Search Results (172)

Local scour and vortex-induced vibrations around cylindrical structures in curved channels pose significant risks to the safety and stability of critical hydraulic infrastructure, such as bridge piers. To address these engineering challenges and elucidate the underlying flow mechanisms, this study conducts numerical simulations of flow past two side-by-side circular cylinders of equal diameter in a curved channel under subcritical conditions at Re = 3900, using the Realizable kε turbulence model. Spectral Proper Orthogonal Decomposition (SPOD) is introduced to quantitatively characterize the energy distribution and dominant coherent structures. Taking the spacing ratio L/D and the placement angle α as key design parameters, the flow field characteristics, modal energy distribution, and coherent structure evolution are systematically investigated for two side-by-side cylinders in three-dimensional straight and curved channels. The numerical results show that, in the straight channel, as L/D increases from 2 to 4, the flow field evolves from strong coupled interference to weak interaction. The vortex shedding frequency structure evolves from a single dominant frequency to a multi-frequency distribution with rich harmonic components, indicating a transition in wake dynamics from energy concentration to multimodal dispersion, accompanied by a significant improvement in flow stability. Under curved channel conditions, the results reveal an asymmetric flow field caused by pronounced energy concentration on the inner side of the channel. SPOD analysis further indicates that as the placement angle α increases from 30° to 90°, the modal energy distribution changes from concentrated to dispersed, the frequency spectrum broadens with enhanced harmonic components, and flow instability gradually intensifies. Overall, the spacing ratio L/D mainly governs the wake-interference pattern, whereas the placement angle α regulates the frequency structure and energy distribution. Among all the cases investigated, relatively favorable flow stability is achieved at L/D = 4 and α = 30°. The SPOD-derived modal energy distributions show that the streamwise fluctuation length of the dominant-mode energy is approximately 0.25 m at α = 30°, compared with 0.5 m at α = 90°, with the energy bandwidth nearly doubling. The combined CFD-SPOD approach effectively captures energy evolution and coherent structure characteristics of complex flows across spatial, temporal, and spectral dimensions. This enables a shift from conventional flow-field description to frequency-based mechanism analysis and provides a theoretical basis for structural layout optimization and scour protection in hydraulic engineering. Full article

Flood discharge from the dam surface and tailwater discharge from the power station directly affect the hydrodynamic processes in the downstream river channel as well as at the entrance area of approaching channel, which are closely related to the navigation stability and safety of vessels entering or leaving the ship lock. To investigate the influence of different dam flood discharge operational scenarios on the hydrodynamic characteristics at the entrance of the downstream ship lock approach channel, a three-dimensional nested coupled CFD model is established for free surface flows with strong nonlinearity in the stilling basin and unsteady turbulent flows in the downstream channel of the Xiangjiaba Hydraulic Project. The model adopts the Reynolds-Averaged Navier–Stokes (RANS) equations for unsteady flows, combined with the Realizable k-ε turbulence model as well as the VOF free surface tracking method for stilling basin flow and the standard k-ε turbulence model for downstream river flow, respectively. Numerical investigations are conducted to clarify characteristics of river flows associated with the discharged flood from dam surface and tailwater from power stations under different flood discharge patterns. The results show that the balanced discharge scenario involving the combined operation of releasing the flood through the crest and middle outlets of both left and right stilling basins can significantly reduce flow velocity and water level fluctuations near the entrance of the approach channel. Optimizing flood discharge scheduling can effectively improve flow conditions at the entrance area, which is beneficial to enhancing navigation safety for ships. Full article

In biomass pellet combustion, the formation of ash layers on particle surfaces severely hinders combustion reactions and heat transfer, while the key parameters governing particle motion behavior and ash pre-separation in rotating cone burners remain insufficiently understood. To address these challenges and to optimize particle mixing and ash separation performance, this study adopts a combined numerical approach. The discrete element method (DEM) coupled with the Hertz–Mindlin (no-slip) contact model is employed to simulate particle motion and mixing dynamics, while a separate cold-state computational fluid dynamics (CFD) model based on the Realizable k-ε turbulence model and the discrete phase model (DPM) with Rosin–Rammler particle size distribution is established to investigate ash separation mechanisms. The Lacey mixing index is used to quantify mixing uniformity, and grid independence verification is performed to ensure numerical reliability. Key findings reveal that the rolling regime (rotational speed: 1.7–11 r/min), a uniform particle size of 25 mm, and a cone inclination angle of 45° collectively optimize particle mixing. Rotational speed is identified as the dominant factor affecting mixing effectiveness. Furthermore, an optimal secondary-to-primary air ratio of approximately 7:3 (within the tested range) balances enhanced centrifugal separation with flow field stability by mitigating backflow and excessive turbulence. This work not only fills the knowledge gap regarding the coupled effects of operational and structural parameters on particle behavior in rotating cone burners but also provides novel, quantitative guidance for the rational design and parameter tuning of such burners to improve combustion efficiency and operational stability. Full article

The use of computational fluid dynamics provides an important tool for the design of supersonic ejectors. Within Reynolds/Favre-averaged simulations, the turbulence model plays an essential role in determining results’ reliability. Existing validation studies show general accuracy problems, whose relevance, partially masked in the double-choked regime, becomes fully evident for the single-choked regime. For this flow regime, errors reported in the literature are strongly erratic, reaching magnitudes higher than 50% in terms of global performance. The absence of clear, unified conclusions by different authors motivates the present work, focused on single-choked ejectors. In the first part, the main ejector flow features are discussed, highlighting the importance of adequately reproducing the turbulence response to different shear intensities. To properly address this point, an original analysis is conducted, exploiting data from previous studies on jets and basic shear flows. The developed analysis explains how the prediction of an ejector jet is influenced by the constitutive relationship of eddy viscosity models and by the modelled balance of the turbulent-dissipation rate. The modelling failures of these two elements are discussed for existing models in common use and addressed through the development of a new Consistent Realizable K − ε model. In Part 2, the analyzed models are used to simulate two test cases, with detailed measurements available. Full article

Nine sets of fin parameter combinations, including a plain tube control group, were modeled. Simulations were performed under steady-state conditions using the EWT Realizable k-ε turbulence model, with benzene and water as working fluids, while accounting for temperature-dependent thermophysical properties. Flow field distribution, temperature profile, Nusselt number, and pressure drop in the shell side of the heat exchanger were analyzed. Response surface methodology was employed to systematically evaluate the coupled effects of fin height and fin spacing on thermal performance. The results indicate that annular fins significantly enhance heat transfer by inducing secondary flow and disrupting the thermal boundary layer. Compared to the smooth tube, the finned tubes increased the Nusselt number (Nu) by up to 28.6% and the total heat transfer rate by 13.55%, while the pressure drop (ΔP) increased by approximately 9.81% to 16.5%. The analysis revealed that fin height is the dominant factor affecting performance, whereas fin spacing plays a regulatory role. As the fins became taller or denser, the temperature field evolved from stable stratification to intense mixing and eventually to local disorder. The study identified an optimal parameter range for engineering applications. A fin height of 2–3 mm combined with a spacing of 10–15 mm achieves the best balance between heat transfer enhancement and flow resistance. Specifically, the combination of h = 3 mm and s = 10 mm yielded the highest Energy Efficiency Coefficient (EEC) of 1.567. This configuration is recommended for large-flow, pressure-drop-sensitive systems, such as those found in petrochemical plants or long-distance heat transmission applications. Full article

High-pressure airless spray coating can atomize high-viscosity, high-solids coatings without compressed air and is widely used for large-scale anticorrosion applications, but robotic operation often produces substantial overspray that increases material waste, environmental burden, and lowers deposition efficiency. In this work, air-curtain blowing is investigated as an overspray control strategy for wall-climbing robotic airless spraying. A validated CFD framework was established using the realizable k–ε turbulence model coupled with a discrete-phase model (DPM) to simulate particle atomization, transport, impact, and escape, and to examine the effects of blowing angle and gap distance on the flow field and particle trajectories. Overspray performance was quantified using the wall deposition rate, hood collection rate, and particle escape rate. Experiments using a transparent spray hood with a mass collection system were conducted to validate the numerical predictions. The CFD results captured the measured trends in deposition and escape across the tested conditions. Among the evaluated parameters, a 60° blowing angle provided the most effective overspray reduction by redirecting particles toward the target surface. Overall, combining CFD analysis with experimental validation offers a practical methodology for designing and optimizing air-curtain systems to improve coating efficiency in automated high-pressure airless spray applications. Full article

This study investigates the factors influencing the performance characteristics of annular jet pumps (AJPs) conveying non-Newtonian fluids, to enhance their suction capability for marine organisms such as jellyfish, which exhibit properties close to non-Newtonian fluids. Based on the power-law fluid model, realizable k-ε model, and volume of fluid (VOF) model, shear-thinning carboxymethyl cellulose (CMC) was selected to simulate marine organisms like jellyfish. Fluent software was employed to numerically simulate the performance characteristics and internal flow field of the annular jet pumps. The results demonstrate that the shear-thinning effect of non-Newtonian fluids reduces the maximum efficiency point of annular jet pumps and decreases the flow rate ratio corresponding to this efficiency point. As the concentration of CMC solution increased to 0.5%, the maximum efficiency point decreased by 5.5%, and the flow rate ratio corresponding to this efficiency point dropped from 1 to 0.8. These findings provide reference and insights for analyzing the full flow field of annular jet pumps pumping shear-thinning non-Newtonian fluids and for structural design of such pumps. Full article

Although air resistance represents a relatively small portion of a ship’s total resistance, it can vary significantly for containerships due to loading conditions and container configurations. As containerships grow, their exposed projected windage area increases, leading to higher wind loads that affect maneuverability, heading control, and operational efficiency. Accurately assessing these aerodynamic effects is therefore crucial for both ship design and operational planning. This study investigates the aerodynamic contribution to ship resistance by evaluating the impact of random container configurations on wind loads for a post-Panamax 6750 TEU containership. Numerical simulations are performed at full scale under open-sea conditions using the Reynolds-averaged Navier-Stokes equations with the Realizable k-ε Two-Layer turbulence model. The resulting aerodynamic forces and moments are expressed as non-dimensional wind load coefficients, following ITTC recommendations. Numerical results are compared with methods provided by Blendermann and Isherwood, which are based on systematic wind tunnel measurements. Full article

Most computational studies of vehicle hydroplaning have emphasized structural realism through fluid–structure interaction, tire deformation, tread geometry, and pavement surface characterization. By contrast, the hydrodynamics governing the flow in the tire vicinity, particularly the role of turbulence, have received comparatively limited attention. In a significant number of studies, the flow has been treated as laminar despite turbulent flow conditions, while in a few other studies turbulence modeling has been adopted without an explicit assessment of its impact on hydroplaning predictions. In this study, we present a simplified three-dimensional computational fluid dynamics (CFD) model designed to isolate the flow regimes governing hydroplaning and to quantify the mean effect of the turbulence modeling on the predicted hydroplaning speed. Using a finite-volume formulation with a volume-of-fluid representation of the air–water interface, the flow around and beneath a smooth 0.7 m-diameter tire sliding in locked-wheel mode over a flooded, nominally smooth pavement is simulated. The tire is represented as a rigid body with an idealized rectangular bottom patch whose area is determined from the tire load and inflation pressure, avoiding the need to prescribe a measured or assumed deformed footprint. Steady-state hydroplaning is modeled for a uniform upstream water film thickness of 7.62 mm with a 0.5 mm gap between the tire and the pavement, over tire inflation pressures ranging from approximately 100 to 300 kPa, and predictions are verified against the empirical NASA hydroplaning equation. For these conditions, simulations without turbulence closure exhibit a consistent, systematic underprediction of the hydroplaning speed of approximately 13.5% relative to the NASA relation. Incorporating turbulence effects through Reynolds-averaged closures substantially reduces this bias, with average deviations of about 6% for the realizable k–ε model and 2.4% for the shear stress transport (SST) k–ω model. An analysis of the results indicates that hydrodynamic lift is dominated by pressure buildup associated with stagnation at the lower leading edge of the tire, with a significant contribution from shear-dominated flow in the thin under-tire gap, and that turbulence acts to moderate the integrated lift from these pressure fields. These results demonstrate that explicitly accounting for turbulence in the tire vicinity is essential for reproducing empirical hydroplaning trends and for avoiding systematic bias in CFD-based hydroplaning predictions. Full article

Drifting fish aggregating devices (DFADs) are central to tropical tuna purse-seine fisheries, yet their hydrodynamic performance under realistic seas has not been adequately addressed, particularly for emerging eco-friendly designs. A three-dimensional framework based on computational fluid dynamics is developed to assess the motion response and mooring loads of full-scale DFADs comprising raft buoys, biodegradable cotton rope, and iron sinkers, using four buoy layouts (Models A to D). Unsteady Reynolds-averaged Navier–Stokes (URANS) simulations are performed with a realizable k–ε closure, volume of fluid (VOF) free-surface capturing, the Euler overlay method, dynamic overset meshes, and catenary mooring coupling. Regular waves representative of operational conditions (T = 1.40 to 2.40 s, H = 0.10 to 0.40 m) are imposed via a VOF wave-forcing technique, and mesh/time-step sensitivity analyses demonstrate the accurate reproduction of the first-order wave elevation (error < 0.8%). Surge drift per cycle and heave response amplitude operators, with the relative mooring force, are evaluated as functions of the relative wavelength (λ/L_a) and wave steepness (H/λ). The results reveal that the buoy layout exerts first-order control on DFAD dynamics, whereas short, steep waves dominate motion and line loads. The intermediate end-point sinker mass achieves a favorable balance between motion suppression and mooring load control, whereas distributing a fixed total sinker mass along the rope reduces heave response and mooring force by improving the tension redistribution and overall stability. Across all sea states, Models A and D reduced motion envelopes and mooring forces, indicating their suitability as robust, low-impact configurations. The proposed framework and design recommendations provide quantitative guidance for optimizing eco-DFAD geometry and deployment strategies, supporting safer and more sustainable DFAD-based tuna fisheries. Full article

The high viscosity of biodiesel fuel, caused by the presence of saturated fatty acid esters, limits its application, particularly at low temperatures. Supercritical fluid extraction (SFE) using carbon dioxide represents a promising method for selective fractionation, enabling the removal of high-viscosity saturated components and the enrichment of the fuel with less viscous unsaturated esters. However, the rational design of such processes requires a deep understanding of the interrelationship between flow hydrodynamics, thermodynamic conditions, and mass transfer in a supercritical medium. In this work, a comprehensive computational fluid dynamics (CFD) modeling study of the fractionation process was performed for a model ethyl oleate/ethyl palmitate mixture (25.28:74.72 wt.%) in supercritical CO₂ at pressures of 11 and 14 MPa and a temperature of 40 °C. A three-dimensional model of a laboratory-scale extractor was developed using the Ansys Fluent software version 2020 R1 environment. Since the target esters are absent from the standard material database, a custom property library and compiled User-Defined Function (UDF) routines were developed. These describe the temperature dependence of density, viscosity, heat capacity, and thermal conductivity for both the individual components and their mixture using established mixing rules. The calculations employed an Eulerian multiphase model, the realizable k–ε turbulence model, and species transport equations. The modeling revealed pronounced selectivity: under the chosen thermodynamic conditions, ethyl palmitate is extracted preferentially over ethyl oleate, with this difference becoming more pronounced as pressure increases. The developed and verified CFD model deepens the fundamental understanding of hydrodynamics and mass transfer during supercritical fractionation and serves as a basis for optimizing process parameters to produce biodiesel with reduced viscosity. The regime at P = 14 MPa and t = 40 °C provides the most favorable thermodynamic and hydrodynamic conditions for the selective removal of saturated esters. Full article

In this study, a three-dimensional simulation of a walking-beam reheating furnace was developed to improve the steel slab reheating process and reduce surface oxidation kinetics using computational fluid dynamics (CFD). Combustion, heat transfer, fluid dynamics, and chemical reaction models were integrated into the numerical framework of this study. In addition, dynamic mesh remeshing was coupled through user-defined functions (UDFs), enabling the simultaneous simulation of slab movement and evolution of the involved transport phenomena. Turbulence was modeled with the realizable k-ε formulation, combustion with the Eddy Dissipation model, and radiation with the P-1 model coupled with WSGGM to include CO₂ and H₂O gas radiation. Scale formation was modeled using customized functions based on Arrhenius-type kinetics and Wagner’s oxidation model, evaluating its growth as a function of time, temperature, and furnace atmosphere. The predicted thermal evolution inside the furnace was validated using industrial data, yielding an average deviation of 5%. Furthermore, the proposed operating conditions led to an average slab temperature of 1289.77 °C at the exit of the homogenization zone, which was 16 °C higher than that under the current operation but still within the target range (1250 ± 50 °C). The reduction in combustion air decreased energy losses and improved product quality, lowering the molar oxygen content in the furnace atmosphere from 4.9 × 10² mol to 6.7 × 10¹ mol. Additionally, annual savings of 4,793,472 kg of natural gas and 13,884 tons of steel were estimated owing to reduced oxidation losses. The proposed air–fuel adjustment led to estimated annual energy savings (equivalent to 4,793,472 kg of natural gas) and a reduction in material loss due to oxidation from 4.5% to 3.75% (an absolute reduction of 0.75 percentage points; relative reduction ≈ 16.7%), which has a significant industrial impact on metal conservation and descaling cost reduction. Although there are CFD studies on plate overheating and scale growth separately, this work presents three main contributions: (1) the integration, within a single numerical framework, of combustion, radiation, species transport, oxidation kinetics, and actual plate movement using a dynamic mesh; (2) validation against continuous industrial records (16 thermocouples) and quantification of operational benefits such as fuel savings and reduced material loss; and (3) a comparative analysis between actual and optimized conditions, which standardize the air–methane ratio. Full article

The Yingxian Wooden Pagoda, a structure with a history spanning a thousand years, currently faces significant wind-induced safety risks. To understand the aerodynamic mechanism behind this issue, this study uses Computational Fluid Dynamics (CFD) with the Realizable k-ε turbulence model to perform high-fidelity transient simulations at wind speeds from 10 to 30 m per second. The results show that the highest positive pressure occurs on the sides of the windward face, while a large low-pressure vortex zone forms on the leeward side. The simulations include both the Kármán vortex street and the measurement of three-dimensional vortex-induced forces, marking a major advancement. A key finding is the synchronized period (ratio ≈ 1) of the along-wind and cross-wind forces, which differs from streamlined cylinders and is due to the pagoda’s unique octagonal shape. The force amplitudes increase exponentially with wind speed, while the average drag and lift have a quadratic relationship. Additionally, a new shape-specific correction factor of 0.875 is introduced to adjust the classical Strouhal formula, which greatly improves prediction accuracy for this type of ancient structure. This study offers both a theoretical foundation and a practical “digital wind tunnel” method for assessing wind-induced risks and supporting the safety monitoring of historic timber structures. Full article

Hydrodynamic Energy-Saving Devices (ESDs) have become effective solutions to improve vessel operational efficiency in maritime applications. A novel toroidal boosted appendage which is installed behind the KP505 propeller, featuring an integrated self-driving turbine and closed-loop blade structure, is proposed to simultaneously enhance propulsion efficiency, rectify wake non-uniformity, and mitigate vortex-induced energy losses. High-fidelity Computational Fluid Dynamics (CFD) simulations are conducted to evaluate the hydrodynamic performance of the device, aiming to minimize side effects such as the generated tip vortices and pressure pulses. Based on the STAR-CCM+ software, the Realizable $k-\epsilon$ turbulence model is adopted to simulate the flow fields of the propeller with and without the novel appendage. This paper focuses on investigating the influence of the new appendage on the propeller’s propulsion performance and conducts open-water performance prediction and wake field comparative analysis under different advance coefficients. The results show that the new appendage significantly improves the wake situation behind the propeller disk, changing from diffusion-flow to constriction-flow and achieving a uniform distribution of the wake field. The propulsion efficiency is increased by up to 7.453% at the design advance coefficient, and the novel toroidal boosted appendage is confirmed to have the potential to enhance the hydrodynamic performance of the propeller. Full article

Flow tubes are key rescue devices used to respond to explosions and fires caused by blowouts. Improperly designed flow tubes can cause buckling failures, which can result in injuries or fatalities, particularly during high-speed blowouts, so optimizing the design based on the mechanism of high-speed blowout flow near the flow tube can improve rescue efficiency and reduce risk. This study investigated the flow control mechanism and analyzed the lift force of variable-diameter flow tubes. Simultaneously, the suction effect generated by the flow tubes was also quantified. The effect of flow tube structure and posture parameters on the flow field near a blowout well was numerically investigated using Fluent CDF software 2020R2, and the realizable k-ε turbulent model was used to account for turbulence. The inlet velocity was set to 300 m/s in order to simulate a high-speed blowout flow. The diameter ratio of the upper and lower parts of the flow tube changed from 1:1 to 1:2.4, and the ratio of the lower part to the total length changed from 1:10 to 3:10. The effects of the diameter ratio and length ratio on the distribution of the velocity and pressure in the flow tube were investigated. A strong negative pressure profile was observed in the equal-diameter flow tube. As the diameter ratio increased from 1:1.6 to 1:2.4, the negative pressure decreased from −1094 Pa to −214 Pa. In addition, the risk of personal suction due to negative pressure at the bottom of the flow tube was evaluated, and the effectiveness of drainage and the capability of flow control were analyzed. When the diameter ratio was increased by approximately 12.5%, the flow rate of entrainment decreased by 4% compared to the equal-diameter tube. Furthermore, the flow tube was subjected to significant upward lift forces during the snapping process, thereby increasing the risk of dislodgment. The effect of the changes in height and angle on the lift forces on the flow tube during buckling-up-installation was examined. It was found that the lift force decreases with height and is sensitive to the angle of inclination. Overall, it was concluded that the diameter ratio of the flow tube and the length of the lower section are key parameters for flow tube design. Full article