Fluids

24 pages, 5875 KB

Open AccessArticle

The Influence of the Installation Angle of a Blade’s Low-Pressure Edge on the Cavitation Performance of Francis Pump-Turbines

by Hui Ruan, Wenxiong Chao, Xiangyang Li, Qingyang Zhang, Lvjun Qing and Chunmei Wei

Fluids 2025, 10(9), 248; https://doi.org/10.3390/fluids10090248 - 22 Sep 2025

Viewed by 291

Abstract

The low-pressure edge of a pump-turbine runner blade is more prone to cavitation than other parts. The installation angle of the blade’s low-pressure edge is one of the key parameters affecting the cavitation performance of the pump-turbine. Based on the installation angle of [...] Read more.

The low-pressure edge of a pump-turbine runner blade is more prone to cavitation than other parts. The installation angle of the blade’s low-pressure edge is one of the key parameters affecting the cavitation performance of the pump-turbine. Based on the installation angle of the blade’s low-pressure edge obtained by the principle of normal outflow of the turbine runner, two other installation angles of the low-pressure edge are constructed by increasing the installation angle of the low-pressure edge toward the band direction. Three types of blades are designed based on the parametric design program of the pump-turbine runner. The Zwart cavitation model is adopted to carry out full-channel steady numerical simulations for the three runners. The efficiencies and internal flow fields of the draft tube under turbine operating conditions are compared. The cavitation characteristics in pump mode, the distribution of the turbulent flow field, and the pressure distribution on the blade surface are analyzed. The influence laws of the installation angle of the blade’s low-pressure edge on pump-turbine performance is summarized. A design method for anti-cavitation of Francis pump-turbine runners has been explored. The results show that the LP1 blade can achieve normal outflow under the turbine’s rated operating condition, but due to the large inflow attack angle under pump operating conditions, the cavitation performance in pump mode is very poor. By increasing the installation angle of the blade’s low-pressure edge toward the band direction, the efficiencies and cavitation performances of the pump mode can be improved. The LP3 blade reduces the inflow attack angle while optimizing the pressure distribution on the blade’s suction surface, thereby reducing the superimposed effect of two phenomena under large-discharge pump operating conditions with low cavitation numbers: flow separations on the pressure surface caused by inflow impact, and flow separations on the suction surface of adjacent blades caused by cavitation. As a result, the cavitation performance of the LP3 blade is significantly better than that of the LP1 and LP2 blades. The proposed anti-cavitation design method is simple and effective and can be applied to the research and modification design of Francis pump-turbine runners. Full article

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21 pages, 2805 KB

Open AccessArticle

Predictive Analysis for U-Tube Transient Flow Events: A Digitalisation Framework

by Edwin A. Martínez-Padilla, Alfonso Arrieta-Pastrana, Oscar E. Coronado-Hernández, Manuel Saba and Vicente S. Fuertes-Miquel

Fluids 2025, 10(9), 247; https://doi.org/10.3390/fluids10090247 - 20 Sep 2025

Viewed by 451

Abstract

This study presents a methodology for the digitalisation process for analysing transient flow phenomena in a U-tube. It comprises several layers, including the characterisation of liquid oscillation dynamics, image segmentation for experimentally determining variations in the meniscus position, and the integration of machine [...] Read more.

This study presents a methodology for the digitalisation process for analysing transient flow phenomena in a U-tube. It comprises several layers, including the characterisation of liquid oscillation dynamics, image segmentation for experimentally determining variations in the meniscus position, and the integration of machine learning techniques with analytical solutions. The position, velocity, and acceleration of the meniscus are obtained using image-processing methods and subsequently compared with the corresponding analytical predictions. The proposed methodology accurately represents the existing hydraulic conditions, incorporating both Newtonian and Ogawa friction models. To assess model performance, the index of agreement was employed to compare analytical and experimental results. The findings indicate a systematic error of 2.2 mm ± 3 pixels when using the Ogawa friction model, which corresponds to the best model for predicting this hydraulic behaviour. Finally, the implementation of machine learning techniques demonstrates considerable potential for predictive analysis, with statistical measures showing coefficients of determination above 0.997 and consistently low Root Mean Square Error values. Full article

(This article belongs to the Section Mathematical and Computational Fluid Mechanics)

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29 pages, 22467 KB

Open AccessArticle

Research on Internal Instability Characteristics of Centrifugal Impeller Based on Dynamic Mode Decomposition

by Xiaoping Fan, Zhuhai Zhong, Hongfen Chen, Yang Chen, Meng Wang and Xiaodong Lu

Fluids 2025, 10(9), 246; https://doi.org/10.3390/fluids10090246 - 19 Sep 2025

Viewed by 266

Abstract

Nitrogen compression requires centrifugal compressors to operate under relatively high ambient pressure. However, the internal instability characteristics of compressors handling high-density working fluids remain unclear. Therefore, this study employs Dynamic Mode Decomposition (DMD) to investigate unsteady flow fluctuations within an isolated centrifugal impeller [...] Read more.

Nitrogen compression requires centrifugal compressors to operate under relatively high ambient pressure. However, the internal instability characteristics of compressors handling high-density working fluids remain unclear. Therefore, this study employs Dynamic Mode Decomposition (DMD) to investigate unsteady flow fluctuations within an isolated centrifugal impeller under both best efficiency and near-stall conditions at high ambient pressure. Results show that as the throttling process progresses, distinct unsteady phenomena emerge within the impeller. Under near-stall conditions, the frequency of the instability is 0.44 times the blade passage frequency (BPF), manifesting as periodic pressure fluctuations throughout the entire blade passage. This instability originates from periodic passage blockages caused by fluctuations in tip leakage flow. Additionally, the pressure fluctuations at the impeller inlet exhibit a noticeable lag compared to those in the latter half of the passage. Through DMD analysis, it is found that after the tip leakage vortex exits the blade, it interacts with the pressure surface of the adjacent blade, affecting the tip loading of the neighboring blade and forming a dynamic cycle. However, this vortex is not the primary flow structure responsible for the instability. These insights into the nature of unsteady disturbances provide valuable implications for future stall warning and instability prediction technologies. Full article

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19 pages, 3918 KB

Open AccessArticle

Numerical Simulation Study on the Performance of a New Gas Burner for Radiant Heating

by Jinyu Lv, Zongbao Li, Li Jia and Yinke Dou

Fluids 2025, 10(9), 245; https://doi.org/10.3390/fluids10090245 - 19 Sep 2025

Viewed by 399

Abstract

Compared with other fossil fuels, the combustion of natural gas releases fewer pollutants, with carbon dioxide being the main emission. As the need for environmental protection increases, gas combustion technology has been progressively developed, working to improve combustion efficiency and reduce harmful emissions. [...] Read more.

Compared with other fossil fuels, the combustion of natural gas releases fewer pollutants, with carbon dioxide being the main emission. As the need for environmental protection increases, gas combustion technology has been progressively developed, working to improve combustion efficiency and reduce harmful emissions. This study utilized computational fluid dynamics to conduct a numerical simulation of gas burners, establishing a physical model on the basis of the standard structural dimensions of the burners. This research focused on investigating the impacts of the excess air ratio, air temperature, and fuel load on combustion characteristics and nitrogen oxide emission levels. These results indicate that although increasing the excess air ratio can effectively reduce nitrogen oxide generation, it adversely affects the combustion efficiency. Additionally, a decrease in air temperature tends to reduce nitrogen oxide emissions, but adaptive adjustments to the combustion system are needed to sustain efficiency. While reducing the fuel load contributes to lower nitrogen oxide emissions, it compromises the combustion efficiency. Full article

(This article belongs to the Section Mathematical and Computational Fluid Mechanics)

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14 pages, 2287 KB

Open AccessArticle

Applicability of Reynolds Analogy and Visualization of Coolant Flow Mixing in Downcomer of Land-Based Water-Cooled SMR

by Anton Riazanov, Sergei Dmitriev, Aleksandr Dobrov, Denis Doronkov, Aleksey Pronin, Tatiana Demkina, Daniil Kuritsin, Danil Nikolaev and Dmitriy Solntsev

Fluids 2025, 10(9), 244; https://doi.org/10.3390/fluids10090244 - 16 Sep 2025

Viewed by 349

Abstract

This article presents an experimental study on the hydrodynamics of coolant flow within the pressure vessel of a small modular reactor (SMR) cooled with water, including areas such as the annular downcomer, bottom chamber, and core-simulating channels that are being developed for use [...] Read more.

This article presents an experimental study on the hydrodynamics of coolant flow within the pressure vessel of a small modular reactor (SMR) cooled with water, including areas such as the annular downcomer, bottom chamber, and core-simulating channels that are being developed for use in land-based nuclear power plants. This paper describes the experimental setup and test model, measurement techniques used, experimental conditions under which this research was conducted, and results obtained. This study was conducted at the Nizhny Novgorod State Technical University (NNSTU) using a high-pressure aerodynamic testing facility and a scale model that included structural components similar to those found in loop-type reactors. Experiments were performed with Reynolds numbers (Re) ranging from 20,000 to 50,000 in the annular downcomer space of the test model. Two independent techniques were used to simulate the non-uniform flow field in the pressure vessel: passive impurity injection (adding propane to the airflow) and hot tracer (heating one of the reactor circulation loops). The axial velocity field at the inlet to the reactor core was also investigated. This study provided information about the spatial distribution of a tracer within the coolant flow in the annular downcomer and bottom chamber of the pressure vessel. Data on the distribution of the contrasting admixture are presented in plots. The swirling nature of the coolant flow within the pressurized vessel was analyzed. It was shown that the intensity of mixing within the bottom chamber of the pressure vessel is influenced by the presence of a central vortex. Parameters associated with the mixing of admixtures within the model for the pressure vessel were estimated. Additionally, the possibility for simulating flow with different temperature mixing processes using isothermal models was observed. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques, 2nd Edition)

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31 pages, 19437 KB

Open AccessInteresting Images

Fringes, Flows, and Fractures—A Schlieren Study of Fluid and Optical Discontinuities

by Emilia Georgiana Prisăcariu, Raluca Andreea Roșu and Valeriu Drăgan

Fluids 2025, 10(9), 243; https://doi.org/10.3390/fluids10090243 - 16 Sep 2025

Viewed by 474

Abstract

This article presents a collection of schlieren visualizations captured using a custom-built, laboratory-based imaging system, designed to explore a wide range of flow and refractive phenomena. The experiments were conducted as a series of observational case studies, serving as educational bloc notes for [...] Read more.

This article presents a collection of schlieren visualizations captured using a custom-built, laboratory-based imaging system, designed to explore a wide range of flow and refractive phenomena. The experiments were conducted as a series of observational case studies, serving as educational bloc notes for students and researchers working in fluid mechanics, optics, and high-speed imaging. High-resolution images illustrate various phenomena including shockwave propagation from bursting balloons, vapor plume formation from volatile liquids, optical surface imperfections in transparent materials, and the dynamic collapse of soap bubbles. Each image is accompanied by brief experimental context and interpretation, highlighting the physical principles revealed through the schlieren technique. The resulting collection emphasizes the accessibility of flow visualization in a teaching laboratory, and its value in making invisible physical processes intuitively understandable. Full article

(This article belongs to the Special Issue Physical and Chemical Phenomena in High-Speed Flows)

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20 pages, 3389 KB

Open AccessArticle

Analytical Modelling of Water Pipeline Start-Up Processes

by Alberto Patiño-Vanegas, Carlos R. Payares Guevara, Enrique Pereira-Batista, Oscar E. Coronado-Hernández and Vicente S. Fuertes-Miquel

Fluids 2025, 10(9), 242; https://doi.org/10.3390/fluids10090242 - 12 Sep 2025

Viewed by 402

Abstract

The start-up process of water-distribution networks has been extensively investigated in recent years, particularly regarding the pressure surges that may occur during such transient events. In this context, researchers have concentrated on exploring physical formulations capable of describing the behaviour of the two [...] Read more.

The start-up process of water-distribution networks has been extensively investigated in recent years, particularly regarding the pressure surges that may occur during such transient events. In this context, researchers have concentrated on exploring physical formulations capable of describing the behaviour of the two interacting phases—water and air—typically resolved through numerical approaches. This paper presents an analytical solution to the nonlinear mathematical model governing the start-up of water pipelines containing a trapped air pocket. The model adopts the rigid water column approximation for the liquid phase and a polytropic gas law to account for the compressibility of the air. The resulting system can be formulated as a second-order nonlinear differential equation. The analytical approach consists of transforming the governing equation into a first-order linear ordinary differential equation, in which the square of the water front velocity is expressed as a function of the water column length. This transformation yields a closed-form solution expressed as a special integral series. The required integrals are evaluated using binomial expansions and incomplete gamma functions, enabling the derivation of a general solution valid within alternating intervals of monotonic motion. A practical application involving an 800 m pipeline is presented. Furthermore, the proposed solution is validated against experimental measurements, demonstrating the accuracy and effectiveness of the analytical approach in capturing the system’s transient behaviour. Full article

(This article belongs to the Special Issue Fluid Mechanics in Water Distribution Systems)

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22 pages, 6874 KB

Open AccessArticle

Numerical Investigation of Ventilated Cavities Around a Rudder-Equipped Axisymmetric Body

by Wanyun Xu, Yipeng Li, Renfang Huang, Weixiang Ye, Liang Hao and Wei Jiang

Fluids 2025, 10(9), 241; https://doi.org/10.3390/fluids10090241 - 10 Sep 2025

Viewed by 320

Abstract

As an efficient drag reduction technique, ventilated cavity technology demonstrates significant application in underwater launch systems. This study employs numerical simulations to systematically examine the ventilated cavity flow characteristics and cavity–rudder interaction mechanisms for a rudder-equipped axisymmetric body. Numerical simulation predicts the gas [...] Read more.

As an efficient drag reduction technique, ventilated cavity technology demonstrates significant application in underwater launch systems. This study employs numerical simulations to systematically examine the ventilated cavity flow characteristics and cavity–rudder interaction mechanisms for a rudder-equipped axisymmetric body. Numerical simulation predicts the gas leakage behavior, cavity geometry, and internal flow structure. The results indicate that the development of the ventilated cavity proceeds through three distinct stages: rapid growth, slow development, and quasi-periodic shedding. During this process, local high pressure at the leading edge of the rudder suppresses cavity growth, while cavity shedding is associated with re-entrant jet effects. Under the influence of the ventilated cavity, the overall load on the entire body and the local load on the rudder exhibit consistent patterns: F_x > F_y > F_z ≈ 0 and T_z > T_x ≈ T_y ≈ 0, with F_y and T_z fluctuating the most violently. The shedding cavity clusters are primarily concentrated at the rudder root during the quasi-periodic shedding stage. Full article

(This article belongs to the Section Mathematical and Computational Fluid Mechanics)

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1 pages, 163 KB

Open AccessCorrection

Correction: Nabwey et al. Effectiveness of Magnetized Flow on Nanofluid Containing Gyrotactic Micro-Organisms over an Inclined Stretching Sheet with Viscous Dissipation and Constant Heat Flux. Fluids 2021, 6, 253

by Hossam A. Nabwey, S.M.M. El-Kabeir, A.M. Rashad and M.M.M. Abdou

Fluids 2025, 10(9), 240; https://doi.org/10.3390/fluids10090240 - 9 Sep 2025

Viewed by 244

Abstract

There was an error in the original publication [...] Full article

21 pages, 3216 KB

Open AccessArticle

Enhancement of Aerodynamic Performance of Two Adjacent H-Darrieus Turbines Using a Dual-Rotor Configuration

by Douha Boulla, Saïf ed-Dîn Fertahi, Maryam Bernatchou, Abderrahim Samaouali and Asmae Arbaoui

Fluids 2025, 10(9), 239; https://doi.org/10.3390/fluids10090239 - 8 Sep 2025

Viewed by 1242

Abstract

Improvements in the aerodynamic performance of the H-Darrieus turbine are crucial to address future energy requirements. This work aims to optimize the behavior of two adjacent turbines through the addition of a dual H-Darrieus rotor. The first rotor is composed of three NACA [...] Read more.

Improvements in the aerodynamic performance of the H-Darrieus turbine are crucial to address future energy requirements. This work aims to optimize the behavior of two adjacent turbines through the addition of a dual H-Darrieus rotor. The first rotor is composed of three NACA 0021 blades, while the second comprises a single Eppler 420 blade. This study focuses on 2D CFD simulation based on the solution of the unsteady Reynolds-averaged Navier–Stokes (URANS) equations, using the sliding mesh method and

k - ω

SST turbulence model. The simulation results indicate a 17% improvement in the efficiency of the two turbines integrating dual rotors, compared to the two isolated turbines, for

α

= 0°. Moreover, the power coefficient

{(C}_{P})

reaches maximum values of 0.49, 0.42, and 0.40 for angles of attack of 30°, 25°, and 20°, respectively, at TSR = 2.51. Conversely, the selection of an optimal angle of attack allows the efficiency of the two H-Darrieus turbines to be increased. It is also shown by the results that the effect of stagnation is reduced and lift is maximized when the optimum distance between two adjacent turbines is chosen. Moreover, the overall aerodynamic performance of the system is enhanced by the potential of a dual-rotor configuration, and the wake between the two turbines is disrupted, which can result in a decrease in energy production within wind farms. Full article

(This article belongs to the Topic Fluid Mechanics, 2nd Edition)

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25 pages, 5300 KB

Open AccessArticle

CFD Analysis of Non-Isothermal Viscoelastic Flow of HDPE Melt Through an Extruder Die

by Aung Ko Ko Myint, Nontapat Taithong and Watit Pakdee

Fluids 2025, 10(9), 238; https://doi.org/10.3390/fluids10090238 - 8 Sep 2025

Viewed by 529

Abstract

The optimization of polymer extrusion processes is crucial for improving product quality and manufacturing efficiency in plastic industries. This study aims to investigate the viscoelastic flow behavior of high-density polyethylene (HDPE) through an extrusion die with an internal mandrel, focusing on the effects [...] Read more.

The optimization of polymer extrusion processes is crucial for improving product quality and manufacturing efficiency in plastic industries. This study aims to investigate the viscoelastic flow behavior of high-density polyethylene (HDPE) through an extrusion die with an internal mandrel, focusing on the effects of die geometry and flow parameters. A two-dimensional (2D) numerical model is developed in COMSOL Multiphysics using the Oldroyd-B constitutive equation, solved using the Galerkin/least-square finite element method. The simulation results indicate that the Weissenberg number (Wi) and die geometry significantly influence the dimensionless drag coefficient (Cd) and viscoelastic stress distribution along the die wall. Furthermore, filleting sharp edges of the die wall surface effectively reduces stress oscillations, enhancing flow uniformity. These findings provide valuable insights for optimizing die design and improving polymer extrusion efficiency. Full article

(This article belongs to the Special Issue Advances in Computational Mechanics of Non-Newtonian Fluids, 2nd Edition)

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29 pages, 4433 KB

Open AccessArticle

Influence of Boundary Conditions and Heating Modes on the Onset of Columnar Convection in Rotating Spherical Shells

by William Seeley, Francesca Coke, Radostin D. Simitev and Robert J. Teed

Fluids 2025, 10(9), 237; https://doi.org/10.3390/fluids10090237 - 5 Sep 2025

Viewed by 530

Abstract

We investigate the linear onset of thermal convection in rotating spherical shells with a focus on the influence of mechanical boundary conditions and thermal driving modes. Using a spectral method, we determine critical Rayleigh numbers, azimuthal wavenumbers, and oscillation frequencies over a wide [...] Read more.

We investigate the linear onset of thermal convection in rotating spherical shells with a focus on the influence of mechanical boundary conditions and thermal driving modes. Using a spectral method, we determine critical Rayleigh numbers, azimuthal wavenumbers, and oscillation frequencies over a wide range of Prandtl numbers and shell aspect ratios at moderate Ekman numbers. We show that the preferred boundary condition for convective onset depends systematically on both aspect ratio and Prandtl number: for sufficiently thick shells or for large Pr, the Ekman boundary layer at the outer boundary becomes destabilising, so that no-slip boundaries yield a lower

{Ra}_{c}

than stress-free boundaries. Comparing differential and internal heating, we find that internal heating generally raises

{Ra}_{c}

, shifts the onset to larger wavenumbers and frequencies, and relocates the critical column away from the tangent cylinder. Mixed boundary conditions with no-slip on the inner boundary behave similarly to purely stress-free boundaries, confirming the dominant influence of the outer surface. These results demonstrate that boundary conditions and heating mechanisms play a central role in controlling the onset of convection and should be carefully considered in models of planetary and stellar interiors. Full article

(This article belongs to the Collection Challenges and Advances in Heat and Mass Transfer)

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23 pages, 9252 KB

Open AccessArticle

Performance Analysis of Multi-Capillary Knudsen Heat Pumps

by Ana Ajuda, Goncalo Silva and Viriato Semiao

Fluids 2025, 10(9), 236; https://doi.org/10.3390/fluids10090236 - 3 Sep 2025

Viewed by 459

Abstract

The present work investigates the theoretical performance of the Knudsen heat pump (KHP), a novel heat pump concept in which the conventional mechanical compressor is replaced by a Knudsen compressor. This modification has the potential to reduce both maintenance requirements and energy consumption. [...] Read more.

The present work investigates the theoretical performance of the Knudsen heat pump (KHP), a novel heat pump concept in which the conventional mechanical compressor is replaced by a Knudsen compressor. This modification has the potential to reduce both maintenance requirements and energy consumption. The flow behavior within the Knudsen compressor, the core element of the KHP, is described using a simplified gas model derived from the formulation originally proposed by Muntz et al. The model predictions are initially validated against well-established data reported in the literature, and subsequently employed to analyze the performance of the KHP, with the final objective of enhancing its operational efficiency. To ensure the practical relevance of the performance assessment, the analysis is conducted using realistic geometrical and operational parameters derived from previously reported experimental studies of Knudsen compressors featuring rectangular or circular cross-sectional geometries. The results of this study suggest that, while the original KHP configuration exhibits limited performance, parametric analysis suggests the possibility to enhance its performance by more than 100% under optimal conditions, with additional factors identified that may enable further gains. Full article

(This article belongs to the Special Issue Physics and Applications of Microfluidics)

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25 pages, 10989 KB

Open AccessArticle

Research on the Relationship Between Pressure Pulsation and Leakage Vortex Intensity in the Blade Tip Clearance Under Various Operational Conditions of Axial Flow Pumps

by Xiaoqi Jia, Zhipeng Gan, Jie Liu, Xiaoqin Li, Zhe Lin and Zuchao Zhu

Fluids 2025, 10(9), 235; https://doi.org/10.3390/fluids10090235 - 3 Sep 2025

Viewed by 425

Abstract

Large underwater vehicles, designed for multiple cruising speeds, are required to operate under diverse conditions such as full speed, surfacing, diving, and hovering. This demands that the axial flow pumps used in these applications have a broad operational range, typically functioning efficiently from [...] Read more.

Large underwater vehicles, designed for multiple cruising speeds, are required to operate under diverse conditions such as full speed, surfacing, diving, and hovering. This demands that the axial flow pumps used in these applications have a broad operational range, typically functioning efficiently from 0.1 times rated flow to 1.5 times rated flow. In the process of adjusting operational conditions, axial flow pumps may experience rotating stall phenomena. Importantly, the presence of tip leakage vortices within the pump markedly influences the internal flow dynamics. To assess the impact of tip leakage vortices on the internal flow field under varied operational states, this study delves into the inherent link between tip leakage vortices and pressure pulsation across three specific scenarios: optimal, critical stall, and deep stall conditions. Analyzing from the perspective of the vorticity transport equation, it is found that the compression–expansion term dictates the core strength of tip leakage vortices, while the viscous dissipation factor determines the frequency of pressure pulsation. With an increase in the core strength of tip leakage vortices, a gradual rise in pressure pulsation is observed; in optimal scenarios, the core of tip leakage vortices progressively shifts toward the interior of the clearance, keeping the pulsation amplitude at each monitoring point within the blade tip clearance at integer multiples of the blade passing frequency. During critical stall and deep stall scenarios, the viscous dissipation effect of tip leakage vortices contributes to the emergence of high-frequency harmonic components within pressure pulsation. Full article

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16 pages, 2967 KB

Open AccessArticle

Effects of the Left Ventricular Mechanics on Left Ventricular-Aortic Interaction: Insights from Ex Vivo Beating Rat Heart Experiments

by Chenghan Cai, Ge He and Lei Fan

Fluids 2025, 10(9), 234; https://doi.org/10.3390/fluids10090234 - 2 Sep 2025

Viewed by 555

Abstract

The interaction between the left ventricle (LV) and aorta is critical for cardiovascular performance, particularly under pathophysiological conditions. However, how changes in LV mechanics, including preload and afterload, affect aortic function via LV–aorta interactions remains poorly understood due to the challenges associated with [...] Read more.

The interaction between the left ventricle (LV) and aorta is critical for cardiovascular performance, particularly under pathophysiological conditions. However, how changes in LV mechanics, including preload and afterload, affect aortic function via LV–aorta interactions remains poorly understood due to the challenges associated with varying loading conditions in vivo. To overcome these limitations, the effects of varying LV preload or afterload on LV and aortic functions via LV–aorta interactions are quantified using ex vivo beating rat heart experiments in this study. In five healthy rat hearts under retrograde Langendorff and antegrade working heart perfusion, LV pressure, volume, aortic pressure, and aortic blood flow were measured. Key findings include the following: (1) under Langendorff perfusion, aortic flow increased linearly with LV developed pressure (DP), with a slope of 4.04 mmHg·min/mL; under working heart constant-pressure perfusion (2) a 12.4% increase in afterload decreased aortic flow by 58.8%, indicating that elevated aortic pressure significantly impedes aortic flow; (3) a 10.4% increase in preload enhanced aortic flow by 44.2%, driven primarily by an increase in LV DP that promoted forward flow. These results suggest that aortic pressure predominantly influences aortic flow under varying afterload conditions, whereas LV DP plays the dominant role in regulating aortic flow under different preload conditions. These findings demonstrate that the heart’s loading conditions strongly impact aortic blood flow. Specifically, elevated LV afterload can severely limit forward blood flow, while increased LV filling with increased LV preload can enhance blood flow, highlighting the importance of managing both afterload and preload in conditions such as hypertension and heart failure with preserved ejection fraction. This pilot study also established the feasibility of experimental platforms for coronary and ventricular function analysis. Full article

(This article belongs to the Special Issue Recent Advances in Cardiovascular Flows)

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30 pages, 19735 KB

Open AccessArticle

Assessing Pedestrian Comfort in Dense Urban Areas Using CFD Simulations: A Study on Wind Angle and Building Height Variations

by Paulo Ulisses da Silva, Gustavo Bono and Marcelo Greco

Fluids 2025, 10(9), 233; https://doi.org/10.3390/fluids10090233 - 1 Sep 2025

Viewed by 950

Abstract

Pedestrian wind comfort is a critical factor in the design of sustainable and livable dense urban areas. This study systematically investigates the effects of surrounding building height and wind incidence angle on pedestrian-level wind conditions, analyzing a nine-building arrangement through validated Computational Fluid [...] Read more.

Pedestrian wind comfort is a critical factor in the design of sustainable and livable dense urban areas. This study systematically investigates the effects of surrounding building height and wind incidence angle on pedestrian-level wind conditions, analyzing a nine-building arrangement through validated Computational Fluid Dynamics (CFD) simulations. Scenarios included neighborhood heights varying from 0L to 6L and wind angles from 0° to 45°. The results reveal that wind angles aligned with urban canyons (0° case) induce a strong Venturi effect, creating hazardous conditions with Mean Velocity Ratio (MVR) peaks reaching 3.42. Conversely, an oblique 45° angle mitigates high speeds by promoting flow recirculation. While increasing neighborhood height generally intensifies channeling, the study also highlights that even an isolated building (0L case) can generate hazardous localized velocities due to flow separation around its corners. The Overall Mean Velocity Ratio (OMVR) analysis identifies that, among the studied cases, a 2L neighborhood height is the most tolerable configuration, striking a balance between sheltering and channeling effects. Ultimately, these findings highlight for urban planners the importance of analyzing diverse geometric configurations and wind scenarios, reinforcing the value of CFD as an essential tool for designing safer and more comfortable public spaces. Full article

(This article belongs to the Special Issue Computational Fluid Dynamics Applied to Transport Phenomena)

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21 pages, 5447 KB

Open AccessArticle

Dynamic Responses of Harbor Seal Whisker Model in the Propeller Wake Flow

by Bingzhuang Chen, Zhimeng Zhang, Xiang Wei, Wanyan Lei, Yuting Wang, Xianghe Li, Hanghao Zhao, Muyuan Du and Chunning Ji

Fluids 2025, 10(9), 232; https://doi.org/10.3390/fluids10090232 - 1 Sep 2025

Viewed by 468

Abstract

This study experimentally investigates the wake-induced vibration (WIV) behavior of a bio-inspired harbor seal whisker model subjected to upstream propeller-generated unsteady flows. Vibration amplitudes, frequencies, and wake–whisker interactions were systematically evaluated under various flow conditions. The test matrix included propeller rotational speed N [...] Read more.

This study experimentally investigates the wake-induced vibration (WIV) behavior of a bio-inspired harbor seal whisker model subjected to upstream propeller-generated unsteady flows. Vibration amplitudes, frequencies, and wake–whisker interactions were systematically evaluated under various flow conditions. The test matrix included propeller rotational speed N_p = 0~5000 r/min, propeller diameter D_p = 60~100 mm, incoming flow velocity U_∞ = 0~0.2 m/s, and separation distance between the whisker model and the propeller L/D = 10~30 (D = 16 mm, diameter of the whisker model). Results show that inline (IL) and crossflow (CF) vibration amplitudes increase significantly with propeller speed and decrease with increasing separation distance. Under combined inflow and wake excitation, non-monotonic trends emerge. Frequency analysis reveals transitions from periodic to subharmonic and broadband responses, depending on wake structure and coherence. A non-dimensional surface fit using L/D and the advance ratio (J = U_∞/(N_pD_p)) yielded predictive equations for RMS responses with good accuracy. Phase trajectory analysis further distinguishes stable oscillations from chaotic-like dynamics, highlighting changes in system stability. These findings offer new insight into WIV mechanisms and provide a foundation for biomimetic flow sensing and underwater tracking applications. Full article

(This article belongs to the Special Issue Marine Hydrodynamics: Theory and Application)

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21 pages, 6273 KB

Open AccessArticle

Numerical Investigation of an Ocean Brick System

by Hari Bollineni, Xiuling Wang and Joshua Toblas

Fluids 2025, 10(9), 231; https://doi.org/10.3390/fluids10090231 - 1 Sep 2025

Viewed by 443

Abstract

A three-dimensional Computational Fluid Dynamics (CFD) model is developed to simulate an Ocean Brick System (OBS) placed in a wave tank. When stacked, ocean bricks are designed to withstand wave forces and ocean currents, enhancing the stability of offshore support structures, such as [...] Read more.

A three-dimensional Computational Fluid Dynamics (CFD) model is developed to simulate an Ocean Brick System (OBS) placed in a wave tank. When stacked, ocean bricks are designed to withstand wave forces and ocean currents, enhancing the stability of offshore support structures, such as base supports of offshore wind turbines. In this study, the commercial software Ansys Fluent 2022 R1 is used for the simulations. A user-defined function (UDF) is developed to generate numerical waves that closely replicate those observed in experimental conditions. The numerical wave model is first validated against theoretical wave data, showing good agreement. The CFD model is then validated using experimental data from OBS tests conducted in the wave tank. Subsequently, the study investigates how OBS structures influence tidal waves—specifically, how they reduce the wave amplitude, and the pressure exerted on the bricks. Specifically, the wave amplitude reduction is more effective for waves with shorter wavelengths than for those with longer wavelengths, achieving up to a 70% reduction for waves with an amplitude of 0.785 m, a period of 5 s. Finally, a modification to the original brick geometry is proposed to further reduce wave amplitude and improve the stability of OBS platforms. For the same wave input, the modified brick geometry reduces wave energy effectively, achieving an 89.2% decrease in wave amplitude. Full article

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25 pages, 7391 KB

Open AccessArticle

Assessment of Transitional RANS Models and Implementation of Transitional IDDES Method for Boundary Layer Transition and Separated Flows in OpenFOAM-V2312

by Sandip Ghimire, Xiang Ni and Yue Wang

Fluids 2025, 10(9), 230; https://doi.org/10.3390/fluids10090230 - 1 Sep 2025

Viewed by 663

Abstract

Traditional hybrid RANS/LES methods often struggle to accurately capture both the boundary layer transition and flow separation simultaneously due to their reliance on fully turbulent RANS models. To address this limitation, the present study first evaluates three transitional RANS models (γ-Re_θt-SST, [...] Read more.

Traditional hybrid RANS/LES methods often struggle to accurately capture both the boundary layer transition and flow separation simultaneously due to their reliance on fully turbulent RANS models. To address this limitation, the present study first evaluates three transitional RANS models (γ-Re_θt-SST, γ-SST, and Kγ-SST) on the E387 airfoil. The results demonstrate that the γ-SST model offers the best balance of accuracy and computational efficiency in predicting laminar separation bubbles (LSBs) and transition points. Building on this, we implement the γ-SST-IDDES model into OpenFOAM-v2312, which integrates the γ-SST transitional RANS model with the Improved Delayed Detached Eddy Simulation (IDDES) approach. This coupling allows for the simultaneous prediction of the laminar-turbulent transition and high-fidelity resolution of separated flows. The γ-SST-IDDES model is rigorously validated across three airfoil cases with distinct separation characteristics: E387 (small separation), DBLN-526 (moderate separation), and NACA 0021 (massive separation). The results show that the γ-SST-IDDES model outperforms conventional methods, capturing leading-edge LSBs with high accuracy compared to fully turbulent IDDES. Additionally, it successfully resolves complex 3D vortical structures in separated regions, whereas unsteady URANS provides only quasi-2D results. Full article

(This article belongs to the Section Turbulence)

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22 pages, 10525 KB

Open AccessArticle

Numerical Study of Transverse Jet in Supersonic Flowfield Using Reynolds Stress Model Based Detached Eddy Simulation

by Zhi-Kan Liu, Yi-Lun Liu, Gang Wang and Tian-Yu Lin

Fluids 2025, 10(9), 229; https://doi.org/10.3390/fluids10090229 - 29 Aug 2025

Viewed by 648

Abstract

This study investigated the aerodynamic structures generated by transverse jet injection in supersonic flows around high-speed vehicles. The unsteady evolution of these structures was analyzed using an improved delayed detached Eddy simulation (IDDES) approach based on the Reynolds stress model (RSM). The simulations [...] Read more.

This study investigated the aerodynamic structures generated by transverse jet injection in supersonic flows around high-speed vehicles. The unsteady evolution of these structures was analyzed using an improved delayed detached Eddy simulation (IDDES) approach based on the Reynolds stress model (RSM). The simulations successfully reproduced experimentally observed shock systems and vortical structures. The time-averaged flow characteristics were compared with the experimental results, and good agreement was observed. The flow characteristics were analyzed, with particular emphasis on the formation of counter-rotating vortex pairs in the downstream region, as well as complex near-field phenomena, such as flow separation and shock wave/boundary layer interactions. Time-resolved spectral analysis at multiple monitoring locations revealed the presence of a global oscillation within the flow dynamics. Within these regions, pressure fluctuations in the recirculation zone lead to periodic oscillations of the upstream bow shock. This dynamic interaction modulates the instability of the windward shear layer and generates large-scale vortex structures. As these shed vortices convect downstream, they interact with the barrel shock, triggering significant oscillatory motion. To further characterize this behavior, dynamic mode decomposition (DMD) was applied to the pressure fluctuations. The analysis confirmed the presence of a coherent global oscillation mode, which was found to simultaneously govern the periodic motions of both the upstream bow shock and the barrel shock. Full article

(This article belongs to the Section Mathematical and Computational Fluid Mechanics)

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18 pages, 1911 KB

Open AccessArticle

Rapid Assessment of Relative Hemolysis Amidst Input Uncertainties in Laminar Flow

by Nasim Gholizadeh, Ryan Wang, Gayatri Gautham and Gautham Krishnamoorthy

Fluids 2025, 10(9), 228; https://doi.org/10.3390/fluids10090228 - 29 Aug 2025

Viewed by 466

Abstract

Predicting absolute values of hemolysis using the power law model to guide medical device design is hampered by uncertainties stemming from four sources of model inputs: incoming/upstream velocity profiles, blood viscosity models, power law hemolysis coefficients, and obtaining accurate stress exposure times. Amidst [...] Read more.

Predicting absolute values of hemolysis using the power law model to guide medical device design is hampered by uncertainties stemming from four sources of model inputs: incoming/upstream velocity profiles, blood viscosity models, power law hemolysis coefficients, and obtaining accurate stress exposure times. Amidst all these uncertainties, enabling rapid assessments and predictions of relative hemolysis would still be valuable for evaluating device design prototypes. Towards achieving this objective, hemolysis data from the Eulerian modeling framework was first generated from computational fluid dynamics simulations encompassing five blood viscosity models, four sets of hemolysis power law coefficients, fully developed as well as developing velocity flow conditions, and a wide range of shear stresses, strain rates, and stress exposure times. Corresponding hemolysis predictions were also made in a Lagrangian framework via numerical integration of shear stress and residence time spatial variations under the assumption of fully developed Newtonian fluid flow. Absolute hemolysis predictions (from both frameworks) were proportional to each other and independent of the blood viscosity model. Further, relative hemolysis trends were not dependent on the hemolysis power law coefficients. However, accuracy in wall shear stresses in developing flow conditions is necessary for accurate relative hemolysis assessments. Full article

(This article belongs to the Special Issue Advances in Computational Mechanics of Non-Newtonian Fluids, 2nd Edition)

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27 pages, 16172 KB

Open AccessArticle

Effects of Airfoil Parameters on the Cavitation Performance of Water Jet Propulsion Pumps

by Yingying Zheng, Yun Long, Churui Wan, Jianping Chen, Youlin Cai and Jinqing Zhong

Fluids 2025, 10(9), 227; https://doi.org/10.3390/fluids10090227 - 28 Aug 2025

Viewed by 569

Abstract

This paper investigates the influence of airfoil parameters on the cavitation performance of water jet propulsion pumps through numerical simulation methods. The effects of a varying inlet pressure and different airfoil structures on the critical net positive suction head (NPSH), head, and efficiency [...] Read more.

This paper investigates the influence of airfoil parameters on the cavitation performance of water jet propulsion pumps through numerical simulation methods. The effects of a varying inlet pressure and different airfoil structures on the critical net positive suction head (NPSH), head, and efficiency were systematically studied. Subsequently, the impact pattern of the airfoil structure on the cavitation performance was analyzed. The results demonstrate that the NACA0009-16_0004-16 airfoil exhibited the lowest required NPSH and superior cavitation resistance relative to the other tested airfoils. Nevertheless, the NACA0009-13_0004-13 airfoil demonstrated an optimal comprehensive performance, balancing the efficiency, head, and cavitation resistance. By extracting a water velocity isosurface of 23.6 m/s, we further investigated the flow characteristics of the suction surfaces of different airfoils at different cavitation conditions and found that the cavitation mainly includes TIP cavitation and sheet cavitation. With an increasing cavitation intensity, the sheet cavitation region progressively develops axially from the blade tip towards the blade outlet, extends radially from the shroud to the hub, and eventually nearly extends over the entire blade surface. The area of the TIP cavitation also expands, spreading downward in the same direction as the impeller rotation. The velocity vector exhibits a significantly higher density near the shroud and blade tips, suggesting potential flow separation and complex vortex structures in these regions. Near the blade leading edge, the water velocity isosurface area contracts, whereas near the trailing edge, it expands. These alterations indicate that the cavitation development modifies the flow field velocity distribution and adversely affects the impeller performance. This study establishes a theoretical foundation and offers practical guidelines for the multi-objective collaborative design of water jet propulsion pumps. Full article

(This article belongs to the Section Turbulence)

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29 pages, 1803 KB

Open AccessReview

Machine Learning in Fluid Dynamics—Physics-Informed Neural Networks (PINNs) Using Sparse Data: A Review

by Mouhammad El Hassan, Ali Mjalled, Philippe Miron, Martin Mönnigmann and Nikolay Bukharin

Fluids 2025, 10(9), 226; https://doi.org/10.3390/fluids10090226 - 28 Aug 2025

Viewed by 2883

Abstract

Fluid mechanics often involves complex systems characterized by a large number of physical parameters, which are usually described by experimental and numerical sparse data (temporal or spatial). The difficulty of obtaining complete spatio-temporal datasets is a common issue with conventional approaches, such as [...] Read more.

Fluid mechanics often involves complex systems characterized by a large number of physical parameters, which are usually described by experimental and numerical sparse data (temporal or spatial). The difficulty of obtaining complete spatio-temporal datasets is a common issue with conventional approaches, such as computational fluid dynamics (CFDs) and various experimental methods, particularly when evaluating and modeling turbulent flows. This review paper focuses on the integration of machine learning (ML), specifically physics-informed neural networks (PINNs), as a means to address this challenge. By directly incorporating governing physical equations into neural network training, PINNs present a novel method that allows for the reconstruction of flow from sparse and noisy data. This review examines various applications in fluid mechanics where sparse data is a common problem and evaluates the effectiveness of PINNs in enhancing flow prediction accuracy. An overview of diverse PINNs methods, their applications, and outcomes is discussed, demonstrating their flexibility and effectiveness in addressing challenges related to sparse data and illustrating that the future of fluid mechanics lies in the synergy between data-driven approaches and established physical theories. Full article

(This article belongs to the Special Issue Machine Learning and Artificial Intelligence in Fluid Mechanics)

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25 pages, 6248 KB

Open AccessArticle

Analysis of Disruption of Airflow and Particle Distribution by Surgical Personnel and Lighting Fixture in Operating Rooms

by Vikas Valsala Krishnankutty, Chandrasekharan Muraleedharan and Arun Palatel

Fluids 2025, 10(9), 225; https://doi.org/10.3390/fluids10090225 - 27 Aug 2025

Viewed by 675

Abstract

Surgical procedures have significantly contributed to the increased life expectancy of the global population. The surgical procedures are carried out in specialised rooms within a healthcare facility normally designated as operating rooms or operating theatres. These rooms require meticulously designed heating, ventilating, and [...] Read more.

Surgical procedures have significantly contributed to the increased life expectancy of the global population. The surgical procedures are carried out in specialised rooms within a healthcare facility normally designated as operating rooms or operating theatres. These rooms require meticulously designed heating, ventilating, and air conditioning systems to ensure optimal thermal comfort, strict sterility, and effective removal of airborne contaminants and anaesthetic gases. The performance of the system directly affects the risk of surgical site infections and associated post-operative complications. This study presents a computational fluid dynamics analysis of disturbance on airflow and particulate distribution within a representative operating room by the surgical staff and lighting fixtures concerning supply air velocity. The removal of the maximum possible particulate matter, precise control of air temperature and humidity, and unidirectional airflow in the surgical field were incorporated as key design strategies. The species transport model simulations revealed that while laminar airflow offers superior protection in terms of surgical site sterility, its performance is sensitive to disruptions caused by surgical lighting configurations and variations in supply air velocity. The findings highlight the complexities involved in maintaining optimal airflow conditions and underscore the need for integrative air conditioning design approaches that account for optimal design of surgical lighting and operational setups. Full article

(This article belongs to the Section Geophysical and Environmental Fluid Mechanics)

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18 pages, 2969 KB

Open AccessArticle

CFD-Based Extensional Stress and Hemolysis Risk Evaluation in the U.S. Food and Drug Administration (FDA) Benchmark Nozzle Configurations

by Mesude Avcı

Fluids 2025, 10(9), 224; https://doi.org/10.3390/fluids10090224 - 27 Aug 2025

Viewed by 599

Abstract

Hemolysis, or the breakdown of red blood cells, observed in medical devices has been a significant concern for many years, particularly when mechanical stress on the cells is considered. This study focuses on evaluating extensional stresses in two configurations of the U.S. Food [...] Read more.

Hemolysis, or the breakdown of red blood cells, observed in medical devices has been a significant concern for many years, particularly when mechanical stress on the cells is considered. This study focuses on evaluating extensional stresses in two configurations of the U.S. Food and Drug Administration (FDA) nozzle: the Gradual Cone (GC) and Sudden Contraction (SC) models. The nozzle geometries were created as 3D models using Ansys Fluent 18.2 and its pre-processing software ICEM CFD. The mesh was constructed with hexahedral elements with O-grid topologies. Effects of varying flow conditions were observed by modeling five experimental cases of the FDA nozzles, including throat Reynolds numbers of 500, 2000, 3500, 5000, and 6500. Hemolysis potentials of FDA nozzle configurations were examined by analyzing the whole domains. Turbulent modeling was used by applying the shear stress transport k-ω (SST k-ω) model. A threshold of 2.8 Pa for extensional stress was observed. Moreover, the most commonly used power law models were applied to the FDA nozzle to see the effect of extensional stress on power law models. Zhang’s power law models gave the lowest standard error, while Giersiepen’s model gave the highest error on hemolysis predictions. Full article

(This article belongs to the Special Issue Advances in Hemodynamics and Related Biological Flows)

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20 pages, 4657 KB

Open AccessArticle

Experimental and Numerical Analysis of Nozzle-Induced Cavitating Jets: Optical Instrumentation, Pressure Fluctuations and Anisotropic Turbulence Modeling

by Luís Gustavo Macêdo West, André Jackson Ramos Simões, Leandro do Rozário Teixeira, Igor Silva Moreira dos Anjos, Antônio Samuel Bacelar de Freitas Devesa, Lucas Ramalho Oliveira, Juliane Grasiela de Carvalho Gomes, Leonardo Rafael Teixeira Cotrim Gomes, Lucas Gomes Pereira, Luiz Carlos Simões Soares Junior, Germano Pinto Guedes, Geydison Gonzaga Demetino, Marcus Vinícius Santos da Silva, Vitor Leão Filardi, Vitor Pinheiro Ferreira, André Luiz Andrade Simões, Luciano Matos Queiroz and Iuri Muniz Pepe

Fluids 2025, 10(9), 223; https://doi.org/10.3390/fluids10090223 - 26 Aug 2025

Cited by 1 | Viewed by 623

Abstract

Cavitation has been widely explored to enhance physical and chemical processes across various applications. This study aimed to model the key characteristics of a cavitation jet, induced by a triangular-orifice nozzle, using both experimental and numerical methods. Optical instrumentation, a pressure transducer and [...] Read more.

Cavitation has been widely explored to enhance physical and chemical processes across various applications. This study aimed to model the key characteristics of a cavitation jet, induced by a triangular-orifice nozzle, using both experimental and numerical methods. Optical instrumentation, a pressure transducer and the Reynolds-Averaged Navier–Stokes (RANS) equations were employed. Optical instrumentation and high-speed photography detected the two-phase flow generated by water vaporization, revealing a mean decay pattern. Irradiance fluctuations and photographic evidence provided results about the light transmission dynamics through cavitating jets. Pressure fluctuations exhibited similar growth and decay, supporting optical instrumentation as a viable method for assessing cavitation intensity. Experimental data showed a strong relationship between irradiance and flow rate (R² = 0.998). This enabled the correlation of the standard deviation of instantaneous pressure measurements and normalized flow rate (R² = 0.977). Furthermore, vapor volume fraction and normalized flow rate reached a correlation coefficient of 0.999. On the simulation side, the SSG-RSM turbulence mode showed better agreement with experimental data, with relative deviations ranging from 2.1% to 6.6%. The numerical results suggest that vapor jet length is related to vapor fraction through a power law, enabling the development of new equations. These results demonstrated that anisotropic turbulence modeling is essential to reproduce experimental observations compared to mean flow properties. Based on the agreement between the numerical model and the experimental data for mean flow quantities, a formulation is proposed to estimate the jet length originating from the nozzle, offering a predictive approach for cavitating jet behavior. Full article

(This article belongs to the Section Turbulence)

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38 pages, 11916 KB

Open AccessArticle

Compressing Magnetic Fields by the Electromagnetic Implosion of a Hollow Lithium Cylinder: Experimental Test Beds Simulated with OpenFOAM

by Victoria Suponitsky, Ivan V. Khalzov, David M. Roberts and Piotr W. Forysinski

Fluids 2025, 10(9), 222; https://doi.org/10.3390/fluids10090222 - 25 Aug 2025

Viewed by 907

Abstract

Electromagnetic implosions of hollow lithium cylinders can be utilized to compress magnetized plasma targets in the context of Magnetized Target Fusion (MTF). Two small-scale experiments were conducted at General Fusion as a stepping stone toward compressing magnetized plasmas on a larger scale. The [...] Read more.

Electromagnetic implosions of hollow lithium cylinders can be utilized to compress magnetized plasma targets in the context of Magnetized Target Fusion (MTF). Two small-scale experiments were conducted at General Fusion as a stepping stone toward compressing magnetized plasmas on a larger scale. The first experiment is an electromagnetic implosion of a lithium ring, and the second is a compression of toroidal magnetic flux by imploding a hollow lithium cylinder onto an hourglass-shaped central structure. Here we present the methodology and results of modelling these experiments with OpenFOAM. Our in-house axisymmetric compressible MHD multi-phase solver was further extended to incorporate: (i) external RLC circuit model for electromagnetic compression coils and (ii) diffusion of the magnetic field into multiple solid materials. The implementation of the external RLC circuit model for electromagnetic coils was verified by comparison with results obtained with FEMM software and with the analytical solution. The solver was then applied to model both experiments and the main conclusions are as follows: (i) modelling solid lithium as a high-viscosity liquid is an adequate approach for the problems considered; (ii) the magnetic diffusivity of lithium is an important parameter for the accurate prediction of implosion trajectories (for the implosion of the lithium ring, higher values of magnetic diffusivity in the range

0.2 \leq η_{ring} [m^{2} / s] \leq 0.5

resulted in a better fit to the experimental data with a relative deviation in the trajectory of

≲ 20 %

); (iii) simulation results agree well with experimental data, and in particular, the toroidal field amplification of 2.25 observed in the experiment is reproduced in simulations within a relative error margin of 20%. The solver is proven to be robust and has the potential to be employed in a variety of applications. Full article

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35 pages, 11851 KB

Open AccessArticle

Numerical Investigation of Concave-to-Convex Blade Profile Transformation in Vertical Axis Wind Turbines for Enhanced Performance Under Low Reynolds Number Conditions

by Venkatesh Subramanian, Venkatesan Sorakka Ponnappa, Madhan Kumar Gurusamy and Kadhavoor R. Karthikeyan

Fluids 2025, 10(9), 221; https://doi.org/10.3390/fluids10090221 - 25 Aug 2025

Viewed by 854

Abstract

Vertical axis wind turbines (VAWTs) are increasingly utilized for decentralized power generation in urban and low-wind settings because of their omnidirectional wind capture and compact form. This study numerically investigates the aerodynamic performance of Darrieus-type VAWT blades as their curvature varies systematically from [...] Read more.

Vertical axis wind turbines (VAWTs) are increasingly utilized for decentralized power generation in urban and low-wind settings because of their omnidirectional wind capture and compact form. This study numerically investigates the aerodynamic performance of Darrieus-type VAWT blades as their curvature varies systematically from deeply convex (−50 mm) to strongly concave (+50 mm) across seven configurations. Using steady-state computational fluid dynamics (CFD) with the frozen rotor method, simulations were conducted over a low Reynolds number range of 25 to 300, representative of small-scale and rooftop wind scenarios. The results indicate that deeply convex blades achieve the highest lift-to-drag ratio (Cl/Cd), peaking at 1.65 at Re = 25 and decreasing to 0.76 at Re = 300, whereas strongly concave blades show lower and more stable values ranging from 0.95 to 0.86. The power coefficient (Cp) and torque coefficient (Ct) similarly favor convex shapes, with Cp starting at 0.040 and remaining above 0.030, and Ct sustaining a robust 0.067 at low Re. Convex blades also maintain higher tip speed ratios (TSR), exceeding 1.30 at Re = 300. Velocity and pressure analyses reveal that convex profiles promote stable laminar flows and compact wakes, whereas concave geometries experience early flow separation and fluctuating torque. These findings demonstrate that optimizing the blade curvature toward convexity enhances the start-up, torque stability, and power output, providing essential design guidance for urban VAWTs operating under low Reynolds number conditions. Full article

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14 pages, 344 KB

Open AccessArticle

Magnetohydrodynamic Turbulence in a Spherical Shell, Part 2: Emergent Magnetic Field from a Turbulent Geodynamo

by John V. Shebalin

Fluids 2025, 10(9), 220; https://doi.org/10.3390/fluids10090220 - 25 Aug 2025

Viewed by 431

Abstract

Using established results, we examine how a turbulent magnetic field in an outer core emerges and manifests itself as the geomagnetic field. Two basic results are demonstrated: First, how the stationary interior magnetic dipole components gain fluctuating parts, leading to polar wander of [...] Read more.

Using established results, we examine how a turbulent magnetic field in an outer core emerges and manifests itself as the geomagnetic field. Two basic results are demonstrated: First, how the stationary interior magnetic dipole components gain fluctuating parts, leading to polar wander of the geomagnetic dipole. Second, how the relation between the interior dipole energy

E_{D}

and magnetic helicity

H_{M}

, i.e.,

E_{D} = k_{m i n} H_{M}

, permits us to estimate the value of

H_{M}

in the outer core from the strength of the geomagnetic dipole field. We also discuss how MHD turbulence with magnetic helicity may be seen as the essential engine of the geodynamo. Full article

(This article belongs to the Section Turbulence)

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25 pages, 3412 KB

Open AccessArticle

Experimental Investigation of the Effects of Blocky Cuttings Transport on Drag and Drive Torque in Horizontal Wells

by Ye Chen, Wenzhe Li, Xudong Wang, Jianhua Guo, Pengcheng Wu, Zhaoliang Yang and Haonan Yang

Fluids 2025, 10(9), 219; https://doi.org/10.3390/fluids10090219 - 22 Aug 2025

Viewed by 563

Abstract

The deposition of large-sized cuttings (or blocky cuttings) is a critical risk factor for stuck pipe incidents during the drilling of deep and extended-reach wells. This risk is particularly pronounced in well sections with long borehole trajectories and low drilling fluid return velocities, [...] Read more.

The deposition of large-sized cuttings (or blocky cuttings) is a critical risk factor for stuck pipe incidents during the drilling of deep and extended-reach wells. This risk is particularly pronounced in well sections with long borehole trajectories and low drilling fluid return velocities, where it poses a substantial threat to wellbore cleanliness and the safe operation of the drill string. This study utilizes a self-developed visual experimental platform to simulate the deposition evolution of large-sized cuttings (20–40 mm in diameter) in the annulus under various wellbore inclinations and drilling fluid parameters. The stable height, lateral distribution characteristics, and response patterns of the resulting cuttings bed under different conditions were quantitatively characterized. Building upon this, a theoretical contact friction model between the drill string and the cuttings bed was employed to investigate how the bed height influences hook load during tripping and rotary torque during top drive operation. A mapping relationship was established between cuttings bed structural parameters and the resulting additional loads and torques. Results reveal significant interactive effects among drilling fluid velocity, fluid density, drill pipe rotation speed, and wellbore inclination on both cuttings bed development and associated drill string loads. Strong correlations were identified among these parameters. Based on these findings, a stuck pipe early-warning indicator system is proposed using frictional load thresholds, with clearly defined safety limits for cuttings bed height. Recommendations for optimizing cuttings transport parameters through coordinated control of fluid velocity, density, and rotary speed are also provided, offering theoretical support and engineering guidance for borehole cleaning strategies and stuck pipe risk prediction in large cuttings scenarios. Full article

(This article belongs to the Special Issue Numerical Modeling and Experimental Studies of Two-Phase Flows, 2nd Edition)

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