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Solar chimney power plants (SCPPs) convert buoyancy-driven airflow into renewable electricity, and their performance strongly depends on geometric design parameters. While chimney height and collector diameter have been extensively investigated, the influence of the collector outlet-to-inlet area ratio (OIAR) remains insufficiently addressed despite its important role in airflow acceleration, heat retention, and pressure generation. A three-dimensional computational fluid dynamics (CFD) analysis of OIAR effects on the thermo-fluid behavior and power performance of a scaled SCPP model is conducted. The incompressible turbulent flow equations were solved in ANSYS Fluent using the Boussinesq approximation for buoyancy, the standard k–ε turbulence model, and the Discrete Ordinates (DO) radiation model. Turbine rotation was represented through a multiple reference frame approach, and the numerical model was validated against experimental data with good agreement in velocity and temperature distributions. Four OIAR configurations (0.5, 1.0, 1.5, and 2.0) were investigated. Results show that OIAR significantly affects pressure distribution, airflow structure, and thermal stratification. At OIAR = 0.5, the collector outlet temperature increased by 18 K, the chimney inlet pressure reached −260 Pa, and the turbine power output attained 428.7 W. In contrast, OIAR = 2.0 produced only a 5.3 K temperature rise and reduced the power output to 134.6 W. The study identifies OIAR = 0.5 as the optimal configuration and highlights OIAR as a key parameter for improving large-scale SCPP performance. By emphasizing the governing influence of collector geometry, this research contributes to improving the understanding and future development of high-performance large-scale solar chimney technologies.
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Natural convection flows near a convectively heated vertical wall have significant interest in the fluid mechanics community because of their wide applications in environmental and industrial processes. The development of transient natural convection boundary layer flows near a vertical wall subjected to convective heating is investigated using scaling analysis. The scaling relations are determined by the three governing parameters: the Rayleigh number (Ra or Ray), the Prandtl number (Pr), and the non-dimensional convective heat transfer coefficient (Ci or Ciy). The transient natural convection boundary layer consists of a viscous inner layer and a thermal boundary layer and evolves through an initial stage and a steady stage. Scaling analysis reveals the flow dynamics and heat transfer characteristics of the transient natural convection boundary layer and establishes scaling relations for the boundary layer velocity, thickness, temperature, and heat transfer. In typical scaling relations, the local non-dimensional thermal boundary layer thickness is inversely proportional to Ciy1/5Ray1/5, the local non-dimensional temperature is proportional to Ciy4/5 and inversely proportional to Ray1/5, the thickness from the wall to the location of maximum velocity is inversely proportional to Ciy1/5Ray1/5, the non-dimensional velocity is proportional to Ciy2/5Ray2/5 and both the Nusselt number and the flow rate are proportional to Ci1/5Ra1/5. These scaling relations are also validated through numerical simulations.
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The draft tube pressure pulsation (DTPP) in the Francis turbine could decrease the whole stability of the hydropower unit and cause fatigue damages. In addition, the propagation and action mechanism of the DTPP in the Francis turbine have not been fully clarified. So, three prototype Francis turbines with capacities of 55 MW, 200 MW and 600 MW were used as objects, and a series of field experiments have been carried out by us. Fortunately, some valuable results have been obtained. Firstly, the frequency components in DTPP were found to be mainly 0.25 fn and its multiples. Secondly, the propagation characteristic of DTPP was found and verified in the large Francis turbine for the first time. Only the component with a frequency of 0.25 fn could be propagated to the upstream flow channel, and it was identified as the 'synchronous pressure pulsation'. Thirdly, it has been verified that the shaft vibration can be affected by the synchronous pressure pulsation. Furthermore, a novel index for evaluating the impacts of the DTPP is proposed and validated. These results could provide guidance for the operation optimization and numerical simulation studies for the Francis turbine.
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The pintle injector is widely employed in propulsion systems owing to its simplicity and high performance. While fixed-geometry designs have been extensively investigated, the impact of structural parameters on adjustable configurations remains unclear. This study conducted experimental and numerical investigations of a planar adjustable pintle injector. Experimentally, spray behavior was analyzed at different throttling rates, capturing spray patterns and angles under various conditions. Numerically, the effects of orifice width, blockage factor, and skip distance on spray development and mixing performance were evaluated. The reliability of the simulation approach was verified through comparison with experimental data. Results revealed that reducing the orifice width blurred the boundary between the kerosene liquid film–dominated area and the crushing and mixing zone, while it slightly increased spray angle, enhanced inter-orifice vortex intensity, and improved kerosene utilization, from 0.443 at 3.6 mm to 0.756 at 1.2 mm. Increasing the blockage factor expanded the kerosene liquid film zone, contracted the mixing zone, fragmented the liquid film structure, accelerated jet deformation, reduced the spray angle, and increased kerosene utilization from 0.562 at 0.35 mm to 0.734 at 0.45 mm. By contrast, variations in skip distance had a negligible influence on spray structure and mixing efficiency, causing a minor increase in spray angle and stable kerosene utilization rates (fluctuating between 0.607 and 0.617, <1% variation as the skip distance increased from 0.048 to 0.12). The findings contribute to the design optimization and performance enhancement of adjustable pintle injectors, facilitating deep, efficient, and stable throttling for rocket engines.
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Ultrashort pulse laser machining is pivotal for fabricating aero-engine turbine blade film-cooling holes because of its superior performance; however, machining residues can block the laser, reduce efficiency, and promote recast layer formation, thereby deteriorating component performance. Gas-blowing aids residue removal, but traditional symmetric nozzles easily form reverse flow vortices at hole entrances in high-aspect-ratio micro-hole machining, leading to secondary residue accumulation and reduced machining efficiency. This study numerically simulates coaxial auxiliary gas-blowing flow fields of symmetric and asymmetric nozzles, investigating the effects of their structural parameters. The results show that the symmetric structure has a stable flow field but limited residue removal improvement, while the asymmetric structure achieves directional flow-field optimization. Opposite-direction angles form a "swirling jet", which suppresses reverse flow vortices, expands the high-velocity region and enhances the flow flushing capacity. Compared with the same-direction angle configuration, the opposite-direction configuration reduces stable entrance recirculation and improves axial flow penetration into the micro-hole. The optimal parameters are m = 2 mm, n = 2 mm, θ1 = 10°, and θ2 = -10°, (m is defined as the throat length, n as the divergent-section length, and θ1 and θ2 as the inclination angles of the two opposite sides of the asymmetric divergent section.) which achieves the maximum flow flushing capacity. Under this condition, the total pressure loss across the shock structure is reduced to 36.44%, indicating weaker shock-induced energy dissipation and improved momentum transport inside the micro-hole. This study provides theoretical support for the high-precision machining of high-aspect-ratio film-cooling holes.
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Variable nozzle turbine can enhance the transient response of engine through the regulation of nozzle opening. Under the operating condition characterized by high expansion ratio and small nozzle opening, strong unsteady shock waves tend to develop at the trailing edge of the nozzle blades. These shock waves introduce significant periodic loadings, which can result in high-cycle fatigue and eventual fracture of the rotor blades. This study investigates the unsteady flow characteristics within a representative variable nozzle turbine, with particular focus on the temporal and spatial evolutions of shock waves and their dynamic coupling with the rotor-stator system. It is found that both the intensity and the spatial structure of the shock waves exhibit periodic variation in response to rotor-induced disturbances. The pressure disturbance with a frequency of 10 kHz induces significant blade loading at 50% span of the rotor blade leading edge, whose maximum root mean square (RMS) amplitude is 17.45 kPa. It substantially increases the probability of high-cycle fatigue for the rotor blade leading edge. These findings can offer meaningful guidance for optimizing and controlling shock waves in variable nozzle turbines.
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To address the persistent issue of uneven fluid distribution within zigzag pneumatic biomass sorting channels, this study develops and evaluates multiple deflector configurations for zigzag pneumatic biomass sorting channels to address uneven fluid distribution. RANS-based CFD simulations show that strategically placed deflectors substantially enhance corner-vortex generation, reduce the peak-to-peak velocity fluctuation amplitude along the channel centerline by 90%, and extend the uniform flow region by 45%. These changes improve the internal flow dynamics of the channel. A movable point-array measurement system was developed for experimental validation. The results show that the Enhanced Wall Treatment (EWT) model predicts near-wall velocities more accurately than Scalable Wall Functions (SWF), with more than 96% agreement with experimental data. This accuracy is attributed to the refined near-wall resolution of EWT. Although the deflectors increase the system pressure drop by 7.6%, the resulting improvements in flow uniformity and stability are expected to enhance the separation efficiency of mixed biomass particles.
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The Central Tire Inflation System (CTIS) is increasingly adopted in buses, trucks, and military vehicles, as it enables adaptive tire pressure control under varying terrain conditions, thereby improving vehicle performance and safety. In defense applications, the use of electronic components within tire systems is prohibited, which necessitates fully mechanical valve designs operating on alternative physical principles. This study presents an experimental and numerical investigation of a dynamic-function vehicle tire air control valve developed by Altan Hydraulic. Experimental results show that the original valve configuration cannot reliably remain open at low airflow rates (&le;135 L/min). To overcome this limitation, a vacuum-assisted operating mechanism was investigated. Region A denotes the vacuum chamber, Region B the high-pressure chamber generating the opening force on the spool, and Region C the main flow passage. The nozzle tip inclination angle was modified from its original value of &alpha; = 90&deg; in 10&deg; increments over a range of 40&deg;&ndash;180&deg;, and the &Oslash;0.2 mm orifice was repositioned to align with regions of maximum flow velocity, limiting the mass flow entering Region A. Flow dynamics and spool force balances were analyzed for Reynolds numbers between 33,874 and 112,913. The redesigned configuration generated a vacuum pressure of approximately &minus;35 kPa in Region A at &alpha; = 120&deg;, enabling the valve to remain open at tire pressures as low as 30 kPa. The results demonstrate that pressurized air from the tire can be used to generate a self-induced vacuum, ensuring sustained valve opening without electronic components. The proposed fully mechanical design significantly enhances CTIS performance by providing a reliable and energy-efficient solution for military and off-road vehicles.
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Unmanned sailing vessels are increasingly utilized for reconnaissance, environmental monitoring, and autonomous transportation; however, they produce considerable stern wake disturbances that diminish propulsive efficiency and degrade stealth performance. Although various strategies, such as stern rounding, fins, and hydrofoil appendages, have been studied, quantitative investigations on the effect of stern-mounted flow-interrupting plates under varying geometric configurations remain limited. To bridge this gap, this study proposes a stern flow-interrupting plate and employs a three-dimensional CFD framework combining the SST k&ndash;&omega; turbulence model with the VOF free-surface method, enabling detailed evaluation of wake evolution, wave height response, and hydrodynamic resistance under varying plate inclination angles and vertical distances from the waterline. Comparative analyses reveal that, with a plate inclination of 2.5&deg; and a distance of 10 mm from the waterline, vortex diffusion is effectively suppressed, wake height is substantially reduced, and additional resistance is limited to approximately 4 N. Specifically, quantitative evaluation in defined downstream regions shows maximum wave height reductions of 20.99% in D1, 57.08% in D2, and 43.85% in D3, demonstrating consistent attenuation of wave amplitude from the near wake to the far-field region. These findings provide compelling evidence for the effectiveness of the proposed flow-interrupting plate in controlling wake formation and evolution. Furthermore, the study offers a practical reference for stealth optimization of unmanned surface vessels and yields valuable insights into the hydrodynamic design of next-generation surface vehicles, underscoring the unique potential of passive stern flow-interrupting structures for quantitative wake suppression.
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Effect of gas composition on aerodynamic performance and internal flow in ethane oxidation dehydrogenation (ODH) centrifugal compressors is investigated. Although inert gases are widely used to reduce explosion risk in ODH processes, the effects of multi-component inert gases on compression thermodynamics and internal flow structures are not fully understood. The novelty of this work lies in a systematic quantification of how different inert-gas compositions change the evolution of internal flow, thereby influencing the loss mechanisms and isentropic efficiency in a high-pressure ODH environment. Three gas mixtures are evaluated: Mixture 1 (C2H6 55%, O2 15%, CO2 15%, 10%, H2 5%), Mixture 2 (C2H6 55%, O2 18%, CO2 10%, 7%, He 5%, N2 5%), and Mixture 3 (C2H6 53%, O2 18%, CO2 8%, N2 15%, 6%). Numerical simulations are conducted using a steady-state mean streamline model coupled with the SST turbulence model. The isentropic efficiency varies non-monotonically with mass flow rate, staying nearly constant from 6.5 to 7.1 kg/s and then decreasing markedly. Detailed flow analysis shows that low-density gases such as H2 and He promote leakage flow and flow separation, increasing energy dissipation; in contrast, N2 reduces separation-related losses. Compared with Mixture 1, the isentropic efficiency increases by 21.65% for Mixture 2 and further improves by 30.25% for Mixture 3 (a cumulative increase of 8.6% over Mixture 2). Overall, the results provide quantitative trends and mechanistic understanding of multi-component inert-gas effects on compressor performance, which can help researchers evaluate and design safer ODH compression systems and motivate further studies under other inert-gas combinations and operating conditions.
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Existing greenhouse root-zone heating systems often suffer from limited thermal storage capacity and poor temperature uniformity, while the combined effects of PCM content and heating pipe configuration on thermal regulation performance remain insufficiently understood. This study evaluates a stereoculture rack growing system (SRGS) integrating phase change material (PCM)-enhanced soil matrix blocks (SMBs) with a closed-loop hydronic heating network for root-zone soil temperature control. Computational fluid dynamics simulations were conducted using STAR-CCM+ and validated experimentally to investigate the effects of PCM content, flow velocity, and pipe configuration on root-zone and canopy thermal performance. The results showed that SMBs containing 20% PCM with a dual-pipe configuration, circulating water supplied at 45 °C, and a flow velocity of 1 m/s achieved the best thermal performance. Under an ambient temperature of 10 °C, the root-zone thermally optimal zone (TOZ) volume reached 4.78 × 10⁻³ m³ after 1800 s, while the canopy thermally suitable volume reached 0.58486 m³, corresponding to 69.62% of the total canopy air volume. Compared with conventional soil, the PCM-enhanced SMBs extended the heating and cooling durations within the 20–26 °C phase transition interval by 3.69 and 4.38 times, respectively, demonstrating a substantial thermal buffering effect. The dual-pipe configuration provided the best balance between heating intensity and temperature uniformity, whereas excessive heating in the triple-pipe configuration reduced the TOZ volume due to local overheating. These findings demonstrate that PCM-enhanced SMBs can effectively improve greenhouse thermal regulation and root-zone temperature stability. However, the present study focuses on thermal performance under controlled operating conditions, and future work should evaluate long-term crop responses and field-scale applications.
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Nonuniform air distribution in passenger-coach air-conditioning ducts commonly causes insufficient distal airflow and reduces cabin thermal comfort. In ducts with uniform cross-sections, continuous air discharge through distributed outlets induces streamwise static-pressure attenuation, resulting in a typical front-dominant, rear-deficient airflow pattern. This study combines theoretical analysis, computational fluid dynamics (CFD), and multiobjective optimization to investigate the flow characteristics and uniform air-supply strategy of a passenger-coach air-conditioning duct. The results demonstrate that static-pressure decay is the primary mechanism responsible for outlet-flow maldistribution and that global outlet-radius reduction provides only limited improvement. In contrast, a streamwise graded-radius scheme, in which the outlet radii generally increase from the upstream to the downstream region with local corrections at selected outlets, effectively compensates for pressure attenuation and enhances distal airflow. Based on CFD data, cubic polynomial correlations between the preliminarily optimized outlet radius and the distance from the fan-side reference position are established for both ducts, with coefficients of determination exceeding 0.90. Guide-vane optimization using Gaussian process regression, support vector regression, and a genetic algorithm further indicates that guide vanes mainly provide local flow regulation, whereas radius gradation governs the global redistribution of airflow. The optimized radius scheme reduces the left-duct flow nonuniformity coefficient by 8.2%. Because the radius fitting and guide-vane optimization are based on limited simulation samples, the resulting correlations should be regarded as preliminary design references that require further validation.
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This study focuses on the use of numerical simulations and experimental investigations to analyze how modifications in jet diffuser geometry affect the heating of cubic components exposed to crossflow at different Reynolds numbers and constant-speed impinging jets. The diffuser configurations analyzed include circular, lobed, and swirling designs. The numerical model was validated against experimental data. The results show that as the crossflow Reynolds number increases, the cube temperature and average Nusselt number decrease significantly, especially at higher values. In the absence of crossflow (ReH = 0), a region of influence forms on the cube due to the impinging jet, with noticeable differences in heat transfer among the diffuser configurations. The lobed and circular designs demonstrated slight improvements in heat transfer efficiency. As the Reynolds number increases, the lobed diffuser provides superior heat distribution, offering a broader heating area and more uniform temperature across the cube. For ReH = 2506, the lobed and circular jets improved heat transfer by 19.13% and 14.69%, respectively, while for ReH = 7518, improvements of 16.29% and 25.70% were observed. Finally, at ReH = 10024, the lobed and circular diffusers achieved superior heating performance, with increases of 13.95% and 35.31%, respectively. Overall, the lobed diffuser emerged as the most effective configuration for heating cubic components across various crossflow Reynolds numbers. Therefore, jet geometry is a critical factor to consider when designing and optimizing jet systems to ensure uniform heating and optimal performance.&nbsp;
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Experimental rotation effects on flow past cylinders have been observed. The cylinders have the porous media coatings (PMC) named PMC1, PMC2, PMC3 and PMC4. For the range of rotation rates, including &alpha; = 0.52, &alpha; = 1.05 and &alpha; = 1.57, the flow characteristics have been presented at a Reynolds number of Re = 2500. In comparison with the experimental results for stationary cases (&alpha; = 0), the zones with lower streamwise velocity values became closer to the bodies as the rotation rates were enhanced. The maximum wake displacements have been obtained as 1.8D for the circular cylinder with no coating. For coated cylinders, these are 1.9D, 1.9D, 1.7D and 1.95D for the PMC1, PMC2, PMC3 and PMC4, respectively. For the same reason, the cross-stream velocity components also approached the circular cylinders. However, asymmetrical distributions for flow patterns have been attained. For this reason, the lower values for cross-stream velocity became more dominant in the wake. Moreover, separated flows were obviously seen by velocity fluctuations. The interaction of separated and wake flows induced these fluctuations. Wake lengths have been reduced compared to those of the cases with no rotation. The highest percentage for wake length reduction has been obtained as 92.9% by &alpha; = 1.05 for the bare cylinder. The maximum reductions have been attained as 89.7% by &alpha; = 1.05 for PMC1, 96.3% by &alpha; = 1.05 for PMC2, 85.7% by &alpha; = 1.57 for PMC3 and 90% by &alpha; = 1.05 for PMC4. The experimental results have also been presented for turbulence values. The highest turbulence kinetic energy value, 0.236, has been obtained at &alpha; = 1.05 for the bare cylinder. These values are 0.271 at &alpha; = 0.52 for the PMC1, 0.271 at &alpha; = 1.05 for PMC2, 0.245 at &alpha; = 1.05 for the PMC3 and 0.217 at &alpha; = 0.52 for the PMC4. As with the flow patterns, the induction by surface coating has been clearly observed through the augmentation of &alpha; values. Since surface movement for coating has also been seen for these situations, boundary layer thickness values have been reduced.
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Straight outlet conduits (SOCs) are widely used in pump stations. However, their performance is limited by residual circulation from the guide-vane outlet and curvature-induced secondary flow in the outlet elbow, which can generate spiral flow, leading to bias flow and additional hydraulic loss. This study investigated spiral-flow evolution in a vertical mixed-flow pump using unsteady numerical simulations validated against model tests, together with theoretical analysis. A simplified theoretical model was established, and the main precession frequency and swirl number were introduced to quantify spiral-flow motion and intensity, respectively. The most pronounced spiral flow was observed under the low-flow condition of 0.85Qdes. The dominant flow region shifted from the upper-right side of the elbow outlet to the upper-left side of the circular-to-square transition section (CTSTS), resulting in an outlet bias flow ratio of 0.91. The outlet elbow hydraulic loss under this condition was 1.21 times the design-condition value. At the elbow inlet, the main precession frequency reached 0.55 times the rotational frequency, while swirl number at the CTSTS outlet was 1.67 times the design-condition value. These findings identify the CTSTS and the division-pier leading-edge region as critical regions for spiral-flow development, biased-flow formation, and hydraulic-loss control. The proposed framework provides a quantitative basis for evaluating spiral-flow evolution in similar low-head pump systems.
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A comprehensive parametric numerical study is performed using the shear stress transport (SST) k-ω turbulence model to systematically investigate the effects of the installation configuration (leading edge (F), middle (M), trailing edge (B), F&B, and F&M&B) and the relative length ratio (L/D = 0.25–1.5) of rigid ancillary plates on the single-degree-of-freedom cross-flow vortex-induced vibration (VIV) of a 10:1 aspect-ratio rectangular box girder. The results indicate that the configuration and length of the ancillary plate substantially affect the vibration response of the box girder. Installing the plate at F or in combinations with F (F, F&B and F&M&B) significantly increases vibration, and the vibration amplitude increases with plate length. Installing ancillary short plates (L/D ≤ 0.5) at M and B results in no vibration, whereas using long plates (L/D ≥ 1) at M leads to strong vibration. Flow field analysis indicates that the ancillary plate modifies the shear layer’s separation and reattachment, as well as vorticity accumulation-release mechanisms, thereby affecting the generation and evolution of the excitation force and the vibration response. The vibration amplitude is strongly correlated with fluctuations in aerodynamic force. The ratio of the lift coefficient’s dominant frequency to the structural natural frequency approaches 1 within the lock-in range, whereas galloping characteristics are observed at high reduced velocities and for long plates.
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Mini-hydrocyclones achieve separation through their internal, complex, and intense vortex flow fields. A comprehensive investigation into the stability of this flow field is crucial. While our prior work explored separation performance under various inlet angles, this study introduces a novel concept (the top-plate angle) to explore a potential breakthrough in the field. Particle Image Velocimetry (PIV) was employed to examine the flow morphology in a 35 mm mini-hydrocyclone with different roof angles (0&deg;, 30&deg;, 45&deg;, and 60&deg;), obtaining contour maps of axial and radial velocities and deriving streamline distributions through calculation. The results reveal that at identical angles, the streamline distribution within the cone-disk structure is more regular and organized. This indicates that integration cone-disk structure, the separation process becomes less susceptible to disturbance from vortices. Our findings identify a 30&deg; inlet roof angle as the optimal configuration. Compared to the conventional design, this configuration reduces short-circuit flow by 2.34%. The optimized mini-hydrocyclone has been successfully implemented in a Methanol To Olefin (MTO) quench water treatment system, enabling efficient separation of fine particles and facilitating wastewater reclamation. Operational data confirm that the separation efficiency of the primary stage remains stable at 60-70%, while the secondary stage maintains an efficiency exceeding 80%.
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The inter-deck clearance height (IDH), defined as the vertical distance between decks, is a key parameter in the aerodynamic design of double-deck bridges. This parameter governs both global aerodynamic characteristics and the allocation of wind loads across different subcomponents (top/bottom decks, windward/leeward trusses) of the girder, yet its effects remain underexplored. This study examines the effects of IDH using five double-deck girders with varying IDH, combining wind tunnel tests and Computational Fluid Dynamics (CFD) simulations. Results show that the drag and lift (absolute) coefficients increase with IDH, notably at high angles of attack. For all investigated girders, roughly half of the average drag is sustained by the windward truss. Furthermore, as the IDH increases, the mean drag force borne by the leeward truss increases, whereas the share carried by the top/bottom decks decreases correspondingly; meanwhile, the primary contributor for the fluctuating drag force shifts from the leeward to the windward truss. In contrast, both the mean and fluctuating lift force are primarily carried by the decks, with the trusses having a negligible contribution, and this partitioning pattern remains unchanged regardless of IDH. Furthermore, Dynamic Mode Decomposition (DMD) reveals that increasing IDH enhances flow instability near the windward truss while suppressing vortex development in the leeward region, leading to a shorter downstream persistence of wake structures. These findings provide insights into the underlying flow mechanisms and offer guidance for the aerodynamic selection and wind-resistant design of double-deck bridges.
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Hydropower valves operate under transient, high-pressure conditions in which local throttling, pressure recovery, vortex evolution, and pressure pulsation govern hydraulic performance, cavitation, erosion, and flow-induced vibration. This review synthesizes recent advances in numerical modeling of these coupled phenomena and links internal-flow evolution with hydraulic response, damage initiation, and structural behavior. The evidence base comprised 90 primary studies and five contextual reviews. Twelve primary studies (13.3%) directly addressed hydropower valves or hydropower valve–system interactions, whereas 78 studies (86.7%) provided transferable mechanistic or methodological evidence from related valve and hydraulic systems, highlighting the limited availability of hydropower-specific validation. Reynolds-averaged Navier–Stokes (RANS)-based computational fluid dynamics (CFD) remains the principal engineering method for global hydraulic quantities, whereas large-eddy simulation (LES), multiphase erosion models, and fluid–structure interaction (FSI) methods are more appropriate for unsteady cavitation, particle-induced damage, and structural response. Quantitatively, one direct hydropower butterfly-valve study reported an approximately 10% mean absolute error between CFD predictions and experimental flow-field data. In a transferable three-way globe-valve study, geometric optimization reduced the simulated percentage of cavitation by 49.5%, from 34.90% to 17.62%, while the response-surface model predicted 19.06%. The former supports case-specific confidence in hydropower-valve FSI modeling, whereas the latter demonstrates transferable optimization potential rather than direct hydropower performance. Major limitations include incomplete uncertainty reporting, uneven numerical verification and model calibration, scarce prototype-scale evidence, and limited long-term multiphysics validation. Standardized benchmarks, prototype measurements, and physics-informed monitoring are required to support transferable design and life-cycle decisions.
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The growing demand for reliable and clean wind energy has renewed interest in vertical axis wind turbines (VAWTs), which are well suited for low speed, highly turbulent urban wind conditions. This study proposes and refines a hybrid VAWT architecture, integrating straight Darrieus outer blades with a helical inner rotor, with the aim of enhancing aerodynamic behavior in these urban wind environments. Using 3D unsteady computational fluid dynamics and a Taguchi orthogonal design, the effects of four inner rotor parameters&mdash;helical angle, radius, height, and chord length&mdash;were evaluated. The inner rotor chord length emerged as the most influential factor. The optimized hybrid turbine achieves substantially higher power coefficients than a Darrieus turbine at low tip-speed ratios, demonstrating gains of up to 17.89%. Vortex analysis indicates that the helical rotor suppresses dynamic stall and reduces negative torque regions, resulting in smoother torque output and improved startup behavior. Overall, optimized hybridization represents a promising pathway for enhancing the efficiency of small-scale VAWTs.
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Conventional methods of computational fluid dynamics require substantial computational resources to solve the Navier-Stokes equations, whereas existing deep-learning-based reduced-order models lack robust cross-domain transfer capabilities. To address these challenges, we propose FlowBERT framework that integrates the bidirectional attention mechanism of bidirectional encoder representations from Transformers (BERT) with proper orthogonal decomposition to predict the temporal flow fields. The study introduces fluid mechanics-oriented text templates by establishing theoretical links between the attention mechanism of BERT and spatiotemporal flow correlations. Furthermore, we developed a multi-head cross-attention mechanism to align the flow data with text embeddings. The experimental evaluations of flow around a cylinder and over airfoils validated the approach. The results demonstrated that FlowBERT achieved over 90% prediction accuracy using only 10% of the training data and reduced computation time from several hours to a few seconds. The cross-scenario transfer learning yielded an average accuracy of 95% and substantially outperformed classical reduced-order methods. Through engineering practice, the study demonstrated that large language model attention mechanisms can be utilised in physical system modelling to create a new paradigm that integrates machine learning methods with scientific computing. The framework holds considerable practical value in aerodynamic optimization and flow control applications.
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To mitigate the prohibitive cost of simultaneously modifying all stages in multistage molten salt pumps used in concentrating solar power plants, address the substantial degradation of hydraulic performance under off-design operating conditions, and resolve the difficulty of tracing cross-stage flow-loss coupling mechanisms, this paper presents a cost-effective retrofit strategy that limits geometric modifications to the first-stage impeller and harnesses the interstage flow-matching effect to improve system-wide performance. The methodology proceeds in four sequential phases. First, a comprehensive sample database is constructed using Latin hypercube sampling combined with high-fidelity computational fluid dynamics simulations. Second, the Adam optimization algorithm is used to train a backpropagation neural network, establishing a computationally efficient surrogate model to replace time-intensive numerical simulations. Third, the surrogate model is coupled with the NSGA-II multi-objective evolutionary algorithm to perform Pareto-front optimization, simultaneously maximizing hydraulic head and efficiency at the design operating point. Finally, based on entropy production theory, the evolution of energy loss before and after optimization is systematically characterized at four hierarchical levels: total entropy production, component-level entropy production, wall entropy production, and vortex structure evolution, through the joint analysis of local entropy production rate distributions and vortex identification using the Q-criterion. The optimization yields notable improvements: the hydraulic efficiency of the first-stage impeller increases from 78.42% to 81.83%; total entropy production under design conditions decreases by 8.63%; and entropy production within the first-stage guide vanes decreases by 45.23% at 1.2 times the design flow rate. Crucially, the second-stage impeller geometry remains unchanged, confirming that first-stage optimization propagates performance gains to all downstream stages through the interstage flow-matching mechanism. By limiting the scope of hydraulic modification from the full pump assembly to a single stage, the proposed framework establishes a practical pathway for low-cost hydraulic optimization of multistage molten salt pumps.
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Surge instability is a critical issue that frequently plagues centrifugal compressors at low flow rates. To address this urgent challenge, high-fidelity computational fluid dynamics (CFD) simulations are employed as the primary method to comprehensively evaluate the effect of active control casing treatment (ACCT) on surge mitigation in a centrifugal compressor. The core objective is to enhance the compressor’s pressure rise and flow stability under near-surge conditions. The results reveal that the application of ACCT significantly enhances flow stability and increases the peak pressure rise of the centrifugal compressor by up to 8.84% under optimal conditions. Furthermore, the optimal injection pressure range for achieving peak stability enhancement is identified as 250-450 kPa, with 350 kPa yielding the greatest improvement. While this qualitative rule provides general reference for other compressors, the optimal injection pressure range may differ due to variations in compressor performance parameters. These results highlight the potential of ACCT as an effective mechanism for enhancing compressor stability and offer practical guidelines for the design of surge control systems.
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This study examines the evolution mechanism of gap-vortex structures and the corresponding vibration excitation under load-rejection conditions in a pump-turbine. Through numerical simulations, we systematically investigate the flow characteristics, vortex dynamics, and pressure pulsations within the clearance region of a pumped-storage unit. The results show that the gap flow transitions from an initially quasi-steady state to a multi-scale unsteady pattern, displaying coupled effects of shear enhancement, flow redistribution, and vortex-induced instability. In the upper crown region, the vortex core develops from localized small-scale disturbances into band-like structures during the early stage, eventually forming concentrated fluctuations in the later phase. In contrast, the lower ring region exhibits more pronounced three-dimensional evolution accompanied by a delayed dynamic response. Spectral analysis of pressure pulsations indicates a gradual broadening from low-frequency dominance to a wideband distribution. The principal frequency component corresponds to the rotor&rsquo;s low-order modal frequency (1.78 fn), implying a potential structural-resonance risk. The emergence of multi-frequency and non-integer harmonic components further reveals complex nonlinear excitation mechanisms and multi-region coupling effects, which may generate intermittent shock-type fluctuations in the local flow field and act as a key source of hydraulic excitation. These findings provide important insights into flow instability and vibration response under load-rejection conditions and offer valuable references for structural optimization and vibration-control strategies in pump-turbine systems.
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To investigate the coupled effects of submergence ratio and gas flow rate on the lifting performance and bubble evolution of a straight-pipe air–water airlift pump, a three-dimensional transient computational fluid dynamics model built upon the Eulerian–Eulerian two-fluid framework incorporated with a Population Balance Model (PBM) was developed and validated against experimental measurements. Over the ranges of submergence ratio of 0.3–0.7 and gas flow rate of 20–50 m³·h⁻¹, the variations in liquid flow rate, gas holdup, Sauter mean diameter, and mass lifting ratio were systematically examined. The results indicate that liquid flow rate generally increases with gas flow rate and submergence ratio, although its marginal increment diminishes significantly under high gas flow rate conditions. Gas holdup increases monotonically with gas flow rate, but such an increase fails to yield a continuous enhancement in liquid lifting capacity. The Sauter mean diameter initially decreases and subsequently increases with gas flow rate, reflecting the competing effects of bubble breakup and coalescence. The mass lifting ratio declines progressively with increasing gas flow rate but rises gradually with submergence ratio. Overall, submergence ratio serves as the fundamental driving force for system operation, whereas gas flow rate modulates two-phase transport by regulating the bubble swarm structure and the gas–liquid interfacial state. Proper matching of operating parameters is therefore critical for the synergistic optimization of lifting capacity and gas utilization efficiency.
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Achieving rapid mixing in microfluidic systems is notoriously difficult at ultra-low Reynolds numbers (Re &lt;&lt; 1), where slow molecular diffusion dominates species transport. This study presents a numerical investigation of an active micromixer designed to overcome this limitation. Our design incorporates five cylindrical obstacles that rotate at alternating angular velocities (100 to 800 RPM) to induce chaotic advection. The mixing efficiency quantified using the Degree of Mixing (DOM) across a critical range of molecular diffusivities (D = 10⁻&sup1;0 to 10⁻&sup1;2 m&sup2;/s) and for Power-Law non-Newtonian fluids with indices from n=0.6 to n=1.4. The results demonstrate that the rotating obstacles successfully achieve DOM &gt; 90% for all tested diffusivities when the angular velocity reaches 800 RPM. However, the most significant finding concerns fluid rheology: contrary to conventional expectations, mixing efficiency consistently improved with increasing Power-Law index (n). Shear-thickening fluids (n &gt; 1) demonstrated superior performance, consistently achieving DOM &gt; 90% across the entire diffusivity range. In contrast, the most shear-thinning fluid (n=0.6) failed to reach the 90% threshold at the lowest diffusivities. Hydrodynamic analysis revealed that this enhancement stems from increased flow asymmetry in shear-thickening fluids, where the peak axial velocity shifted approximately 5.5% from the centerline, thereby strengthening the convective transport crucial for efficient mixing. This work provides practical operational and design guidelines for achieving high-efficiency mixing (&ge;90%) under severe diffusional limitations and complex rheological conditions. All simulations were conducted in the creeping-flow regime. An analysis based on a rotational Reynolds number confirms that inertial effects and flow instabilities remain negligible even at the highest rotation rates considered.