundary condition of the rotating domain was updated at each time step through a custom UDF routine. The numerical model was validated against model-test data, and good agreement was obtained for both pump head and efficiency. Based on the validated model, the flow-angle distribution, vortex stretching term, blade-surface pressure, rotor mechanical response, radial-force time–frequency characteristics and blade-loading variation were analyzed. The results show that the internal flow in the main pump section evolves from a strongly unsteady swirling state to an axially dominated quasi-steady state. In the early stage, obvious pre-swirl, local backflow and strong vortex stretching occur near the impeller inlet, blade-tip clearance and impeller–guide-vane interaction region. With increasing rotational speed and flow rate, the disordered vortical structures are gradually suppressed, and the internal flow becomes more organized. The rotor response exhibits clear stage-dependent characteristics, and the radial force is more sensitive to local flow instability than the axial force and torque. Continuous wavelet transform and variational mode decomposition further indicate that the radial-force signal is dominated by low-frequency transient excitation in the early stage, while medium- and high-frequency modulation components appear in the later stage. This study reveals the coupling mechanism between transient internal-flow evolution and rotor dynamic response during pump start-up, providing guidance for improving the start-up stability of siphon vertical axial-flow pump systems. Full article

The cleaning system of combine harvesters is an important component whose efficiency directly depends on the uniform distribution of the air velocity profile on the sieve surface. However, the experimental evaluation of the air flow rate is often limited by the high cost and the impossibility of simultaneous multi-point acquisition using commercial anemometers. This paper presents the design, development, and calibration of a low-cost anemometric sensor, based on an NTC thermistor, intended for aerodynamic optimization in the design and laboratory testing phase of cleaning systems. The proposed system replaces the need to use Wheatstone bridges by combining a constant current source, a 16-bit ADC converter, and an RC filter, allowing it to resolve fine voltage variations at low velocity. The algorithm integrates temperature compensation in the 3rd degree polynomial equation by simultaneously reading the environmental temperature using a digital sensor with an accuracy of ±0.1 °C. Experimental validation on the bench against the reference anemometer testo 405i showed excellent agreement (R² = 0.998, mean bias = 0.017 m/s, and a maximum error of ±0.08 m/s), falling within the tolerance of the reference anemometer. By the ability to use multiple sensors and parallel acquisition in real time, the proposed solution offers an alternative for 2D/3D aerodynamic mapping of sieves under controlled laboratory conditions for the design of combine harvester cleaning systems. Full article

Frigatebirds achieve exceptional long-endurance flight by efficiently utilizing marine atmospheric updrafts, offering a valuable biological prototype for low-energy bionic aircraft. Most existing studies focus on uniform inflow and single-axis attitudes, lacking systematic analysis of coupled-attitude aerodynamics and flow-field mechanisms in updraft environments. This work numerically investigates both clean-wing and fuselage-equipped frigatebird-inspired configurations under pitch-alone, pitch–yaw coupled, and yaw–roll coupled attitudes using the RANS/k-$\omega$ SST method. Results show that updraft effects exhibit strong pitch-angle dependence: significant drag reduction (maximum 0.0984) and lift augmentation occur within $-{12}^{\circ }<\alpha <4^{\circ }$, while above $\alpha =4^{\circ }$ updrafts suppress full-span deep stall and extend the stall angle by more than $8^{\circ }$, with a maximum lift increment of 220% near the wingtip. Under pitch–yaw coupling, a high-efficiency aerodynamic window emerges with a physically meaningful peak lift-to-drag ratio of 9.55. For yaw–roll coupling, the rolling moment is fundamentally driven by sideslip-induced asymmetric separation bubbles rather than by roll angle itself, as confirmed by surface limiting streamlines, skin friction distributions, and velocity vector plots. The bionic wing also achieves a near-elliptical spanwise lift distribution with an Oswald efficiency of approximately 0.994. This study provides key support for aerodynamic design, attitude control, and updraft energy harvesting of bird-like aerial vehicles. Full article

The Rheinstahl–Heraeus with ladle bottom-blowing (LBB-RH) vacuum refining process has been developed to enhance circulation flow and decarburization efficiency for ultra-low carbon steel production. However, current numerical models rely on the steady-state assumption for Ar-molten steel flow, neglecting the transient evolution of CO bubbles generated by decarburization in LBB-RH. Thus, a transient coupled model integrating unsteady CO-Ar-molten steel flow with decarburization kinetics is established in this paper. The transient distributions of CO gas, flow velocity, circulation flow rate, and carbon mass concentration are systematically investigated. The results show that LBB-RH achieves higher velocity on the main longitudinal section but lower velocity at the vacuum chamber free surface. The circulation flow rate of LBB-RH reaches 1.58 times that of conventional RH at the 25th minute. Under identical argon flow rates, the carbon mass concentration in LBB-RH is less than that in conventional RH, but the difference diminishes to 4 × 10⁻⁶ at the 25th minute as the decarburization rate decays significantly. At the 4th minute, the average carbon mass concentration in LBB-RH is only 73.26% of that in conventional RH. The proposed transient coupled model quantitatively elucidates the transfer mechanism in LBB-RH, and offers a theoretical foundation for the industrial production of ultra-low carbon steel with enhanced overall performance. Full article

Preliminary aerodynamic assessment provides an efficient means of investigating internal flow organization in aircraft gas turbine combustors before more computationally demanding three-dimensional and reactive-flow simulations are undertaken. This study presents a preliminary CFD investigation of cold-flow phenomena in the annular combustion chamber of the F100-PW-229 low-bypass turbofan engine. A two-dimensional planar model was intentionally adopted to identify dominant flow structures and assess their response to representative engine operating conditions at reduced computational cost. Numerical simulations were performed in ANSYS Fluent using a pressure-based coupled solver and the k-ω SST turbulence model for idle, cruise, and maximum rotational speed conditions. Additional simulations were performed at cruise conditions for an altitude of 11 km to examine the influence of reduced ambient pressure and air density on the internal flow field. The results show that increasing engine rotational speed primarily changes the intensity of the aerodynamic field while preserving its dominant spatial organization. The maximum velocity at idle was approximately 55% lower than that at maximum rotational speed, while the corresponding value at cruise was 20.1% lower. Similarly, the maximum stagnation pressure at idle was approximately 67.1% lower than that at maximum rotational speed, while the difference between cruise and maximum speed was 24.2%. A distinct vortex structure was identified in the outer annular passage between the liner and casing, with its location shifting downstream as the operating condition changed. At 11 km altitude, the calculated pressure level was approximately 81% lower and the characteristic flow velocity approximately 9% lower than under corresponding ground-level conditions, while the dominant flow topology remained similar. These results highlight the role of combustor geometry and secondary-air distribution in governing global pressure redistribution, velocity development, and recirculation behavior. Within the scope of the adopted two-dimensional non-reacting formulation, the obtained flow patterns provide an initial aerodynamic reference for identifying regions of interest for subsequent high-fidelity simulations. The proposed approach is therefore intended as a computationally efficient preliminary engineering tool rather than a substitute for validated three-dimensional combustor modelling. Full article

Aircraft icing can substantially alter leading-edge geometry and degrade aerodynamic performance, highlighting the need for computationally efficient prediction methods during preliminary aircraft design. This study presents a two-dimensional reduced-order framework for airfoil ice-accretion prediction that couples a Hess–Smith potential-flow solver with Lagrangian droplet tracking, surface collection-efficiency reconstruction, a low-order freezing model, and an iterative geometry-update procedure. After each ice-accretion increment, the aerodynamic flow field and droplet trajectories are recomputed over the updated geometry, thereby capturing the coupled effects of ice growth, local flow acceleration, and downstream droplet impingement. A convergence study was performed to establish suitable surface and particle discretization. The predictive capability of the framework was assessed against four experimental NACA 23012 ice-accretion geometries representing streamwise and roughness-dominated configurations. The numerical predictions reproduced the location, extent, and principal morphological characteristics of the measured leading-edge deposits. Parametric investigations showed that the collection-efficiency distribution is governed primarily by the angle of attack, median volumetric diameter, and freestream velocity, whereas the maximum ice thickness is controlled predominantly by liquid water content and ambient temperature. Two-parameter response maps further revealed nonlinear interactions among droplet inertia, aerodynamic transport, incident water flux, freezing conditions, and geometry evolution. The proposed framework provides a practical, low-cost tool for preliminary icing assessment, sensitivity analysis, and rapid screening of atmospheric and operating conditions prior to higher-fidelity investigation. Full article

CO₂ plume geothermal systems (CPGS) hold significant potential for geothermal energy extraction due to their dual benefits of carbon sequestration and efficient heat transfer. Existing studies primarily focus on macroscopic models of geothermal systems, with limited evaluation of real thermal reservoir cores in terms of CPGS heat extraction performance. This study investigates the seepage and heat transfer characteristics of CO₂ under various injection conditions through a combination of experiments and numerical simulations. Experimental results indicate that under the tested conditions, CO₂ exhibits 20–50% higher heat transfer capacity compared to distilled water. The heat transfer capacity increases with injection flow rate, while injection temperature shows a relatively limited influence under high-velocity seepage conditions. Specifically, at low injection flow rates, lower injection temperatures result in higher heat transfer capacity due to the increased thermal driving force between CO₂ and the reservoir rock. Microscale seepage-heat transfer simulations reveal that an increase in injection flow rate enhances CO₂ flow velocity within the pore network, expands the swept volume, and consequently strengthens convective heat transfer and increases the effective heat exchange area. Additionally, higher thermal reservoir temperatures establish a greater temperature gradient between the working fluid and rock, thereby enhancing heat transfer between CO₂ and the reservoir rock. The findings of this study provide valuable insights for optimizing CPGS design, particularly in understanding the impact of injection conditions on heat transfer capacity, with both practical engineering and theoretical implications. Full article

This paper presents a systematic reward engineering methodology for training a Proximal Policy Optimization (PPO) quadrotor navigation policy in the Webots simulator, using a hierarchical architecture in which a PID controller handles low-level stabilization and a PPO policy issues velocity commands. We document the complete evolution of a composite ten-term reward function across seven versions (v5 through v11) and retrain the key versions with multiple independent training seeds. The multi-seed study revises the single-seed history: penalty-dominated configurations (v8, v10) fail across all seeds, while the strongest historical version proves seed-sensitive (v11: 32.2 +/− 15.8%). An ablation removing the continuous distance-shaping term from v11 yields 0% success across seven seeds, identifying that term as necessary for convergence. We further isolate a previously hidden co-factor: with the library-default exploration-noise initialization (sigma_0 = 1.0), sampled actions saturate the bounded action space, the exploration variance receives no learning gradient, and curriculum progression deadlocks regardless of reward design; initializing sigma_0 = 0.37 restores gradient flow. With this correction and a deterministic evaluation-gated curriculum, the final configuration is evaluated across the full curriculum rather than at a single operating point: across five independent training seeds under a deterministic protocol, it attains 95.0% ± 6.2% navigation success at Stage 0 conditions (2 m targets, no obstacles), 89.6% ± 6.9% at Stage 1 conditions (4 m, one obstacle), and 48.4% ± 10.8% at Stage 2 conditions (7 m, three obstacles). Reporting this difficulty curve, rather than a single headline value, exposes a substantial generalization gap whose dominant failure mode is obstacle collision (45–52% of episodes at Stage 2). Matched retraining of Soft Actor-Critic and TD3 baselines under identical reward and curriculum conditions yields one completed seed each both baselines show non-monotonic difficulty curves, and at Stage 2 conditions, TD3 (64.0%) exceeds PPO (48.4% ± 10.8%) while SAC (43.0%) falls just below it, whereas at Stage 0, PPO (95.0%) leads both, so the ranking is operating point-dependent on the current single-seed evidence. We conclude that a continuous shaping gradient and the exploration-noise initialization, interacting with the curriculum advancement criterion, determine convergence in continuous control deep reinforcement learning, and that reward polarity by itself does not. Full article

Focusing on the complex aerodynamic interference and attitude response during multibody separation in the low-pressure and low-density environment of near space, a numerical investigation of the effects of different initial separation conditions on submunition motion is conducted. A three-dimensional unsteady flow model is established by coupling the six-degree-of-freedom rigid-body equations with the overset mesh technique. The effects of the free stream Mach number, initial angle of attack, initial separation velocity, and separation altitude on the motion characteristics of the submunition are systematically analyzed. The results show that, compared with conventional low-altitude conditions, aerodynamic forces have a weaker corrective effect on the separation motion and attitude in near space, making the initial separation parameters more influential. Increasing the free stream Mach number and initial angle of attack enhances the radial separation capability but intensifies the attitude response. As the separation altitude increases, the aerodynamic forces weaken, and the displacement and attitude variations in the submunition decrease accordingly. The initial axial separation velocity V_x0 has relatively little influence on the radial separation distance and pitching response. Increasing the initial radial separation velocity V_y0 increases the radial separation distance and reduces pitch oscillations, whereas increasing the magnitude of the initial lateral separation velocity $\left|V_{z0}\right|$ significantly amplifies the roll and yaw responses. The results provide a reference for the design of initial parameters for multi-body separation systems operating in near space. Full article

The stable boundary layer (SBL) over land exhibits pronounced vertical decoupling and structural heterogeneity that remain challenging to diagnose using routine operational monitoring. Here, we utilize a three-year continuous multi-level tower dataset ($N=\mathrm{40,630}$ quality-controlled 15-min records at 2, 10, 20, and 50 m) over the Horqin Grassland to characterize stability-dependent modifications of surface-layer wind structure and near-surface kinetic energy. Using the bulk Richardson number ($R{i}_b$) and raw thermal gradients ($\Delta T_v/\Delta z$), we identify a stability-driven reduction in vertical wind coupling, with a statistical change-point cluster centered near $R{i}_b\approx 0.30$ (95% CI: 0.09–0.52). Cross-level correlation analysis empirically localizes a structural transition zone to the 10–20 m interval, separating a faster, shear-driven upper layer from a dynamically suppressed near-surface flow under strong stability. Concurrently, near-surface horizontal kinetic energy ($HK{E}_{02}$) undergoes a 47% (nighttime) to 55% (full-record) median reduction, while horizontal wind-direction variability (${\sigma }_{\theta \_02}$) broadens due to low-wind meandering, and sub-hourly vertical velocity variance (${\sigma }_{w,\mathrm{LF}}^2$) is suppressed by 57–72% across all heights. Multidimensional scaling (MDS) reveals a statistically significant phase-space compaction under high stability (${\tilde{d}}_{\mathrm{high}}=2.13$ vs. ${\tilde{d}}_{\mathrm{low}}=2.67$, $p<0.001$), with adjusted odds ratios highlighting thermal gradient ($\mathrm{OR}=5.80$), wind shear ($\mathrm{OR}=1.52$), and directional variability ($\mathrm{OR}=1.34$) as dominant positive predictors. These empirical patterns remain robust under temporal-holdout validation (Year-1 calibration vs. Years 2–3 validation; consensus breakpoint $R{i}_b=0.161$ vs. $0.156$) and seasonal/wind-speed stratifications. The results demonstrate that routine 15-min multi-level tower networks provide valuable observational constraints on surface-layer structural transitions, informing boundary-layer parameterizations in numerical weather prediction models and near-surface dispersion assessments. Full article

This study presents a full-scale steady CFD methodology for evaluating flow quality in a large-scale low-speed wind tunnel (8 m × 6 m test section, 130 m/s). The tunnel geometry is resolved at 1:1 scale, the damping screens and honeycomb are modeled as porous media, and results are validated against wind tunnel test data and the GJB 1179A-2012 acceptance criteria. The baseline configuration reproduces the axial static pressure gradient within the acceptance criterion but substantially overpredicts turbulence intensity, dynamic pressure coefficient, and both velocity direction deviation angles. Damping screens, modeled as porous jumps, provide the dominant correction, bringing all metrics within the acceptance limits. Adding honeycomb yields incremental improvement: porous zone modeling preserves or improves all metrics, whereas a porous jump representation pushes velocity direction deviations beyond the limit. Between RNG k-ε and SST k-ω, only turbulence intensity is closure-dependent, with RNG k-ε closer to experiment. At Ma ≈ 0.38, compressibility does not alter the flow quality assessment, confirming that incompressible assumption is sufficient. By replacing costly physical trials with a validated CFD workflow, these findings provide a practical, resource-efficient reference for the CFD-based evaluation, design, and retrofit of wind tunnel infrastructure that underpins renewable-energy and energy-efficiency research. Full article

Extreme precipitation events have grown more common as a result of global climate change, and conventional static hazard assessments find it difficult to account for the dynamic progression of flood disasters. This study considers extreme precipitation factors for different return times and creates different extreme precipitation scenarios based on multiyear historical precipitation data and actual storm events. The study proposes a method for the dynamic assessment of regional flood hazard that takes extreme rainfall scenarios into account by simulating the dynamic flood inundation processes under each scenario using the Accumulated Runoff and Flood Estimation Model (AccRo v.1.0), iterative flow accumulation, and hydrological calculations. A dynamic assessment and zoning of flood hazards was carried out in Laiyuan County, Hebei Province. The results reveal that high-hazard zones coincide with the distribution of historically badly damaged townships, concentrated in the river valley plains along the Juma River. The results show that spatial patterns are simultaneously influenced by precipitation, terrain, and the river network. In the temporal dimension, under Scenario 3, the superimposition of the 50-year return period daily maximum rainfall at the 12th hour increased the high-hazard area by approximately 110% compared with that at the 11th hour. In addition, the non-uniform multi-peak rainfall pattern in Scenario 4 represented the rise, peak, and recession stages of the flood process. A combined assessment of water depth and flow velocity can effectively distinguish between two disaster-causing modes—deep water with low flow velocity and shallow water with high flow velocity—thereby addressing the underestimation of hazard in transition zones associated with the use of water depth as a single indicator. Full article

Inefficient xanthate transport and poorly defined circulation paths can limit the conditioning of low-grade lead–zinc tailings. A Venturi-assisted composite conditioning tank was therefore evaluated using ANSYS Fluent 2025 R2 (Ansys Inc., Canonsburg, PA, USA). The volume of fluid (VOF) model described the air–slurry free surface, the renormalization group (RNG) k–ε model and multiple reference frame (MRF) approach represented the impeller-induced mean flow, and a transient user-defined scalar (UDS) described the transport of a generic normalized xanthate tracer. The slurry phase was represented as a generic homogeneous equivalent medium rather than as a fully characterized solid–liquid suspension. The Venturi throat generated a local low-pressure region and stable reagent suction; at a tailings inlet velocity of 2.50 m·s⁻¹, the reagent-branch inlet mass flow rate was 2.825 × 10⁻⁴ kg·s⁻¹. Parameter comparisons identified 350 r·min⁻¹, an impeller installation height of 370 mm, and an annular gap width of 60 mm as the preferred combination. The calculated impeller power increased from 0.211 kW at 200 r·min⁻¹ to 2.304 kW at 400 r·min⁻¹. Increasing the speed from 350 to 400 r·min⁻¹ raised power consumption by 60.9% but average velocity by only 5.8%. Compared with a conventional tank, the composite tank formed a coherent impeller–lower connecting–annular upflow–upper recirculation pathway, providing more favorable hydrodynamic conditions for xanthate transport and potential reagent–particle contact. Full article

Reliable electricity supply in Colombia’s Non-Interconnected Zones requires sustainable and low-cost energy technologies. Vertical-axis hydrokinetic turbines are promising for this purpose; however, their relatively low power coefficient remains a major challenge. This study combines transient 2D and 3D $k$–$\omega$ SST computational fluid dynamics (CFD) simulations with hydraulic channel experiments to investigate a hybrid Darrieus–Savonius turbine. A 27-case Design of Experiments (DoE) based on 2D CFD was first applied to screen the effects of rotor radius ratio (RR), attachment angle (AA), and water velocity. Within the investigated design space, the configuration with $\mathrm{R}\mathrm{R}=0.5$ and $\mathrm{A}\mathrm{A}=0^{\circ }$ produced the most favorable average performance. The selected configuration was subsequently analyzed using 3D CFD and experimentally evaluated at TSR values of 1.0, 1.1, and 1.2. At TSR = 1.0, the 3D model predicted ${\mathrm{C}}_{\mathrm{P}}=0.1525$, closely matching the experimental value of 0.1541 with a relative error of 1.05%. The results demonstrate that 2D CFD is useful for computationally efficient parameter screening and qualitative trend identification, but it overpredicts absolute performance because it neglects blade tip vortices, spanwise flow, and volumetric wake interactions. Three-dimensional CFD is therefore required for reliable performance prediction and analysis of the complex flow structures governing hybrid hydrokinetic turbine behavior. Full article