Fluids

22 pages, 21770 KB

Open AccessArticle

Thrust Characteristics of a Ducted Fan of a Quadcopter in Various Flight Modes

by Pavel Bulat and Pavel Chernyshov

Fluids 2026, 11(6), 135; https://doi.org/10.3390/fluids11060135 - 29 May 2026

Ducted fans are widely used in vehicles with a high engine power per unit swept area, including hovercraft propulsors and vertical take-off aircraft. Computational fluid dynamics (CFD) is a powerful tool for selecting the aerodynamic configuration of new aircraft and engines and for [...] Read more.

Ducted fans are widely used in vehicles with a high engine power per unit swept area, including hovercraft propulsors and vertical take-off aircraft. Computational fluid dynamics (CFD) is a powerful tool for selecting the aerodynamic configuration of new aircraft and engines and for determining their optimal operating conditions. The full Navier–Stokes equations, closed by the Shear Stress Transport (SST) and Spalart-Allmaras (SA) turbulence models, are used to simulate airflow induced by the rotating blades of quadcopter ducted-fan propulsors. Thrust characteristics of the ducted fan are analyzed based on numerical simulations in different flight modes, such as hovering and oblique inflow. Tip clearance and inner-wall effects on thrust and power are reported. For the studied four-blade ducted fan, varying the blade angle of attack from

16^{\circ}

to

32^{\circ}

raises the thrust coefficient from 0.27 to 0.84 and the power coefficient from 0.18 to 0.50. At a constant shaft power of 3750 W, the optimal relative tip clearance for moderately loaded blades is

1.5 %

(

16^{\circ}

angle). For heavily loaded blades (

32^{\circ}

angle), maximum thrust occurs at zero clearance. However, even at

0.8 %

clearance, losses are less than

0.1 %

compared to the closed-tip configuration. For technological reasons, a small clearance is generally preferred. Full article

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

16 pages, 3262 KB

Open AccessArticle

Multiphysics Modeling and Analysis of Droplet Impact on Tea Plant Leaves

by Qingmin Pan and Yongguang Hu

Fluids 2026, 11(6), 134; https://doi.org/10.3390/fluids11060134 - 29 May 2026

Abstract

The impact of warmer droplets on cold leaves in sprinkler anti-frost is a case of agricultural engineering involving multiphysics. This study models the leaf as an elastic body of finite thickness, incorporates the temperature field, and establishes a fluid–solid–thermal multiphysics coupling model. The [...] Read more.

The impact of warmer droplets on cold leaves in sprinkler anti-frost is a case of agricultural engineering involving multiphysics. This study models the leaf as an elastic body of finite thickness, incorporates the temperature field, and establishes a fluid–solid–thermal multiphysics coupling model. The effects of droplet velocity, droplet diameter, and initial temperature are analyzed accordingly. The results show that the higher the Weber number (We) of the droplet, the higher the droplet spreading coefficient and the leaf stress. The maximum spreading coefficient and maximum leaf strain at We of 1583.1 are 1.58 and 4.75 times those at We of 1055.4, respectively. There is a gradual decrease in the leaf deformation, a very rapid process, a cycle of about 10% of the spreading time. The temperature at the impact point on the leaf surface increased with the droplet’s initial temperature but could be influenced by an air bubble trapped at the droplet’s bottom. The modeling and analysis of the dynamics of droplet impact on plant leaves enabled a better understanding of the mechanisms of sprinkler frost protection. Full article

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

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19 pages, 1850 KB

Open AccessArticle

Hydrodynamic Synchronization of Two Oscillators in a Newtonian Fluid

by Tomé A. F. da Silva, Brendon O. Pontes, Elias S. Lima and Rodrigo C. V. Coelho

Fluids 2026, 11(6), 133; https://doi.org/10.3390/fluids11060133 - 29 May 2026

Abstract

Particles moving in a fluid interact through the flow field they generate, which can lead to complex nonlinear dynamics. One important example is the synchronization of oscillatory motion in biological systems, such as the coordinated beating of cilia or flagella. In this work, [...] Read more.

Particles moving in a fluid interact through the flow field they generate, which can lead to complex nonlinear dynamics. One important example is the synchronization of oscillatory motion in biological systems, such as the coordinated beating of cilia or flagella. In this work, we investigate the synchronization of two oscillators interacting through a Newtonian fluid using numerical simulations based on the lattice Boltzmann method. The oscillators are modeled as solid particles undergoing periodic motion, while hydrodynamic interactions are resolved explicitly through the surrounding flow. We analyze how synchronization depends on key physical parameters, including the fluid viscosity, the distance between the oscillators, the natural oscillation frequency, and the initial phase difference. The results are compared with predictions from the Kuramoto model in order to relate the hydrodynamic interaction to an effective phase coupling. We find that the coupling strength required for synchronization increases with both the oscillation frequency and the fluid viscosity, while it decreases with the distance between the oscillators. These results provide insight into the mechanisms underlying fluid-mediated synchronization and help bridge microscopic hydrodynamic models with reduced phase-oscillator descriptions. Full article

(This article belongs to the Special Issue 10th Anniversary of Fluids—Recent Advances in Fluid Mechanics)

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18 pages, 9323 KB

Open AccessArticle

RIM-PIV Measurements of Turbulent Flow over a Rough Porous Bed

by Zeeshan Qadir Memon and James Liburdy

Fluids 2026, 11(6), 132; https://doi.org/10.3390/fluids11060132 - 27 May 2026

Abstract

Flow over permeable beds is important in sediment transport and mixing processes, yet detailed velocity and stress measurements remain difficult to obtain, particularly close to the sediment–water interface (SWI). In this work, we use refractive-index-matched PIV to study turbulent open-channel flow over and [...] Read more.

Flow over permeable beds is important in sediment transport and mixing processes, yet detailed velocity and stress measurements remain difficult to obtain, particularly close to the sediment–water interface (SWI). In this work, we use refractive-index-matched PIV to study turbulent open-channel flow over and within a permeable bed composed of monodisperse borosilicate glass beads. Measurements are reported for three low-

R e_{K}

cases,

R e_{K} = 0.224

,

R e_{K} = 0.335

, and

R e_{K} = 0.360

, to resolve the mean velocity structure and the associated viscous, turbulent, Reynolds, and dispersive stress distributions. The results show that both the mean velocity and the turbulence intensity decrease rapidly below the SWI, indicating strong damping within the porous bed. Above the bed, the flow retains a boundary-layer structure, and increasing

R e_{K}

enhances the turbulence intensity without changing the overall regime. The results indicate a shift from turbulent transport above the bed to viscous control within the porous layer, while dispersive stresses peak near the interface. Overall, the SWI controls momentum exchange within a thin region and the porous bed suppresses turbulence penetration into the subsurface. Full article

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28 pages, 1533 KB

Open AccessPerspective

From Black-Box Optimization to Importance-Guided Control: A Perspective on Explainable Deep Reinforcement Learning for Drag Reduction

by Belén Reverte-Badillo, Clara Trillo-Yagüe, Andrés Cremades, Ricardo Vinuesa and Sergio Hoyas

Fluids 2026, 11(6), 131; https://doi.org/10.3390/fluids11060131 - 26 May 2026

Abstract

Fluid-dynamic drag accounts for a substantial fraction of energy consumption across air, ground, and maritime transport systems, making its reduction a critical lever for decarbonizing mobility. While active flow control (AFC) strategies have demonstrated significant drag reduction potential, their design remains constrained by [...] Read more.

Fluid-dynamic drag accounts for a substantial fraction of energy consumption across air, ground, and maritime transport systems, making its reduction a critical lever for decarbonizing mobility. While active flow control (AFC) strategies have demonstrated significant drag reduction potential, their design remains constrained by heuristic physical assumptions about dominant flow structures. Recent developments in deep reinforcement learning (DRL) have emerged as a transformative paradigm, capable of autonomously discovering control strategies in high-dimensional turbulent environments. This perspective traces the evolution of drag reduction approaches from classical passive and active control approaches toward data-driven methods based on DRL. A particularly promising direction is the integration of explainable artificial intelligence (XAI) with DRL, which provides physically interpretable information about flow regions associated with drag generation and guides the learning process toward physically meaningful actuation schemes. As a result, XAI-guided DRL controllers have been shown in canonical configurations to achieve comparable or improved drag reduction with substantially lower actuation power than controllers trained directly for drag minimization. This transition from opaque optimization toward flow control informed by dynamical causal relationships represents a key step for the development of energy-efficient and sustainable flow-control solutions for transport systems. Full article

(This article belongs to the Special Issue Flow Control Techniques: Advances in Flow System Analysis, Modeling and Applications, 2nd Edition)

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2 pages, 125 KB

Open AccessEditorial

Editorial: Future Trends and Challenges in High-Performance Computing for Turbulence

by Yonghua Yan and Yong Yang

Fluids 2026, 11(6), 130; https://doi.org/10.3390/fluids11060130 - 25 May 2026

Abstract

Turbulence remains one of the most persistent challenges in fluid dynamics and engineering [...] Full article

(This article belongs to the Special Issue Future Trends and Challenges in High Performance Computing for Turbulence)

26 pages, 40068 KB

Open AccessArticle

Hydrodynamic Analysis of Flow Inside a Novel Design for a Submerged Entry Nozzle for Steel Continuous Casting

by Jesus Gonzalez-Trejo, Cesar A. Real-Ramirez, Ruslan Gabbasov, Fernando Aragon-Rivera and Carlos E. Alvarado-Rodriguez

Fluids 2026, 11(6), 129; https://doi.org/10.3390/fluids11060129 - 23 May 2026

Abstract

In slab continuous casting, the internal hydrodynamics of the submerged entry nozzle (SEN) play a determining role in mold flow stability and product quality, particularly when external electromagnetic flow-control technologies are not employed. This study analyzes a novel bifurcated SEN design intended to [...] Read more.

In slab continuous casting, the internal hydrodynamics of the submerged entry nozzle (SEN) play a determining role in mold flow stability and product quality, particularly when external electromagnetic flow-control technologies are not employed. This study analyzes a novel bifurcated SEN design intended to promote stable, highly symmetric outlet jets under asymmetric inlet flow conditions produced by typical flow-control devices. The proposed configuration combines three geometric modifications: a square-section bore, a flow-divider bottom wall derived from a rotated mountain-type geometry, and two bell-shaped protrusions that act as flow modulators positioned immediately above the outlet ports. The hydrodynamic behavior inside the nozzle was investigated using complementary experimental and numerical approaches. Physical modeling was conducted in a scaled water model using particle image velocimetry (PIV) to characterize time-averaged velocity fields and flow fluctuations. In parallel, three-dimensional large-eddy simulations (LESs) were performed to resolve transient flow structures and quantify jet characteristics at the nozzle exits. Both approaches show consistent results. The combined action of the flow modulators and the flow-divider bottom wall robustly induces the formation of two nearly identical counter-rotating vortices in the lower region of the SEN. This flow structure suppresses stagnation and recirculation zones near the outlet ports, mitigates inlet-induced asymmetries, and enhances flow evacuation efficiency. Quantitative analysis of the outlet jets indicates a significant reduction in angular dispersion and a flow-rate imbalance below 0.2%, markedly lower than that observed in conventional SEN configurations. The results demonstrate that appropriate internal geometric design can effectively stabilize SEN hydrodynamics without active control systems, offering a feasible and scalable strategy for improving mold flow stability in industrial continuous casting operations. Full article

(This article belongs to the Special Issue Pipe Flow: Research and Applications, 2nd Edition)

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18 pages, 3467 KB

Open AccessArticle

Orientation-Dependent Drag Crisis and Flight Response of the FIFA World Cup Match Ball Trionda

by Sungchan Hong and Takeshi Asai

Fluids 2026, 11(5), 128; https://doi.org/10.3390/fluids11050128 - 21 May 2026

Abstract

Surface orientation can influence the aerodynamic response of modern soccer balls, particularly in the drag crisis regime. This study quantified the orientation-dependent aerodynamic characteristics of the FIFA World Cup match ball Trionda using a single specimen and examined how these differences affect simulated [...] Read more.

Surface orientation can influence the aerodynamic response of modern soccer balls, particularly in the drag crisis regime. This study quantified the orientation-dependent aerodynamic characteristics of the FIFA World Cup match ball Trionda using a single specimen and examined how these differences affect simulated flight at sea level and 1500 m altitude. Two reproducible reference orientations were defined: a red-panel-centered orientation (Series A) and a seam-junction-centered orientation (Series B). Each reference orientation was rotated by 0°, 90°, and 180°, resulting in six fixed-orientation conditions. Wind tunnel measurements were repeated three times per condition to obtain drag, lift, and side-force coefficients, and two-dimensional non-spinning flight simulations were performed for representative long-kick and free-kick conditions. All six orientations exhibited drag crisis behavior, but the transition response magnitude, subcritical drag level, and supercritical drag state differed among conditions. The representative transition region occurred at approximately Re = 2.0 × 10⁵ to 2.5 × 10⁵. Among the tested conditions, B-90 showed the lowest full-range mean drag coefficient (0.231), whereas A-90 showed the highest (0.266). In the simulations, lower drag orientations consistently produced longer flight ranges, and the B-90 > A-90 ordering was preserved across representative launch conditions and the expanded parametric comparison. These findings indicate that the aerodynamic response of Trionda cannot be represented adequately by a single mean drag coefficient and that surface orientation should be considered in aerodynamic characterization and flight prediction. Full article

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25 pages, 7186 KB

Open AccessArticle

Effects of Permeability and Gravity on Capillary Imbibition in Filter Paper

by Josefina Janeth Miranda-Blancas, José Martínez-Trinidad, Abraham Medina-Ovando, Luis Alfonso Moreno-Pacheco, Fernando Alonso-Cruz, Osvaldo Quintana-Hernández and Ricardo Andrés García-León

Fluids 2026, 11(5), 127; https://doi.org/10.3390/fluids11050127 - 21 May 2026

Abstract

Capillary imbibition is the process by which liquids are absorbed into porous materials as a result of capillary pressure differences at the pore scale. Accurate characterization of imbibition dynamics, particularly in the presence of gravitational potential, is essential for understanding fluid transport in [...] Read more.

Capillary imbibition is the process by which liquids are absorbed into porous materials as a result of capillary pressure differences at the pore scale. Accurate characterization of imbibition dynamics, particularly in the presence of gravitational potential, is essential for understanding fluid transport in diverse systems such as soil, fractured rocks, filtration media, and plant roots. This study presents systematic imbibition experiments using filter papers with pore sizes of 2.5 µm, 11 µm, and 20 µm, each inclined at 80° to quantify the influence of gravitational potential on imbibition behavior. For horizontally positioned samples, the imbibition front propagated radially and symmetrically, exhibiting a power law dependence on time. The measured temporal exponents ranged from 0.386 to 0.403, consistently lower than the theoretical value of 1/2 predicted by the Lucas–Washburn law. With increasing permeability, the temporal exponent approached the Washburn limit, indicating a marked dependence of imbibition dynamics on pore structure. For the inclined configuration at an 80° angle, the imbibition fronts remained nearly circular but exhibited a pronounced displacement of the front center toward gravity. This displacement increased with permeability, from approximately 0.497 cm for the 11 µm filter paper to 3545 cm for the 20 µm filter paper, highlighting the combined effects of permeability and gravitational potential on fluid movement. Furthermore, the advance of the imbibition front was significantly slower in the smallest pores (2.5 µm) compared to the larger ones. Experimental results were evaluated against a theoretical model proposed by Medina, demonstrating moderate quantitative agreement at early times, when gravitational potential effects are less significant. These findings confirm that both the temporal scaling exponent and the spatial evolution of the imbibition front are governed by the porous medium’s permeability and inclination angle, providing experimental evidence of deviations from ideal Washburn behavior in real porous systems. Full article

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20 pages, 5906 KB

Open AccessArticle

Numerical Simulation of Separation Characteristics of Particles Enhanced by Synergistic Extraction–Shearing

by Kai Wu, Lixia Hu, Zhanghao Wan, Fupeng Liu, Tao Jiang, Qiang Zhou and Li Luo

Fluids 2026, 11(5), 126; https://doi.org/10.3390/fluids11050126 - 20 May 2026

Abstract

This study utilizes computational fluid dynamics (CFD), numerical simulation of particle separation characteristics enhanced by synergistic extraction–shearing is performed, and the two-phase flow in a liquid–solid stirred tank is simulated using the Eulerian–Eulerian two-fluid model and the standard

k - ε

model. The [...] Read more.

This study utilizes computational fluid dynamics (CFD), numerical simulation of particle separation characteristics enhanced by synergistic extraction–shearing is performed, and the two-phase flow in a liquid–solid stirred tank is simulated using the Eulerian–Eulerian two-fluid model and the standard

k - ε

model. The effects of impeller speed, the hole arrangement pattern of the annular shroud, and the hole area on the multiphase fluid dynamics behavior and stirring power inside the tank are systematically studied. The results show that stirring speed is a key operating parameter affecting turbulence intensity and particle mixing uniformity. When the stirring speed increases from 2000 r/min to 4000 r/min, the overall tank turbulence increases significantly, but the stirring power increases from 4.69 kW to 36.57 kW. The annular cover at the bottom is arranged with vertical openings, which enables full energy transfer within the tank and effectively enhances the turbulence intensity in the middle and lower sections of the flow field; the horizontal opening form is more conducive to the radial diffusion of particles in the middle layer. Reducing the hole area by half increases the fluid jet velocity and local shear stress, effectively improving particle distribution uniformity, while the stirring power decreases by 43.75%, thereby achieving the collaborative optimization of mixing efficiency and energy consumption. Full article

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20 pages, 5970 KB

Open AccessArticle

An Investigation into Dry Gas Seals with Different Groove Structures

by Yu-Wei Wang, Bin-Bin Wu, Wen-Qing Li, Shuai Xu, Zhe-Hui Ma, Tian-Xiao Zhang, Chuang Liu and Jin-Yuan Qian

Fluids 2026, 11(5), 125; https://doi.org/10.3390/fluids11050125 - 20 May 2026

Abstract

Dry gas seals (DGSs) are currently the preferred sealing method for high-speed rotating machinery, widely used in the fields of petrochemicals and energy and power. This study analyzes the effect of groove structure and operating parameters (rotary ring speed and inlet pressure) on [...] Read more.

Dry gas seals (DGSs) are currently the preferred sealing method for high-speed rotating machinery, widely used in the fields of petrochemicals and energy and power. This study analyzes the effect of groove structure and operating parameters (rotary ring speed and inlet pressure) on the performance of the sealing system. The results show that a swallowtail-like groove demonstrates a dual effect of improving film stability and reducing leakage under specific working conditions. Specifically, under the inlet pressure of 4.5852 MPa and rotational speed of 10,380 rpm, the swallowtail-like groove achieves a 1.84% reduction in leakage and a 0.32% increase in opening force compared with a conventional spiral groove. Rotational speed has the greatest impact on the gas film stability of the cluster spiral groove. Increasing inlet pressure enhances the dynamic stabilization of gas film. Dynamic analysis indicates that the opening force demonstrates a linear proportionality with inlet pressure, whereas leakage follows an exponential growth. This work can provide guidance for optimizing the groove structure in dry gas sealing systems. Full article

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15 pages, 1542 KB

Open AccessArticle

Ester and Amide Functionalization of Maleated Polyolefins as Pour Point Depressants for Kumkol Waxy Crude Oil

by Assel Begimova, Zhanna Nadirova, Kazim Nadirov, Gulmira Bimbetova and Berik Sakybayev

Fluids 2026, 11(5), 124; https://doi.org/10.3390/fluids11050124 - 20 May 2026

Abstract

Pour point depressants (PPDs) based on functionalized polyolefins were obtained and evaluated for their efficiency in pour point reducing of Kumkol waxy crude oil (Kazakhstan), which contains 15.2 wt.% paraffin and has a pour point of +17 °C. An ethylene–propylene copolymer (EPR-505A) was [...] Read more.

Pour point depressants (PPDs) based on functionalized polyolefins were obtained and evaluated for their efficiency in pour point reducing of Kumkol waxy crude oil (Kazakhstan), which contains 15.2 wt.% paraffin and has a pour point of +17 °C. An ethylene–propylene copolymer (EPR-505A) was treated through grafting of maleic anhydride (MA-g-PO) and then converted into three different derivatives that had an identical polymer backbone: an ester-functionalized, an amide-functionalized, and a combined ester–amide additive. The obtained products were tested at 500 g/t through kinematic viscosity measurements, equilibrium and kinetic interfacial tension analysis, pour point determination, cooling curve analysis, and optical microscopy. The ester derivative reduced the pour point by 7 °C, the amide derivative did so by 5 °C, and the combined additive achieved a 10 °C pour point reduction and a more than twofold decrease in kinematic viscosity at 0 °C. Interfacial tension measurements and adsorption kinetics allowed us to assume that ester groups govern macromolecular solubility and diffusion mobility, while amide groups enhance adsorption affinity at paraffin crystal surfaces. Their combined action shifts crystallization from a collective to a dispersed regime. These findings establish structure–activity relationships between polar group architecture and PPD efficiency. Full article

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19 pages, 9738 KB

Open AccessArticle

Evaluation of Surface Roughness, Cutting Forces, and Tool Wear Under MQL Using Different Nano Cutting Oils in Milling Hastelloy C276 Superalloy

by Nguyen The Doan, Ngo Minh Tuan, Vu Lai Hoang and Tran The Long

Fluids 2026, 11(5), 123; https://doi.org/10.3390/fluids11050123 - 19 May 2026

Abstract

This paper presents a study on evaluating the effectiveness of nanofluid Minimum Quantity Lubrication (NF MQL) in machining Hastelloy C276 alloy—a difficult-to-cut material. The study compares NF MQL using different types of nanoparticles (Al₂O₃, MoS₂, SiC, and [...] Read more.

This paper presents a study on evaluating the effectiveness of nanofluid Minimum Quantity Lubrication (NF MQL) in machining Hastelloy C276 alloy—a difficult-to-cut material. The study compares NF MQL using different types of nanoparticles (Al₂O₃, MoS₂, SiC, and GrP) with dry and pure MQL conditions in terms of surface roughness, cutting force components, and especially the variation of cutting forces over time. Experimental results indicate that the graphene-containing nanofluid MQL showed the most superior performance in terms of surface roughness R_a with 54.3% and 34% reduction, followed by MoS₂ and Al₂O₃ nanofluid MQL conditions. Regarding the active cutting force F_a, Al₂O₃ nanofluid MQL achieves the largest reduction of about 18.4% and 22.1% when compared to dry and pure MQL, followed by GrP nanofluid MQL, MoS₂ nanofluid MQL, and then SiC nanofluid MQL. Meanwhile, GrP nanofluid MQL shows the highest percentage of F_z reduction at about 13.4% and 26% when compared to the dry and pure MQL conditions, followed by MoS₂ nanofluid MQL. Furthermore, the application of NF MQL also significantly improves tool life and extends about 36.4 ÷ 61.1% and 18.2 ÷ 50% compared to dry and pure MQL, respectively. Notably, through in-depth analysis of the variation of cutting forces, the study has elucidated the superior lubrication and cooling mechanism of the NF MQL method, confirming its potential application in machining advanced materials. Full article

(This article belongs to the Section Flow of Multi-Phase Fluids and Granular Materials)

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36 pages, 9371 KB

Open AccessArticle

Multi-Scenario Resistance Optimisation of an Indonesian Pioneer Vessel Through Response Surface Method

by Muhammad Iqbal, Andi Trimulyono, Ammarunissa Noor Asiyah Raihannanda, Azka Maulana Widestra, Berlian Arswendo Adietya and Ahmad Firdhaus

Fluids 2026, 11(5), 122; https://doi.org/10.3390/fluids11050122 - 18 May 2026

Abstract

Improving ship hydrodynamic efficiency is an important strategy for reducing fuel consumption and operational costs. This study investigates the optimisation of ship resistance through a combined approach involving hull form modification and operational trim adjustment. The research focuses on a pioneer vessel model, [...] Read more.

Improving ship hydrodynamic efficiency is an important strategy for reducing fuel consumption and operational costs. This study investigates the optimisation of ship resistance through a combined approach involving hull form modification and operational trim adjustment. The research focuses on a pioneer vessel model, where hydrodynamic performance is analysed using Computational Fluid Dynamics (CFD) simulations coupled with Central Composite Design (CCD) and the Response Surface Methodology (RSM). Prior to the optimisation analysis, the CFD model was verified through a grid convergence study and validated against towing tank experimental data, showing good agreement. The optimisation was conducted through three scenarios: hull form optimisation, trim optimisation, and integrated optimisation, which combined both strategies. The statistical analysis revealed that longitudinal parameters play a dominant role in resistance reduction. In particular, the longitudinal centre of buoyancy (LCB) was identified as the most influential parameter in hull form optimisation, while the longitudinal centre of gravity (LCG) was the dominant parameter in trim optimisation. The results show that hull form optimisation alone reduced resistance by approximately 6%, while trim optimisation achieved a reduction of about 4%. The integrated optimisation strategy produced the greatest improvement, resulting in resistance reduction of nearly 10% compared with the baseline configuration. The findings highlight the importance of integrating design-stage optimisation and operational optimisation in improving ship hydrodynamic performance. However, the optimisation was limited to calm-water conditions. Full article

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22 pages, 4294 KB

Open AccessReview

Active Flow Control for High-Speed Trains: From Local Flow Manipulation to Mission-Adaptive Aerodynamic Control

by Li Sheng, Kaimin Wang, Xiaodong Chen, Yujun Liu and Tanghong Liu

Fluids 2026, 11(5), 121; https://doi.org/10.3390/fluids11050121 - 17 May 2026

Abstract

High-speed train aerodynamics have mainly been improved by passive design methods, such as streamlined noses, local fairings, and surface smoothing. These methods have achieved clear benefits, but several important aerodynamic problems remain difficult to solve by geometry optimization alone. Open-air drag is still [...] Read more.

High-speed train aerodynamics have mainly been improved by passive design methods, such as streamlined noses, local fairings, and surface smoothing. These methods have achieved clear benefits, but several important aerodynamic problems remain difficult to solve by geometry optimization alone. Open-air drag is still affected by tail flow separation, base-pressure recovery, and disturbances around bogies and the underbody; crosswind safety is influenced by unsteady leeward-side separation and wake asymmetry; slipstream behavior depends on wake vortices, boundary-layer development, and complex near-ground underbody flow; and tunnel-related pressure transients arise from compression-wave generation, propagation, and reflection. These coupled effects mean that one fixed train shape cannot perform optimally in all operating conditions. For this reason, this review proposes that active flow control (AFC) should not be regarded only as a drag-reduction or stability-improvement technique for high-speed trains. Instead, it should be understood as a mission-adaptive aerodynamic control framework, in which different control actions are used for different operating scenarios. This paper first clarifies that passive optimization is increasingly subject to diminishing returns under multi-objective and engineering constraints. It then reviews AFC studies on drag reduction, base-pressure recovery, wake and slipstream control, underbody flow conditioning, crosswind mitigation, and tunnel pressure-wave suppression. Related AFC studies on bluff bodies, road vehicles, and other separated flows are included only when their physical relevance to trains is clear. The review further distinguishes gross aerodynamic improvement from net energy gain and identifies actuator power, durability, maintainability, acoustic impact, validation level, and full-scale transferability as decisive feasibility factors. Current research is still dominated by open-loop numerical studies with simplified actuation. Future work should therefore move toward multi-objective, closed-loop, energy-aware, sensor–actuator-integrated, and explainable machine-learning-assisted AFC. The main message is that the next step in train aerodynamics is not simply a better fixed shape, but a control-enabled train that can selectively redistribute aerodynamic authority across its mission profile. Full article

(This article belongs to the Special Issue Open and Closed-Loop Control Systems for Active Flow Control)

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20 pages, 5722 KB

Open AccessArticle

Development of Methods for Real-Time In-Line Monitoring of Yield Stress for Non-Newtonian Fluid Using Pressure Drop and Liquid Rise Method During the Transfer of Radioactive Waste

by Anirban Saha, Michael Poirier and Dwayne McDaniel

Fluids 2026, 11(5), 120; https://doi.org/10.3390/fluids11050120 - 15 May 2026

Abstract

Real-Time In-Line Monitoring (RTIM) of rheological properties such as slurry yield stress is important in different industries for its various benefits such as significant time savings and increased safety/efficiency of processes while reducing secondary waste due to sampling or inaccurate procedures. This paper [...] Read more.

Real-Time In-Line Monitoring (RTIM) of rheological properties such as slurry yield stress is important in different industries for its various benefits such as significant time savings and increased safety/efficiency of processes while reducing secondary waste due to sampling or inaccurate procedures. This paper discusses two methods for characterizing yield stress in real time: the Pressure Loss method and the Liquid Rise method. The Liquid Rise method uses the height of the slurry in a vertical column and the pressure difference to quantify the yield stress. The Pressure Loss method uses the drop of pressure in a laminar flow of slurry to determine the yield stress. Kaolin–water slurry is used as a simulant of the non-Newtonian fluid. An experimental setup is built to demonstrate the methods, and data obtained from the experimental setup is compared with the yield stress obtained from a conventional table-top rheometer (baseline rheology). The results show a good agreement between the experimental yield stress and baseline rheology. Full article

(This article belongs to the Special Issue 10th Anniversary of Fluids—Recent Advances in Non-Newtonian and Complex Fluids)

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26 pages, 4501 KB

Open AccessArticle

Transient CFD Study of Aerodynamic Interaction Between Heavy-Duty Trucks During Highway Merging and Platoon Formation Under Crosswind

by Daniela Delia Alic, Imre Zsolt Miklos and Cristina Carmen Miklos

Fluids 2026, 11(5), 119; https://doi.org/10.3390/fluids11050119 - 15 May 2026

Abstract

Highway merging and platoon formation are critical scenarios in heavy-duty vehicle aerodynamics. This study presents a transient computational fluid dynamics (CFD) analysis of two trucks undergoing a merging maneuver and subsequent platoon formation. A three-dimensional unsteady Reynolds-Averaged Navier–Stokes (uRANS) approach with the SST [...] Read more.

Highway merging and platoon formation are critical scenarios in heavy-duty vehicle aerodynamics. This study presents a transient computational fluid dynamics (CFD) analysis of two trucks undergoing a merging maneuver and subsequent platoon formation. A three-dimensional unsteady Reynolds-Averaged Navier–Stokes (uRANS) approach with the SST k–ω turbulence model is employed under zero-crosswind and yawed inflow conditions. The present work provides a time-resolved characterization of truck–truck aerodynamic interactions during dynamic spacing evolution, enabling the capture of unsteady wake effects that are not accessible in steady-state formulations commonly used in cooperative driving studies. Unlike previous steady analyses, the approach resolves transient wake development, vortex shedding, and their direct impact on instantaneous aerodynamic loads. Results identify three interaction regimes: weak interaction, strong wake interaction during wake impingement, and wake recovery at larger spacing. Under zero-crosswind conditions, significant drag reduction is observed, confirming platooning benefits. However, crosswind conditions substantially reduce this benefit and increase lateral loads due to asymmetric pressure distribution and wake deflection. A non-linear spacing–drag relationship is observed, governed by wake evolution and shear-layer interaction. These findings provide quantitative insight into transient aerodynamic interactions and highlight the importance of accounting for unsteady and crosswind effects in platoon performance assessment. Full article

(This article belongs to the Special Issue Industrial CFD and Fluid Modelling in Engineering, 3rd Edition)

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25 pages, 14434 KB

Open AccessArticle

Haematocrit Distribution in Coronary Arteries: A ROM-PINN and Data-Driven Approach for Predicting Multiphase Flow

by Bharath Sharma, William Fox, Jianhua Chen, Daniel M. Espino and Marco Castellani

Fluids 2026, 11(5), 118; https://doi.org/10.3390/fluids11050118 - 14 May 2026

Abstract

Blood is a multiphase fluid, constituted of a plasma phase and a red blood cell (RBC) phase. Predicting the distribution of the RBC phase has applications in terms of medical device design, and for the characterisation of the risk of thrombus formation where [...] Read more.

Blood is a multiphase fluid, constituted of a plasma phase and a red blood cell (RBC) phase. Predicting the distribution of the RBC phase has applications in terms of medical device design, and for the characterisation of the risk of thrombus formation where atherosclerosis is present on coronary arteries. Computational fluid dynamics (CFD) can be used to simulate the multiphase flow of blood, but is time-consuming and requires a high level of technical expertise. This study evaluates the use of artificial neural networks (ANNs), as an alternative to CFD, to predict RBC distribution as part of blood flow through a coronary artery bifurcation model, both including and excluding stenosis. ANNs were trained on a dataset of 80 simulations generated using steady-state multiphase CFD. The initial data-driven ANNs encountered issues with overfitting and high errors in velocity component predictions. A physics-informed neural network (PINN) was employed, using a reduced order model (ROM), to enhance velocity component predictions, achieving average percentage error (APE) within 8.5% of CFD. These improved predictions were integrated into a hybrid model combining the PINN and the data-driven ANN to predict RBC distribution more effectively. The hybrid model achieved APEs ranging from 0.04% to 0.05%. Moreover, the hybrid model’s predictions were 14 times faster than CFD transient runs, demonstrating potential for translation into clinical use. In conclusion, a combined ROM-PINN and data-driven approach enables fast high-accuracy predictions of flow for multiphase fluids such as blood when compared to CFD. Full article

(This article belongs to the Special Issue Advances in Hemodynamics and Related Biological Flows, 2nd Edition)

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15 pages, 1259 KB

Open AccessArticle

A Calculation Method and Application Research in Gas-Lift Reverse Circulation Bottom-Hole Pressure Based on Gas–Liquid Two-Phase Flow Theory

by Pu Liu, Chuanhua Ge, Ruiqi Zhang, Ruifeng Tan and Shanquan Fan

Fluids 2026, 11(5), 117; https://doi.org/10.3390/fluids11050117 - 14 May 2026

Abstract

Gas-lift reverse circulation drilling technology is one of the typical “bottom-hole negative pressure” drilling technologies. This technology can significantly reduce wellbore circulation pressure loss, alleviate the bottom-hole pressure holding effect, and effectively lower the probability of lost circulation. The core theory underlying this [...] Read more.

Gas-lift reverse circulation drilling technology is one of the typical “bottom-hole negative pressure” drilling technologies. This technology can significantly reduce wellbore circulation pressure loss, alleviate the bottom-hole pressure holding effect, and effectively lower the probability of lost circulation. The core theory underlying this technology is multiphase flow in the wellbore. Based on gas–liquid two-phase flow theory, this paper develops a method for calculating bottom-hole pressure during gas-lift reverse circulation. The effects of key operational parameters on bottom-hole pressure were analyzed. The results show that bottom-hole pressure decreases as gas injection rate increases and as the gas injection point deepens. Moreover, the deeper the gas injection point, the greater the pressure reduction. Compared with the results from gas-lift reverse circulation drilling design and monitoring software applied to a shale gas well in southern Sichuan, the two sets of data differ by approximately 3%. The proposed calculation method can predict bottom-hole pressure under gas-lift reverse circulation conditions, overcoming the low accuracy of empirical formulas traditionally used in such operations. This has significant implications for advancing gas-lift reverse circulation technology in oil and gas well drilling. Full article

(This article belongs to the Special Issue Fluids Flow in Mining Engineering)

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