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Volume 11
Issue 5
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Fluids, Volume 11, Issue 5 (May 2026) – 24 articles

Cover Story (view full-size image): A critical factor affecting ionic liquid (IL) performance is the absorbed water. The impact of absorbed moisture on Tg, ionic conductivity, and molecular dynamics was investigated in two levulinate-based ILs: 1-ethyl-1-methylpyrrolidinium levulinate ([C2C1Pyr]Lev) and 1-butyl-1-methylpyrrolidinium levulinate ([C4C1Pyr]Lev). The cations differ only in the length of the alkyl chain attached to the pyrrolidinium nitrogen, while the anion remains the same. The stronger Tg depression in [C2C1Pyr]Lev arises from direct water–cation interactions, whereas in [C4C1Pyr]Lev, water primarily perturbs alkyl-chain packing, leading to a smaller Tg decrease. View this paper
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18 pages, 3467 KB  
Article
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
Viewed by 6274
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  
Article
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
Viewed by 439
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  
Article
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
Viewed by 294
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  
Article
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
Viewed by 646
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  
Article
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
Viewed by 306
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  
Article
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
Viewed by 632
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 Ra with 54.3% and 34% reduction, followed by MoS2 and Al2O3 nanofluid MQL conditions. Regarding the active cutting force Fa, 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 Fz 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  
Article
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
Viewed by 363
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  
Review
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
Viewed by 516
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  
Article
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
Viewed by 516
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  
Article
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
Cited by 1 | Viewed by 1087
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  
Article
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
Viewed by 1538
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  
Article
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
Viewed by 436
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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43 pages, 6624 KB  
Review
An Overview of Highly Viscous Fluid Flows in Straight and Elbow Pipes: II Gas–Liquid Flows
by Enrique Guzmán, Leonardo Di G. Sigalotti, Lizeth Torres and Jaime Klapp
Fluids 2026, 11(5), 116; https://doi.org/10.3390/fluids11050116 - 12 May 2026
Viewed by 943
Abstract
This review summarizes the latest research concerning the horizontal flow of two-phase mixtures with viscosities ranging from 0.2 Pa·s to 6.4 × 104 Pa·s. Although our survey is concerned with Newtonian fluids, a short section is included to briefly discuss certain rheological [...] Read more.
This review summarizes the latest research concerning the horizontal flow of two-phase mixtures with viscosities ranging from 0.2 Pa·s to 6.4 × 104 Pa·s. Although our survey is concerned with Newtonian fluids, a short section is included to briefly discuss certain rheological aspects that should be generally considered. In contrast with previous work reporting on the progress in specific domains (e.g., in the oil and gas, chemical, or geophysical contexts), we seek to provide a comprehensive overview of the methods and results used in different contexts. Accordingly, the scope is widened to encompass a broader range of industrial applications and naturally occurring flows. The interest in high-viscosity flows is motivated by the operational challenges occurring in certain systems, most notably in the oil and gas industry, where the production of heavy and extra-heavy crude oils reduces the margins for a safe and efficient operation. Furthermore, this review underlines the cross-field analogies appearing in a broad range of scales and applications. It emphasizes the fundamental role of viscosity in determining the flow patterns, as experimental evidence suggests that the transition boundaries are largely altered at higher viscosities. Some gaps that could be addressed in future work are briefly discussed. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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30 pages, 7834 KB  
Article
Boundary Conditions and Algorithms for 2D and 3D Lattice Boltzmann Simulations of Heterogeneous Reactions
by Claudius Stockinger, Antonio Raiolo, Ulrich Nieken, Abdellah Hadjadj and Mostafa Safdari Shadloo
Fluids 2026, 11(5), 115; https://doi.org/10.3390/fluids11050115 - 6 May 2026
Viewed by 587
Abstract
In this paper, the formulation and method of implementation of boundary conditions for heterogeneous reactions in porous media are elaborated. These are implemented into a previously validated lattice Boltzmann model for the simulation of heterogeneous reactions in porous media, extending it on multiple [...] Read more.
In this paper, the formulation and method of implementation of boundary conditions for heterogeneous reactions in porous media are elaborated. These are implemented into a previously validated lattice Boltzmann model for the simulation of heterogeneous reactions in porous media, extending it on multiple fronts. The formulation of the boundary conditions is validated thoroughly. The conversion of solid carbon to CO and CO2 is chosen as a specific case of application. An extensive parametric study is conducted with a specific geometry consisting of spherical substrate particles, coated with a reactive soot layer, to highlight the capability of the code. The code was able to capture the expected evolution of a combustion front and the influence of process parameters onto its propagation velocity. The propagation speed linearly increased with an increase in the reactant mass fraction and exponentially increased with Péclet number. Also, the CO/CO2 ratio obtained from experimental data could be reproduced with good accuracy. Furthermore, an algorithm for the correct evaluation of the specific surface is presented, which is necessary for evolving solid domains based on realistic geometries containing enclosed cavities. The method of implementation, computational overhead and acceleration technique are discussed. Finally, the model and all boundary conditions are extended to 3D and validated. Full article
(This article belongs to the Special Issue Lattice Boltzmann Methods: Fundamentals and Applications, 2nd Edition)
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4 pages, 145 KB  
Editorial
Editorial for Mass Transfer in Multiphase Reactors
by Stoyan Nedeltchev
Fluids 2026, 11(5), 114; https://doi.org/10.3390/fluids11050114 - 3 May 2026
Viewed by 511
Abstract
Gas–liquid reactors (especially bubble columns (BCs)) are the most widely used in both the chemical and biochemical industries [...] Full article
(This article belongs to the Special Issue Mass Transfer in Multiphase Reactors)
16 pages, 5216 KB  
Article
Air Knives: Going Beyond the Classical Midspan Pressure Distributions
by Celia Miguel-González, Aitor Vega-Valladares, Manuel García-Díaz, Alejandro Rodrígurez de Castro, José González Pérez and Bruno Pereiras
Fluids 2026, 11(5), 113; https://doi.org/10.3390/fluids11050113 - 30 Apr 2026
Viewed by 435
Abstract
Air knives are extensively employed in many cold rolling or tin plate production lines for drying purposes. Generally, these systems are oversized, resulting in excessive energy consumption, a consequence of insufficient understanding of their performance. Considering this deficiency, an empirical exploration was initiated [...] Read more.
Air knives are extensively employed in many cold rolling or tin plate production lines for drying purposes. Generally, these systems are oversized, resulting in excessive energy consumption, a consequence of insufficient understanding of their performance. Considering this deficiency, an empirical exploration was initiated to analyze the functionality of an air knife oriented perpendicularly to a given surface. Given the scarcity of information within the current body of literature, particular emphasis was placed on the regions affected by the finite dimensions of the device. Impingement pressure distributions were measured at the midspan plane and planes parallel to the midspan but extending beyond the projection of the air knife. The midspan impingement pressure profile aligned with the established bell-shaped distribution, whereas the outcomes beyond the air knife’s projection conformed to an analytically fitted similarity principle. Consequently, the mathematical formulations introduced in this study facilitate the mapping of the impingement pressure within the whole impingement plane, encompassing areas influenced by the finite length of the air knife, thereby representing the innovative contribution of this research. Full article
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23 pages, 10173 KB  
Article
Comparative Study of the Performance Characteristics of Annular Jet Pumps Conveying Newtonian and Shear-Thinning Non-Newtonian Fluids
by Tianle Li, Peng Wang, Wang Zheng, Donghua Lu, Xin Xia, Hanghui Zhou and Qiaorui Si
Fluids 2026, 11(5), 112; https://doi.org/10.3390/fluids11050112 - 30 Apr 2026
Viewed by 702
Abstract
This study investigates the factors influencing the performance characteristics of annular jet pumps (AJPs) conveying non-Newtonian fluids, to enhance their suction capability for marine organisms such as jellyfish, which exhibit properties close to non-Newtonian fluids. Based on the power-law fluid model, realizable k [...] Read more.
This study investigates the factors influencing the performance characteristics of annular jet pumps (AJPs) conveying non-Newtonian fluids, to enhance their suction capability for marine organisms such as jellyfish, which exhibit properties close to non-Newtonian fluids. Based on the power-law fluid model, realizable k-ε model, and volume of fluid (VOF) model, shear-thinning carboxymethyl cellulose (CMC) was selected to simulate marine organisms like jellyfish. Fluent software was employed to numerically simulate the performance characteristics and internal flow field of the annular jet pumps. The results demonstrate that the shear-thinning effect of non-Newtonian fluids reduces the maximum efficiency point of annular jet pumps and decreases the flow rate ratio corresponding to this efficiency point. As the concentration of CMC solution increased to 0.5%, the maximum efficiency point decreased by 5.5%, and the flow rate ratio corresponding to this efficiency point dropped from 1 to 0.8. These findings provide reference and insights for analyzing the full flow field of annular jet pumps pumping shear-thinning non-Newtonian fluids and for structural design of such pumps. Full article
(This article belongs to the Special Issue Advances in Computational Mechanics of Non-Newtonian Fluids, 2nd Edition)
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16 pages, 2434 KB  
Article
Developing Bingham Fluid Flow in the Entrance Region Between Parallel Plates
by Rachid Chebbi
Fluids 2026, 11(5), 111; https://doi.org/10.3390/fluids11050111 - 29 Apr 2026
Viewed by 713
Abstract
Bingham fluids, also called Bingham plastics, are used in different industries including the production of food, pharmaceuticals, household products, construction and oil and gas drilling. The behavior of Bingham fluids is viscous above a critical shear stress and rigid-body below the threshold stress [...] Read more.
Bingham fluids, also called Bingham plastics, are used in different industries including the production of food, pharmaceuticals, household products, construction and oil and gas drilling. The behavior of Bingham fluids is viscous above a critical shear stress and rigid-body below the threshold stress value. Knowledge of the size of the entrance region has several applications including hemodynamics and microfluidics. A model for steady Bingham fluid flow in the entrance region between parallel plates is developed using the inlet-filled region concept. A boundary layer model is used to solve the fluid flow dynamics in the inlet region up to the point where the critical shear stress is reached at the edge of the boundary layer. Beyond that point, the boundary layer does not grow, while the velocity profile keeps readjusting in the filled region to asymptotically reach the fully developed flow. The results include boundary layer thickness profiles, dimensionless pressure drop, centerline velocity, friction factor and inlet and entrance region sizes as functions of the Bingham number. The results are validated against the results for the Newtonian fluid case (Bingham fluid yield stress equal to zero) and CFD results, using the finite element method, for nonzero Bingham numbers. In addition, the results are found to asymptotically reach the fully developed flow values for the general Bingham fluid flow case. The effects of the Bingham number are addressed and compared with the literature. The present model is largely analytical, requiring minor numerical tasks. Full article
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28 pages, 2650 KB  
Article
Frequency Variations of Two-Mode Interference Patterns Due to Internal Soliton Waves in Shallow Water
by Matthias Ehrhardt, Sergey Pereselkov, Venedikt Kuz’kin, Sergey Tkachenko and Alexey Pereselkov
Fluids 2026, 11(5), 110; https://doi.org/10.3390/fluids11050110 - 29 Apr 2026
Cited by 1 | Viewed by 413
Abstract
This paper presents a theoretical analysis of frequency shifts in broadband acoustic field interference structures caused by an internal soliton wave in shallow water. It analyzes the spectral signature of interference-maxima frequency shifts within a coupled-mode framework that describes the scattering of acoustic [...] Read more.
This paper presents a theoretical analysis of frequency shifts in broadband acoustic field interference structures caused by an internal soliton wave in shallow water. It analyzes the spectral signature of interference-maxima frequency shifts within a coupled-mode framework that describes the scattering of acoustic normal modes under soliton-induced perturbations. Using the weak coupling approximation, analytical expressions are obtained for modal phase variations and the spectral peak frequency associated with the temporal evolution of frequency shifts induced by internal soliton waves. The analytical estimates obtained in the weak coupling approximation are extensively validated using numerical simulations under realistic ocean conditions without invoking it. This paper’s theoretical analysis demonstrates that internal soliton wave-induced mode coupling produces frequency shift spectrum signatures that strongly depend on soliton parameters. These results suggest that it is potentially feasible to estimate key soliton parameters, such as propagation direction, velocity, and effective amplitude, from measured frequency shifts. Numerical simulations demonstrate the feasibility of solving this inverse problem. These findings highlight the potential of frequency shift analysis as a practical, robust tool for remote sensing of internal wave dynamics in ocean acoustics. Full article
(This article belongs to the Section Geophysical and Environmental Fluid Mechanics)
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31 pages, 2570 KB  
Article
Statistical Analysis of Velocity Skewness and Kurtosis Under Adverse Pressure Gradients in Turbulent Boundary Layers
by Omid Farghadani, Abdolamir Bak Khoshnevis and Morteza Bayareh
Fluids 2026, 11(5), 109; https://doi.org/10.3390/fluids11050109 - 29 Apr 2026
Viewed by 751
Abstract
Skewness (S) and kurtosis (K) are statistical measures that provide insights into the characteristics of turbulence. This paper investigates the effects of adverse pressure gradients (APG) on S and K for mean and fluctuating velocities in the turbulent boundary layer (TBL), using the [...] Read more.
Skewness (S) and kurtosis (K) are statistical measures that provide insights into the characteristics of turbulence. This paper investigates the effects of adverse pressure gradients (APG) on S and K for mean and fluctuating velocities in the turbulent boundary layer (TBL), using the probability distribution function (PDF) and cumulative distribution function (CDF). The velocity distributions in the TBL are obtained experimentally. The experiments are conducted at Re ~ 1.12 × 105. According to the Clauser criterion, the APG parameter is β = 0.62. Two test sections are examined: a straight duct (zero pressure gradient) and a straight diffuser with a divergence angle of 6° and a cross-sectional area ratio of 1:4. Measurements are performed at five streamwise stations (x/c = 1, 1.5, 2, 3, and 4, where c = 100 mm). The results show that the APG does not influence the maximum or minimum values of the PDFs for mean and fluctuating velocities. Compared to the third and fourth moments, variations in the first and second moments are minimal. It is found that S values for the straight duct are lower than those for the straight diffuser. The largest difference is observed in the fourth moment of the PDF, i.e., K. Additionally, four PDF curve-fitting equations are presented for the mean velocity and velocity fluctuations in the TBL for both the straight duct and the straight diffuser. Differential entropy analysis indicates that the decrease in entropy resulting from wall shear and the turbulent boundary layer in the straight channel is more pronounced than the reduction in mean velocity entropy caused by the APG in the diffuser channel. Full article
(This article belongs to the Section Turbulence)
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17 pages, 1422 KB  
Article
Cation-Dependent Role of Water on the Dynamics and Ionic Conductivity of Levulinate-Based Ionic Liquids
by Georgios Tsonos, Sotiria Kripotou, Georgios Mavroeidis, Christos Tsonos, Lorenzo Guazzelli, Luca Guglielmero, Ilias Stavrakas and Konstantinos Moutzouris
Fluids 2026, 11(5), 108; https://doi.org/10.3390/fluids11050108 - 27 Apr 2026
Viewed by 1086
Abstract
The effect of water on the dynamics and ionic conductivity of the ionic liquids 1-ethyl-1-methylpyrrolidinium levulinate ([C2C1Pyr]Lev) and 1-butyl-1-methylpyrrolidinium levulinate ([C4C1Pyr]Lev) was investigated using differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) over a [...] Read more.
The effect of water on the dynamics and ionic conductivity of the ionic liquids 1-ethyl-1-methylpyrrolidinium levulinate ([C2C1Pyr]Lev) and 1-butyl-1-methylpyrrolidinium levulinate ([C4C1Pyr]Lev) was investigated using differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) over a wide temperature range. Although both ILs share the same levulinate anion, water induces markedly different dynamical responses depending on cation structure. In both systems, water acts as a plasticizer, lowering the glass transition temperature; however, the extent of plasticization and the resulting relaxation dynamics are cation-dependent. Stronger water–cation interactions are observed in [C2C1Pyr]Lev, whereas in [C4C1Pyr]Lev, water primarily disrupts alkyl-chain packing, enhancing ionic mobility. Increasing hydration shifts the main relaxation to higher frequencies and increases liquid fragility, while translational ionic motion remains decoupled from structural relaxation. These results demonstrate that water plays a cation-specific and mechanistically distinct role in levulinate-based ILs, providing new insights into hydration-controlled glassy dynamics and charge transport relevant to the design of IL-based electrolytes under non-anhydrous conditions. Full article
(This article belongs to the Section Heat and Mass Transfer)
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19 pages, 4540 KB  
Article
The Development of a Data-Driven Surrogate Model for Enhancing Electric Vehicle Cabin Airflow Analysis
by Mirza Popovac, Thomas Bäuml, Dominik Dvorak and Dragan Šimić
Fluids 2026, 11(5), 107; https://doi.org/10.3390/fluids11050107 - 25 Apr 2026
Cited by 1 | Viewed by 970
Abstract
This paper presents a data-driven surrogate model for predicting cabin airflow and its integration into system-level electric vehicle simulations for energy management analysis. The model employs a graph-based neural network with a mirror-symmetric predictor–corrector architecture and is trained on a dataset generated using [...] Read more.
This paper presents a data-driven surrogate model for predicting cabin airflow and its integration into system-level electric vehicle simulations for energy management analysis. The model employs a graph-based neural network with a mirror-symmetric predictor–corrector architecture and is trained on a dataset generated using computational fluid dynamics (CFD) covering a defined range of inlet velocities and temperatures. The surrogate appropriately reconstructs temperature fields and captures the dominant airflow structures at significantly lower computational cost than CFD. Quantitative evaluation shows high accuracy in passenger-relevant regions, while localized discrepancies remain confined mainly to shear-layer zones. The model enables near-real-time inference and is coupled with a system-level modeling framework for control-oriented simulations that are impractical with CFD. The study is tailored to a specific geometry and operating range, showing that targeted training strategies and physics-based extensions improve robustness, particularly under limited data conditions. Full article
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31 pages, 2040 KB  
Review
Active Flow Control Techniques: Classification, Analysis, and Future Trends for Automotive Applications
by Marco Robert Herberg, Stefano De Pinto, Marco Donato de Tullio and Giuseppe Pascazio
Fluids 2026, 11(5), 106; https://doi.org/10.3390/fluids11050106 - 23 Apr 2026
Cited by 1 | Viewed by 1053
Abstract
Active flow control represents a key enabling technology for advancing aerodynamic performance, offering significant potential improvements in drag reduction, lift enhancement, and overall efficiency. This paper reviews state-of-the-art active flow control techniques originally developed for aerospace applications and evaluates their applicability to automotive [...] Read more.
Active flow control represents a key enabling technology for advancing aerodynamic performance, offering significant potential improvements in drag reduction, lift enhancement, and overall efficiency. This paper reviews state-of-the-art active flow control techniques originally developed for aerospace applications and evaluates their applicability to automotive systems, considering constraints such as packaging, efficiency, cost, and integration. A structured classification of fluidic, surface-based (including morphing), and plasma-based approaches is presented, followed by a comparative and decision-oriented assessment of their performance, technological maturity, and feasibility. The results indicate that synthetic jet actuators and morphing-based solutions provide the most balanced compromise between aerodynamic effectiveness and practical implementation. In contrast, conventional fluidic methods are limited by low system efficiency, while plasma-based techniques, although highly responsive, face challenges related to scalability and integration. 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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13 pages, 3028 KB  
Article
A Neural Network Approach for the Simulation of Real Fluid Two-Phase Combustion Using a Multi-Species (H2/O2) Mechanism
by Bruno Delhom, Chaouki Habchi, Olivier Colin and Julien Bohbot
Fluids 2026, 11(5), 105; https://doi.org/10.3390/fluids11050105 - 22 Apr 2026
Viewed by 846
Abstract
Fully compressible two-phase flow configurations present many challenges for numerical modelling, requiring the development of Real Fluid Models (RFMs) able to simulate flows in subcritical, transcritical and supercritical regimes. Such an RFM has been recently developed at IFPEN based on physical properties lookup [...] Read more.
Fully compressible two-phase flow configurations present many challenges for numerical modelling, requiring the development of Real Fluid Models (RFMs) able to simulate flows in subcritical, transcritical and supercritical regimes. Such an RFM has been recently developed at IFPEN based on physical properties lookup tables, mainly for binary and ternary chemical systems. This paper proposes an Artificial Neural Network (ANN) approach to overcome the limitations of lookup tables of thermodynamic properties and to apply RFM to multi-species combustion. A methodology for generating an optimized data set by combining a vapor–liquid equilibrium (VLE) thermodynamic solver and the in situ adaptive tabulation (ISAT) method is developed. It aims to improve the neural network training process for two-phase combustion simulations where many species are present. This ANN methodology has been implemented in the CONVERGE CFD solver and validated using a mixing layer (LOX/GH2) benchmark from the literature relevant to rocket conditions, and an academic gaseous (H2/O2) case relevant to hydrogen combustion. The results show that this ANN approach makes H2 combustion simulation possible when coupled to the RFM framework and using a 10-species kinetic mechanism. Full article
(This article belongs to the Special Issue Machine Learning and Artificial Intelligence in Fluid Mechanics, 2nd Edition)
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