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Volume 10
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Fluids, Volume 10, Issue 2 (February 2025) – 32 articles

Cover Story (view full-size image): The 3D Lattice Boltzmann method with a deformable boundary (LBM-DB) is extended for the parallel computations of the full-volume colonic flow of the Newtonian fluid driven by the peristaltic segmented circular contractions which obey the “intestinal law'' ((i) deflation, (ii) inflation, and (iii) elastic relaxation) without constraint of fluid volume conservation. The LBM-DB accurately prescribes a deforming surface on the regular grid through compact Dirichlet velocity schemes, and this method could contribute to a better understanding of the intestinal physiology and pathology through its straightforward extension to the non-Newtonian chyme rheology and anatomical geometry. In this work, the LBM-DB simulations in silico extend the experiments performed in vitro on the Dynamic Colon Model (DCM, 2020) to highly occlusive contractile waves. View this paper
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19 pages, 5644 KiB  
Article
Simulation of Transpiration Cooling with Phase Change Process in Porous Media
by Aroua Ghedira, Zied Lataoui, Adel M. Benselama, Yves Bertin and Abdelmajid Jemni
Fluids 2025, 10(2), 52; https://doi.org/10.3390/fluids10020052 - 19 Feb 2025
Cited by 1 | Viewed by 659
Abstract
Phase change modeling in porous media is among the important challenges in many essential engineering problems, including thermal management, energy conservation or recovery, and heat transfer. One particularly efficient method of dissipating heat in a porous material is transpiration cooling with phase change. [...] Read more.
Phase change modeling in porous media is among the important challenges in many essential engineering problems, including thermal management, energy conservation or recovery, and heat transfer. One particularly efficient method of dissipating heat in a porous material is transpiration cooling with phase change. It is one of the most innovative cooling methods available for removing excessive heat flux from engine components such as combustors or gas turbine blades. There is, however, a lack of in-depth understanding of the interconnected mechanisms involved in such an application. In this work, an innovative numerical solver built on the OpenFOAM environment is constructed in order to explore the phase change process in a porous medium. The volume-of-fluid method and the Lee phase change model are applied in this numerical approach. The effects of coolant flow mass rate, heat flux, and porosity of porous structure on temperature and saturation distribution are investigated and discussed. The effects of both the external heat flux and the coolant mass flow rate under fixed porosity are also studied. The phase change is then delayed in the porous matrix when the amount of the injected coolant is increased. It reduces the area of two-phase and vapor regions. Also, a considerable rise in the upper surface temperature is obtained when the input heat flux or the porosity is separately enhanced. Full article
(This article belongs to the Special Issue Numerical Modeling and Experimental Studies of Two-Phase Flows, 2nd Edition)
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26 pages, 6212 KiB  
Article
The Photocatalytic Degradation of Benzo[a]pyrene in Aqueous Solution by Zinc Oxide Utilizing an Annular Reactor with an Immobilized Catalyst: A CFD Analysis
by Hana Ritchel Abracia, Mary Lesley Berina and Joseph Albert Mendoza
Fluids 2025, 10(2), 51; https://doi.org/10.3390/fluids10020051 - 14 Feb 2025
Viewed by 1300
Abstract
Increasing pollution and public health concerns over persistent pollutants necessitate efficient methods like photocatalytic degradation. Despite its potential in air and water treatment, the scale-up of this technology is limited due to insufficient modeling studies. This research explores the photocatalytic degradation of benzo[a]pyrene [...] Read more.
Increasing pollution and public health concerns over persistent pollutants necessitate efficient methods like photocatalytic degradation. Despite its potential in air and water treatment, the scale-up of this technology is limited due to insufficient modeling studies. This research explores the photocatalytic degradation of benzo[a]pyrene (BaP) using immobilized zinc oxide (ZnO) photocatalysts in a 500 mm length annular reactor. The reactor has a 150 mm porous ZnO domain and a UV lamp. Process variables such as the BaP concentration, residence time, surface irradiance, and catalyst zone length were modeled using computational fluid dynamics (CFD). CFD simulations using a pseudo-first-order kinetic model revealed that optimizing these parameters significantly improved the degradation efficiency. The results revealed that optimizing these parameters enhanced the degradation efficiency by over thirteen times compared to the initial setup. The increased residence time, reduced BaP concentration, and improved surface irradiance allowed for more efficient pollutant breakdown, while a longer catalyst zone supported more complete reactions. However, challenges like the high recombination rates of electron–hole pairs and susceptibility to photo-corrosion persist for ZnO. Further studies are recommended to address these challenges. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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19 pages, 4885 KiB  
Article
Rheological Characterization of Magnetorheological Fluids for Brake Applications: An Experimental Procedure
by Guglielmo Peruzzi, Giovanni Imberti, Henrique de Carvalho Pinheiro, Lucia Tsantilis, Rajandrea Sethi and Ezio Santagata
Fluids 2025, 10(2), 50; https://doi.org/10.3390/fluids10020050 - 13 Feb 2025
Viewed by 508
Abstract
This work focused on the development of a complete laboratory procedure for the rheological characterization of magnetorheological fluids (MRFs) by combining information gained from the literature and practical work performed in the laboratory. The procedure developed involves all the experimental stages for a [...] Read more.
This work focused on the development of a complete laboratory procedure for the rheological characterization of magnetorheological fluids (MRFs) by combining information gained from the literature and practical work performed in the laboratory. The procedure developed involves all the experimental stages for a comprehensive analysis, starting from the sample preparation, choice of the optimal measuring system, definition of the test, and rheometric parameters for the execution of an accurate and reliable analysis. Magnetorheology is a critical approach for studying the field-dependent properties of MRFs; however, measurement errors, often stemming from inappropriate rheometric test parameters, pose significant challenges. Additionally, sedimentation, caused by the large density mismatch between magnetic particles and the carrier fluid, presents a major obstacle that can compromise the analysis. Extensive work was performed for selecting the test parameters as well as defining the most relevant type of analysis to conduct with the defined procedure for the definition of the essential properties of an MRF for braking applications. This work is essential for the development of accurate card material for simulation tools, paving the way for broader utilization of MRFs in cutting-edge technologies. Full article
(This article belongs to the Section Non-Newtonian and Complex Fluids)
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15 pages, 7060 KiB  
Article
Investigation of Microjet Engine Inlet Pressure Distortions at Angled Inflow Velocity Conditions
by Santiago Sanchez Villacreses, Jun Yao, Yufeng Yao and Budi Chandra
Fluids 2025, 10(2), 49; https://doi.org/10.3390/fluids10020049 - 13 Feb 2025
Viewed by 626
Abstract
The Armfield CM14 microjet axial flow turbine engine has been tested in open space at ambient conditions with engine inlet pressure at the aerodynamic interface plane (AIP) measured by a built-in pressure sensor for validating computational fluid dynamics (CFD) studies. A three-dimensional computational [...] Read more.
The Armfield CM14 microjet axial flow turbine engine has been tested in open space at ambient conditions with engine inlet pressure at the aerodynamic interface plane (AIP) measured by a built-in pressure sensor for validating computational fluid dynamics (CFD) studies. A three-dimensional computational domain of the test engine intake duct configuration is defined, followed by mesh convergence studies. The latter results in a fine mesh of 5.7 million cells on which CFD-predicted engine inlet pressures are in good agreement with the experimental measurements at the AIP face for 20–100% throttles. CFD studies are continued to investigate the engine inlet pressure distortions at two inflow velocities of 35 m/s and 70 m/s, and various inflow angles ranging from 0° to 30° with a step of 5°, to evaluate their impacts on engine inlet pressure distortions. It is found that pressure distortions increase with the inflow angle, with severe pressure distortions occurring at higher inflow angles above 15°. At the same flow conditions of inflow angle and velocity, pressure distortions from an intake with a flat lip are overall higher than those of a bell-mouth round lip. This is primarily due to a rapid geometry change at the intake entrance causing large vortical flow motions, accompanied by local flow separations at higher inflow angles, therefore impacting the downstream flow field towards the engine inlet. Full article
(This article belongs to the Special Issue 10th Anniversary of Fluids—Recent Advances in Fluid Mechanics)
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19 pages, 5752 KiB  
Article
Numerical Investigation of Flow and Heat Transfer from Twin Circular Cylinders Present in Double Forward-Facing Step
by Parthasarathy Rajesh Kanna, Yaswanth Sivakumar, G. V. Durga Prasad, Dawid Taler, Tomasz Sobota and Jan Taler
Fluids 2025, 10(2), 48; https://doi.org/10.3390/fluids10020048 - 12 Feb 2025
Viewed by 546
Abstract
A numerical simulation of the circular cylinder as an obstacle in a double forward-facing (DFFS) step was performed. The size and position of the upstream cylinder (c1) and downstream cylinder (c2) were varied to explore their role [...] Read more.
A numerical simulation of the circular cylinder as an obstacle in a double forward-facing (DFFS) step was performed. The size and position of the upstream cylinder (c1) and downstream cylinder (c2) were varied to explore their role in heat transfer in both laminar and turbulent conditions. Comparative results of the upper and lower half of the downstream cylinder were plotted as results to understand the heat transfer and flow characteristics around the downstream cylinder due to the effect of the upstream cylinder’s dimension and position. For Re = 800, when the c1 is placed near the bottom of the wall, it results in a pair of rear-side symmetrical vortices, and the c2 cylinder vortices become larger when the c1 is shifted towards the top wall. Additional flow separation happens adjacent to the steps when c1 is greater than c2. These vortices strongly influence the convection heat transfer from the step. However, when Reynolds number (Re) is increased from 800 to 80,000, these vortices’ size is decreased. When c1 moves from 0.375H to 0.75H, the average Nusselt number is increased significantly. Moreover, a hike in Re results in a higher average Nusselt number irrespective of the position of obstacles. The upstream cylinder significantly enhances the Nusselt number when it is placed near the top wall rather than the bottom wall. Full article
(This article belongs to the Section Heat and Mass Transfer)
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19 pages, 4754 KiB  
Article
Computational Analysis of Flow Separation in Non-Transferred Plasma Torch: Causes, Impacts and Control Methods
by Sai Likitha Siddanathi, Lars-Göran Westerberg, Hans O. Åkerstedt, Per Gren, Henrik Wiinikka and Alexey Sepman
Fluids 2025, 10(2), 47; https://doi.org/10.3390/fluids10020047 - 12 Feb 2025
Viewed by 555
Abstract
In a non-transferred plasma torch, the working gas becomes ionized and forms plasma as it interacts with the electric arc at the cathode tip. However, in certain cathode shapes, particularly flat ones, and under specific conditions, the gas flow can separate at the [...] Read more.
In a non-transferred plasma torch, the working gas becomes ionized and forms plasma as it interacts with the electric arc at the cathode tip. However, in certain cathode shapes, particularly flat ones, and under specific conditions, the gas flow can separate at the cathode tip, forming a vortex region. While this flow separation is influenced by geometric factors, it occurs in the critical zone where plasma is generated. Understanding the causes of this separation is essential, as it may significantly impact torch performance. If the separation proves detrimental, it is important to identify ways to mitigate it. This paper presents a computational analysis of a non-transferred plasma torch to investigate the physics behind flow separation. The results highlight the location and causes of the separation, as well as its potential advantages and disadvantages. Finally, the paper explores theoretical approaches to address flow separation in plasma torches, offering practical insights for enhancing their design and efficiency. Full article
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24 pages, 6073 KiB  
Article
Measurements of Wake Concentration from the Continuous Release of a Dense Fluid Upstream of a Cubic Obstacle
by Romana Akhter and Nigel B. Kaye
Fluids 2025, 10(2), 46; https://doi.org/10.3390/fluids10020046 - 11 Feb 2025
Viewed by 659
Abstract
Results are presented from a series of small-scale laboratory experiments designed to model dense gas dispersion around an isolated cuboid building. Experiments were conducted for a broad range of flow Richardson numbers and source discharge rates, and the concentration field in the wake [...] Read more.
Results are presented from a series of small-scale laboratory experiments designed to model dense gas dispersion around an isolated cuboid building. Experiments were conducted for a broad range of flow Richardson numbers and source discharge rates, and the concentration field in the wake of the building was measured using light-induced fluorescence (LIF). Results show that, for low Richardson numbers, the concentration of dense fluid in the wake decreases slightly with distance above the ground. However, for Richardson numbers above Ri≈3, the vertical variation is qualitatively different, as a dense lower layer forms in the wake and the concentration above the layer is much lower than for the lower Ri experiments. For these higher Richardson number flows, the primary mechanism by which dense fluid is flushed from the building wake is by the wake flow skimming dense fluid from the top of the lower layer and then moving it upstream toward the building’s leeward face. It is then transported up the leeward face of the building and then downstream. The results also generally show that, as the release rate of dense fluid increases, the density and thickness of the lower layer increases. The LIF measurements and a series of visualization experiments highlight the complex interaction of a dense fluid discharge with the wake structure behind a building. Full article
(This article belongs to the Special Issue 10th Anniversary of Fluids—Recent Advances in Fluid Mechanics)
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17 pages, 1103 KiB  
Article
Numerical Evaluation of the IMERSPEC Methodology and Spalart–Allmaras Turbulence Model in Fully Developed Channel Flow Simulations
by Laura Augusta Vasconcelos de Albuquerque, Mariana Fernandes dos Santos Villela and Felipe Pamplona Mariano
Fluids 2025, 10(2), 45; https://doi.org/10.3390/fluids10020045 - 11 Feb 2025
Viewed by 879
Abstract
This study evaluates the performance of the IMERSPEC methodology combined with the Spalart–Allmaras turbulence model for simulating fully developed turbulent flows in a plane channel. Turbulent flows, known for their complexity, require numerical methods that balance computational efficiency with accuracy. The IMERSPEC approach, [...] Read more.
This study evaluates the performance of the IMERSPEC methodology combined with the Spalart–Allmaras turbulence model for simulating fully developed turbulent flows in a plane channel. Turbulent flows, known for their complexity, require numerical methods that balance computational efficiency with accuracy. The IMERSPEC approach, recognized for its spectral accuracy and efficiency, was applied alongside the Spalart–Allmaras model, valued for its simplicity and robustness in representing turbulence, particularly in scenarios where flow over solid surfaces is critical. Simulations were conducted at Reynolds numbers (Reτ) of 180, 550, and 1000, with results validated against direct numerical simulation (DNS) data. The study investigated various grid resolutions, revealing that finer meshes substantially enhance accuracy by mitigating velocity profile oscillations and reducing the L2 error norm. Key findings highlight the method’s ability to accurately replicate turbulent flow characteristics, including velocity distributions and shear stress profiles, while maintaining a favorable computational cost-to-accuracy ratio. This work provides valuable insights into turbulence modeling, demonstrating the potential of the IMERSPEC methodology for practical engineering applications. Full article
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20 pages, 3734 KiB  
Article
Design and Pressure Pulsation Analysis of Pure Rolling External Helical Gear Pumps with Different Tooth Profiles
by Zhen Chen, Yingqi Li, Xiaoping Xiao, Chao He, Kai Zhu, Yangzhi Chen and Alfonso Fuentes-Aznar
Fluids 2025, 10(2), 44; https://doi.org/10.3390/fluids10020044 - 10 Feb 2025
Viewed by 499
Abstract
This paper investigates the design methodologies of pure rolling helical gear pumps with various tooth profiles, based on the active design of meshing lines. The transverse active tooth profile of a pure rolling helical gear end face is composed of various function curves [...] Read more.
This paper investigates the design methodologies of pure rolling helical gear pumps with various tooth profiles, based on the active design of meshing lines. The transverse active tooth profile of a pure rolling helical gear end face is composed of various function curves at key control points. The entire transverse tooth profile consists of the active tooth profile and the Hermite curve as the tooth root transition, seamlessly connecting at the designated control points. The tooth surface is created by sweeping the entire transverse tooth profile along the pure rolling contact curves. The fundamental design parameters, tooth profile equations, tooth surface equations, and a two-dimensional fluid model for pure rolling helical gears were established. The pressure pulsation characteristics of pure rolling helical gear pumps and CBB-40 involute spur gear pumps, each with different tooth profiles, were compared under specific working pressures. This comparison encompassed the maximum effective positive and negative pressures within the meshing region, pressure fluctuations at the midpoints of both inlet and outlet pressures, and pressure fluctuations at the rear sections of the inlet and outlet pressures. The results indicated that the proposed pure rolling helical gear pump with a parabolic tooth profile exhibited 42.81% lower effective positive pressure in the meshing region compared to the involute spur gear pump, while the maximum effective negative pressure was approximately 27 times smaller than that of the involute gear pump. Specifically, the pressure pulsations in the middle and rear regions of the inlet and outlet pressure zones were reduced by 33.1%, 6.33%, 57.27%, and 69.61%, respectively, compared to the involute spur gear pump. Full article
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43 pages, 35982 KiB  
Article
A Class of Finite Difference Schemes with Adaptive Controllable Dispersion and Low Dissipation for Compressible Turbulence
by Jianxin Hao and Qiang Wang
Fluids 2025, 10(2), 43; https://doi.org/10.3390/fluids10020043 - 10 Feb 2025
Viewed by 553
Abstract
The dispersion and dissipation properties of a numerical scheme are critical in simulating flow fields involving a wide range of length scales. In this study, we highlight the common oversight of focusing merely on controlling dispersion error without considering the importance of appropriate [...] Read more.
The dispersion and dissipation properties of a numerical scheme are critical in simulating flow fields involving a wide range of length scales. In this study, we highlight the common oversight of focusing merely on controlling dispersion error without considering the importance of appropriate dispersion and scalability in computational efficiency. This study demonstrates that adjusting dispersion to match the local flow field near discontinuities is more effective in suppressing oscillations than simply minimizing dispersion. This proposed high-order finite difference scheme with adaptive dispersion minimized dissipation (ADMD) achieves adaptive controllable dispersion near flow field discontinuities, known as the ADMD scheme. This scheme, derived as a fourth-order finite difference scheme with seven points based on Taylor expansion, comprises a basic central component, additional dissipation component, and dispersion component. By exploring the effect of dispersion on numerical oscillations and the importance of adjusting dispersion according to the local flow field, a discontinuity detection function was established to enable the dispersion properties to adapt to the local flow field. Drawing inspiration from flow field smoothing in the weighted essentially non-oscillatory (WENO) scheme, efforts were made to minimize scheme dissipation. The main benefits of the ADMD scheme over several WENO-type schemes are robustness and efficiency, as the ADMD scheme saves at least 40–90% CPU time compared to the same-order WENO-type schemes for some numerical examples. Additionally, the numerical scheme proves advantageous in terms of simulating the decaying isotropic turbulence problem of three-dimensional compressible turbulence. Full article
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23 pages, 1724 KiB  
Article
An Experimental Study on a Blockage Effect of Sting-Free AxialCircular Cylinders
by Hiroyuki Okuizumi, Yuki Wajima, Yasufumi Konishi, Hideo Sawada, Shigeru Obayashi and Naofumi Ohnishi
Fluids 2025, 10(2), 42; https://doi.org/10.3390/fluids10020042 - 9 Feb 2025
Viewed by 554
Abstract
The correction of the blockage effect is crucial in wind tunnel experiments, particularly for the drag forces on bluff bodies with a large flow separation at the leading edge. Maskell’s wake blockage correction is a widely used method for bluff bodies. However, the [...] Read more.
The correction of the blockage effect is crucial in wind tunnel experiments, particularly for the drag forces on bluff bodies with a large flow separation at the leading edge. Maskell’s wake blockage correction is a widely used method for bluff bodies. However, the correction method is limited to the separated flow that does not reattach. In this study, the blockage effect and correction methods were investigated due to the presence or absence of the leading-edge-separated shear layer reattachment in a flow field with non-support interference produced using a magnetic suspension and balance system (MSBS). The drag coefficient without a wall constraint (CDL) was evaluated by varying the fineness ratio in the models with L/D=1.0 and 2.0. For L/D=1.0, where the leading-edge-separated shear layer does not reattach, the solid blockage correction following the wake blockage correction reduced the difference to within ±0.01. However, as S/C changed, the corrected drag coefficient was not constant. Further investigation is needed to evaluate the possibility of overcorrection. For L/D=2.0, the reattachment occurs; the wake blockage correction alone is insufficient; therefore, a solid blockage correction must apply along with the wake blockage correction. It leads the difference to become less than ±0.02 compared to CDL. The results of this study suggest that both wake and solid blockage corrections are effective regardless of the presence or absence of the reattachment of the leading-edge-separated shear layer on the cylinder. Full article
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24 pages, 5992 KiB  
Review
The Impact of Polydimethylsiloxane (PDMS) in Engineering: Recent Advances and Applications
by Rui A. Lima
Fluids 2025, 10(2), 41; https://doi.org/10.3390/fluids10020041 - 9 Feb 2025
Cited by 1 | Viewed by 1937
Abstract
Since the introduction of polydimethylsiloxane (PDMS) microfluidic devices at the beginning of the 21st century, this elastomeric polymer has gained significant attention in the engineering community due to its biocompatibility, exceptional mechanical and optical properties, thermal stability, and versatility. PDMS has been widely [...] Read more.
Since the introduction of polydimethylsiloxane (PDMS) microfluidic devices at the beginning of the 21st century, this elastomeric polymer has gained significant attention in the engineering community due to its biocompatibility, exceptional mechanical and optical properties, thermal stability, and versatility. PDMS has been widely used for in vitro experiments ranging from the macro- to nanoscale, enabling advances in blood flow studies, biomodels improvement, and numerical validations. PDMS devices, including microfluidic systems, have been employed to investigate different kinds of fluids and flow phenomena such as in vitro blood flow, blood analogues, the deformation of individual cells and the cell free layer (CFL). The most recent applications of PDMS involve complex hemodynamic studies such as flow in aneurysms and in organ-on-a-chip (OoC) platforms. Furthermore, the distinctive properties of PDMS, including optical transparency, thermal stability, and versality have inspired innovative applications beyond biomedical applications, such as the development of transparent, virus-protective face masks, including those for SARS-CoV-2 and serpentine heat exchangers to enhance heat transfer and energy efficiency in different kinds of thermal systems. This review provides a comprehensive overview of the current research performed with PDMS and outlines some future directions, in particular applications of PDMS in engineering, including biomicrofluidics, in vitro biomodels, heat transfer, and face masks. Additionally, challenges related to PDMS hydrophobicity, molecule absorption, and long-term stability are discussed alongside the solutions proposed in the most recent research studies. Full article
(This article belongs to the Special Issue Physics and Applications of Microfluidics)
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21 pages, 7912 KiB  
Article
Visualization and Parameters Determination of Supersonic Flows in Convergent-Divergent Micro-Nozzles Using Schlieren Z-Type Technique and Fluid Mechanics
by Reyna Judith Mendoza-Anchondo, Cornelio Alvarez-Herrera and José Guadalupe Murillo-Ramírez
Fluids 2025, 10(2), 40; https://doi.org/10.3390/fluids10020040 - 3 Feb 2025
Viewed by 2850
Abstract
Small-scale and supersonic convergent-divergent type micro-nozzles with characteristic sizes of around a few centimeters and exit and throat radii of tenths of millimeters were the subjects of this study. Using the schlieren Z-type optical technique, the supersonic airflows established at the exit of [...] Read more.
Small-scale and supersonic convergent-divergent type micro-nozzles with characteristic sizes of around a few centimeters and exit and throat radii of tenths of millimeters were the subjects of this study. Using the schlieren Z-type optical technique, the supersonic airflows established at the exit of seven nozzles were visualized. The dependence of the shock cell characteristics on the nozzle pressure ratio (NPR), defined as the ratio of stagnation pressure to atmospheric pressure, was analyzed. The dependence of the nozzle thrust and the specific impulse on the NPR ratio and the mass flow rate was also studied using a simple device based on concepts of fluid mechanics. The results obtained are in agreement with similar results obtained in recently published research on double-bell nozzles. The thrust of all nozzles depends linearly on the shock-cell spacing, which is one of the most relevant findings of this research. In other words, the output airflow structure determines the performance of the nozzles, such as the thrust or the specific impulse they produce. These small nozzles offer significant advantages over conventional nozzles in low energy consumption and lower manufacturing cost, making them suitable for scientific research in space micro-propulsion and cooling microelectronic systems, among other applications. Full article
(This article belongs to the Special Issue Flow Control Techniques: Advances in Flow System Analysis, Modeling and Applications)
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25 pages, 4476 KiB  
Article
A Machine Learning Approach to Volume Tracking in Multiphase Flow Simulations
by Aaron Mak and Mehdi Raessi
Fluids 2025, 10(2), 39; https://doi.org/10.3390/fluids10020039 - 2 Feb 2025
Viewed by 693
Abstract
This work presents a machine learning (ML) approach to volume-tracking for computational simulations of multiphase flow. It is an alternative to a commonly used procedure in the volume-of-fluid (VOF) method for volume tracking, in which the phase interfaces are reconstructed for flux calculation [...] Read more.
This work presents a machine learning (ML) approach to volume-tracking for computational simulations of multiphase flow. It is an alternative to a commonly used procedure in the volume-of-fluid (VOF) method for volume tracking, in which the phase interfaces are reconstructed for flux calculation followed by volume advection. Bypassing the computationally expensive steps of interface reconstruction and flux calculation, the proposed ML approach performs volume advection in a single step, directly predicting the volume fractions at the next time step. The proposed ML function is two-dimensional and has eleven inputs. It was trained using MATLAB’s (R2021a) Deep Learning Toolbox with a grid search method to find an optimal neural network configuration. The performance of the ML function is assessed using canonical test cases, including translation, rotation, and vortex tests. The errors in the volume fraction fields obtained by the ML function are compared with those of the VOF method. In ideal conditions, the ML function speeds up the computations four times compared to the VOF method. However, in terms of overall robustness and accuracy, the VOF method remains superior. This study demonstrates the potential of applying ML methods to multiphase flow simulations while highlighting areas for further improvement. Full article
(This article belongs to the Special Issue Advances in Multiphase Flow Simulation with Machine Learning)
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17 pages, 5866 KiB  
Article
An Application of Upwind Difference Scheme with Preconditioned Numerical Fluxes to Gas-Liquid Two-Phase Flows
by Tianmu Zhao and Byeongrog Shin
Fluids 2025, 10(2), 38; https://doi.org/10.3390/fluids10020038 - 1 Feb 2025
Cited by 1 | Viewed by 598
Abstract
A time-consistent upwind difference scheme with a preconditioned numerical flux for unsteady gas-liquid multiphase flows is presented and applied to the analysis of cavitating flows. The fundamental equations were formulated in general curvilinear coordinates to apply to diverse flow fields. The preconditioning technique [...] Read more.
A time-consistent upwind difference scheme with a preconditioned numerical flux for unsteady gas-liquid multiphase flows is presented and applied to the analysis of cavitating flows. The fundamental equations were formulated in general curvilinear coordinates to apply to diverse flow fields. The preconditioning technique was applied specifically to the numerical dissipation terms in the upwinding process without changing the time derivative terms to maintain time consistency. This approach enhances numerical stability in unsteady multiphase flow computations, consistently delivering time-accurate solutions compared to conventional preconditioning methods. A homogeneous gas-liquid two-phase flow model, third-order Runge-Kutta method, and the flux difference splitting upwind scheme coupled with a third-order MUSCL TVD scheme were employed. Numerical tests of two-dimensional gas-liquid single- and two-phase flows over backward-facing step with different step height and flow conditions successfully demonstrated the capability of the present scheme. The calculations remained stable even for flows with a very low Mach number of 0.001, typically considered incompressible flows, and the results were in good agreement with the experimental data. In addition, we analyzed unsteady cavitating flows at high Reynolds numbers and confirmed the effectiveness and applicability of the present scheme for calculating unsteady gas-liquid two-phase flows. Full article
(This article belongs to the Special Issue Advances in Multiphase Flow Science and Technology, 2nd Edition)
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19 pages, 5127 KiB  
Article
Towards Efficient Bio-Methanation: A Comparative Analysis of Disperser Designs and Process Optimization in Bubble Columns
by Florian Klapal and Mark Werner Hlawitschka
Fluids 2025, 10(2), 37; https://doi.org/10.3390/fluids10020037 - 31 Jan 2025
Viewed by 620
Abstract
This study aims to contribute to the optimization of bio-methanation in bubble columns, making it a more viable alternative to stirred tank reactors. The primary challenge to be addressed is the enhancement of mass transfer, which strongly depends on parameters such as bubble [...] Read more.
This study aims to contribute to the optimization of bio-methanation in bubble columns, making it a more viable alternative to stirred tank reactors. The primary challenge to be addressed is the enhancement of mass transfer, which strongly depends on parameters such as bubble size and gas hold-up. Various disperser designs were examined in a 0.14 mm diameter column, comparing their performance in terms of bubble diameter distribution and gas hold-up. The results indicate that an optimized plate disperser featuring a porous structure outperformed other designs by maintaining high gas retention without significant coalescence. Additionally, newly developed plug-in dispersers allowed for counter-current flow operation, enhancing process flexibility. Commercially available porous pin dispersers produced smaller bubbles compared to the other designs, yielding high gas hold-ups at lower gas velocities. Correlations between disperser type and column design parameters were established, laying the foundation for apparatus optimization. The findings contribute to the development of digital twin models, facilitating the refinement of bio-methanation processes within bubble columns for increased efficiency. Full article
(This article belongs to the Special Issue Mass Transfer in Multiphase Reactors)
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10 pages, 4801 KiB  
Article
Hydrological Response of Land Use and Climate Change Impact on Sediment Rate in Upper Citarum Watershed
by Evi Anggraheni, Abdul Halim Hamdany, Farouk Maricar, Neil Andika, Dian Sisinggih, Fransiskus Sean Tanlie and Fransiskus Adinda Rio Respati
Fluids 2025, 10(2), 36; https://doi.org/10.3390/fluids10020036 - 31 Jan 2025
Viewed by 622
Abstract
The Citarum Watershed is indeed a critical water resource in Indonesia, playing a significant role in providing water to Jakarta and other areas in West Java. However, it faces severe environmental stress due to land use changes and climate changes. The Upper Citarum [...] Read more.
The Citarum Watershed is indeed a critical water resource in Indonesia, playing a significant role in providing water to Jakarta and other areas in West Java. However, it faces severe environmental stress due to land use changes and climate changes. The Upper Citarum Watershed, considered to be a conservation area, has experienced rapid development due to human activities and economic growth. Climate change not only affects the rainfall value but also the rainfall patterns and sediment flow. The sedimentation process significantly impacts the soil characteristics around the river body. Several factors such as topography, flow velocity, and soil texture influence the sediment characteristics. Given the critical condition of climate and land use change, this study aims to analyse the impacts of the hydrological response of land use and climate change on the sediment rate in the Upper Citarum Watershed. The land use change analysis was conducted by comparing the land use data in 2000, 2010, and 2023 in the Upper Citarum Watershed. The deposition process of solid particles such as sand, silt, and gravel that are transported in the Upper Citarum River were examined in a soil investigation. The sediment rate and deposition by river flow were analysed using HEC-RAS quasi-unsteady flow. The impact of climate change in this study was assessed by simulating the discharge in three conditions, with the first simulation using the discharge from 2000 to 2010, the second simulation using the discharge from 2011 to 2023, and the last simulation using the discharge from 2000 to 2023. Due to the land use change, the developed area increased from 4% to 24% between 2000 until 2023. The magnitude of low flow during the simulation step for three discharge gauges (Majalaya, Dayeuhkolot, and Nanjung) decreased up to 48%, but, on other hand, the sediment rate increased by 20% in Dayeuhkolot. Full article
(This article belongs to the Special Issue Environmental Hydraulics, Turbulence and Sediment Transport, 3rd Edition)
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16 pages, 14747 KiB  
Article
Analysis of Flow Past a Double-Slanted Ahmed Body
by Matthew Aultman and Lian Duan
Fluids 2025, 10(2), 35; https://doi.org/10.3390/fluids10020035 - 31 Jan 2025
Viewed by 939
Abstract
For this study, Improved Delayed Detached-Eddy Simulations (IDDES) were used to analyze the wake of a modified Ahmed body with varying upper and lower slants. The modified geometry produced a constant projected vertical base area, ensuring that the base and slant drag were [...] Read more.
For this study, Improved Delayed Detached-Eddy Simulations (IDDES) were used to analyze the wake of a modified Ahmed body with varying upper and lower slants. The modified geometry produced a constant projected vertical base area, ensuring that the base and slant drag were a function of the pressure caused by the wake structures. Except at extreme slant angles, the general structures of the wake were a base torus with two pairs of streamwise-oriented vortices on each slant. These structures strongly correlated with the drag contribution of the rear surfaces: the torus with the vertical base and the streamwise-oriented vortices with the slants. As such, the base drag was minimized when the torus was most centrally aligned with the base, producing the largest stagnation region. Two slant-drag minima developed corresponding to two regimes of vortical flow on opposing slants. On one slant, the vortices were attached, and the drag correlated with the size and strength of the vortices. On the other slant, the vortices separated, and the drag correlated with the slant normal due to a more uniform pressure. This demonstrates a rich and complex set of interactions that must be managed in the development of base drag caused by wake flows. Full article
(This article belongs to the Special Issue Aerodynamics and Aeroacoustics of Vehicles, 4th Edition)
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17 pages, 2011 KiB  
Article
AI-Driven Optimization of Breakwater Design: Predicting Wave Reflection and Structural Dimensions
by Mohammed Loukili, Soufiane El Moumni and Kamila Kotrasova
Fluids 2025, 10(2), 34; https://doi.org/10.3390/fluids10020034 - 30 Jan 2025
Viewed by 817
Abstract
Coastal defense structures play a crucial role in mitigating wave impacts; yet, existing breakwater designs often face challenges in balancing wave reflection, energy dissipation, and structural stability. This study leverages machine learning (ML) to predict the optimal 2D dimensions of rectangular breakwaters in [...] Read more.
Coastal defense structures play a crucial role in mitigating wave impacts; yet, existing breakwater designs often face challenges in balancing wave reflection, energy dissipation, and structural stability. This study leverages machine learning (ML) to predict the optimal 2D dimensions of rectangular breakwaters in two configurations: submerged at the bottom of a wave tank and positioned at the free surface. Further, the objective is to achieve controlled wave reflection allowing a specific wave run-up and optimized energy dissipation, while ensuring maritime stability. Thus, we used an analytical equation modeling the reflection coefficient versus relative water depth (KH), for different immersion ratios of obstacle (h/H), and relative length (l/H). Two datasets of 32,000 data points were generated for underwater and free-surface breakwaters, with an additional 10,000 data points for validation, totaling 42,000 data points per case. Five ML algorithms—Random Forest, Support Vector Regression, Artificial Neural Network, Decision Tree, and Gaussian Process—were applied and evaluated. Results demonstrated that Random Forest and Decision Tree balanced accuracy with computational efficiency, while the Gaussian Process closely matched analytical results but demanded higher computational resources. These findings support ML as a powerful tool to optimize breakwater design, complementing traditional methods and contributing to more sustainable and resilient coastal defense systems. Full article
(This article belongs to the Special Issue Machine Learning and Artificial Intelligence in Fluid Mechanics)
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24 pages, 12569 KiB  
Article
Flow of Fluids with Pressure-Dependent Viscosity in Between Intersecting Planes
by Rhameez S. Herbst, Charis Harley and Kumbakonam R. Rajagopal
Fluids 2025, 10(2), 33; https://doi.org/10.3390/fluids10020033 - 30 Jan 2025
Viewed by 617
Abstract
The flow of an incompressible power-law fluid through a convergent channel is considered, where the viscosity is chosen to be pressure dependent. Instead of utilizing the classical similarity transformation traditionally employed when considering Jeffery-Hamel flow, allowing for purely radial solutions for the velocity [...] Read more.
The flow of an incompressible power-law fluid through a convergent channel is considered, where the viscosity is chosen to be pressure dependent. Instead of utilizing the classical similarity transformation traditionally employed when considering Jeffery-Hamel flow, allowing for purely radial solutions for the velocity field, we allow for flow in both the radial and angular directions. We develop a numerical scheme that conserves the pressure-dependent viscosity at each cell in the computational grid. We recover the classical solution to the problem, and through our numerical solutions, we observe not only that the tangential velocities are not negligible, but also that flow reversal occurs, as illustrated by solutions with varying flow regimes. Decreasing the angle of the channel causes the magnitude of the velocity to decrease, while shorter channels lead to an increase in the magnitude of the radial and tangential velocities. In the case of the latter, this could indicate that in shorter channels, the tangential velocity has a larger impact on the occurrence of flow reversal. For more varied flow regimes, the magnitude of the radial and tangential velocities increases. Full article
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19 pages, 1043 KiB  
Article
Sustaining Vaccine Potency in Cold Chain Logistics: Numerical Analysis of Extended Cooling Duration in Glycerol-Infused n-Tetradecane Phase-Change Materials
by Tapasvi Bhatt, Naman Jain and Eddie Yin Kwee Ng
Fluids 2025, 10(2), 32; https://doi.org/10.3390/fluids10020032 - 29 Jan 2025
Viewed by 930
Abstract
Vaccination cold chains depend critically on maintaining temperatures within the 2–8 °C range, with phase-change materials (PCMs) like n-tetradecane offering substantial potential due to their high latent heat and optimal melting characteristics. Despite extensive research on PCM melting enhancement, strategies to extend [...] Read more.
Vaccination cold chains depend critically on maintaining temperatures within the 2–8 °C range, with phase-change materials (PCMs) like n-tetradecane offering substantial potential due to their high latent heat and optimal melting characteristics. Despite extensive research on PCM melting enhancement, strategies to extend melting duration and thermal stability remain underexplored. This pioneering numerical study investigates the impact of incorporating 5% glycerol additive in n-tetradecane, aiming to decelerate the melting rate and sustain the desired temperature range over prolonged periods. This study numerically assesses the effect of a 5% glycerol additive on n-tetradecane, revealing a substantial 20.6 h extension in safe temperature maintenance, from 123.3 h in pure n-tetradecane (T) to 143.9 h with the additive (T + G). Although T reaches full melting in 121.7 h, the air temperature inside the cold box breaches 8 °C only 1.6 h after; in contrast, T + G reaches this threshold 2.2 h before full melting, resulting in an effective extension of 20.6 h. Entropy analysis shows a delayed rise in T + G, indicating enhanced thermal stability, while temperature contours confirm T + G sustains cooling until day 6, a full day beyond T. These findings highlight glycerol’s potential to modulate thermal dynamics within PCM-based cold boxes, offering a cost-effective improvement in vaccine transport sustainability. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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41 pages, 26974 KiB  
Article
Spurious Aeroacoustic Emissions in Lattice Boltzmann Simulations on Non-Uniform Grids
by Alexander Schukmann, Viktor Haas and Andreas Schneider
Fluids 2025, 10(2), 31; https://doi.org/10.3390/fluids10020031 - 28 Jan 2025
Cited by 1 | Viewed by 858
Abstract
Although there do exist a few aeroacoustic studies on harmful artificial phenomena related to the usage of non-uniform Cartesian grids in lattice Boltzmann methods (LBM), a thorough quantitative comparison between different categories of grid arrangement is still missing in the literature. In this [...] Read more.
Although there do exist a few aeroacoustic studies on harmful artificial phenomena related to the usage of non-uniform Cartesian grids in lattice Boltzmann methods (LBM), a thorough quantitative comparison between different categories of grid arrangement is still missing in the literature. In this paper, several established schemes for hierarchical grid refinement in lattice Boltzmann simulations are analyzed with respect to spurious aeroacoustic emissions using a weakly compressible model based on a D3Q19 athermal velocity set. In order to distinguish between various sources of spurious phenomena, we deploy both the classical Bhatnagar–Gross–Krook and other more recent collision models like the hybrid recursive-regularization operator, the latter of which is able to filter out detrimental non-hydrodynamic mode contributions, inherently present in the LBM dynamics. We show by means of various benchmark simulations that a cell-centered approach, either with a linear or uniform explosion procedure, as well as a vertex-centered direct-coupling method, proves to be the most suitable with regards to aeroacoustics, as they produce the least amount of spurious noise. Furthermore, it is demonstrated how simple modifications in the selection of distribution functions to be reconstructed during the communication step between fine and coarse grids affect spurious aeroacoustic artifacts in vertex-centered schemes and can thus be leveraged to positively influence stability and accuracy. Full article
(This article belongs to the Special Issue Lattice Boltzmann Methods: Fundamentals and Applications)
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20 pages, 2071 KiB  
Article
Instability of a Film Falling Down a Bounded Plate and Its Application to Structured Packing
by Giulio Croce and Nicola Suzzi
Fluids 2025, 10(2), 30; https://doi.org/10.3390/fluids10020030 - 27 Jan 2025
Cited by 1 | Viewed by 560
Abstract
The instability of a film falling down a vertical plate with lateral walls, which is the base configuration describing the structured packing geometry, is numerically investigated via the lubrication theory. The solid substrate wettability is imposed through the disjoining pressure, while the assumption [...] Read more.
The instability of a film falling down a vertical plate with lateral walls, which is the base configuration describing the structured packing geometry, is numerically investigated via the lubrication theory. The solid substrate wettability is imposed through the disjoining pressure, while the assumption of a tiny, precursor film thickness allows for modelling a moving contact line. Contact angles up to 60∘, which falls in the range of structured packing applications, are investigated, thanks to the full implementation of the capillary pressure instead of the small slope approximation. Parametric computations are run for a film falling down a vertical plate bounded by lateral walls, changing the plate width and the flow characteristics. An in-house, finite volume method (FVM) code, previously developed in FORTRAN language and validated in the case of film instability and rivulet flow, is used. The number of observed rivulets, triggered by the instability induced by the lateral walls, is traced for each computation. The numerical results suggest that rivulets with a given wavelength, equal to the one provided by the linear stability analysis, are generated, but only those characterized by a wavelength greater than a minimum threshold, which depends on the substrate wettability, induce partial dewetting of the domain. This allowed for the development of a simplified, statistically based model to predict the effective interface area and the rivulet holdup (required to estimate the mass transfer rate in absorption/distillation applications). Compared to the literature models of the structured packing hydrodynamics, which usually assume a continuous wetting layer, the influence of the flow pattern (continuous film or ensemble of rivulets) on the liquid holdup and on the interfacial area is introduced. The predicted flow regime is successfully verified with evidence from the literature, involving a flow down a corrugated sheet. Full article
(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)
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14 pages, 462 KiB  
Review
Updated Review on the Available Methods for Measurement and Prediction of the Mass Transfer Coefficients in Bubble Columns
by Stoyan Nedeltchev
Fluids 2025, 10(2), 29; https://doi.org/10.3390/fluids10020029 - 27 Jan 2025
Viewed by 766
Abstract
This review summarizes the most important measurement techniques for determination of the volumetric liquid-phase mass transfer coefficient kLa. In addition, the main empirical correlations (with their applicability ranges) for kLa estimation are presented. It is clearly underlined that [...] Read more.
This review summarizes the most important measurement techniques for determination of the volumetric liquid-phase mass transfer coefficient kLa. In addition, the main empirical correlations (with their applicability ranges) for kLa estimation are presented. It is clearly underlined that in tall bubble columns, a system of two differential equations (involving the gas and liquid axial dispersion coefficients) should be solved in order to obtain the accurate kLa value. The semi-empirical correlations for kLa prediction based on the correction of the penetration theory are also summarized. The need for a correction of the penetration theory is explained. The different definitions of the gas–liquid contact time, including the one based on the local isotropic turbulence theory, are presented. Finally, the various chemical methods for the determination of the gas–liquid interfacial area are summarized. Additionally, the main correlation for the prediction of the interfacial area is reported. The effects of pressure, temperature, and viscosity on the interfacial area and kLa are discussed. Full article
(This article belongs to the Special Issue Mass Transfer in Multiphase Reactors)
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18 pages, 10143 KiB  
Article
Features of Supersonic Flow Around a Blunt Body in the Area of Junction with a Flat Surface
by T. A. Lapushkina, E. V. Kolesnik, N. A. Monahov, P. A. Popov and K. I. Belov
Fluids 2025, 10(2), 28; https://doi.org/10.3390/fluids10020028 - 26 Jan 2025
Viewed by 540
Abstract
This work studies the influence of a growing boundary layer on the process of supersonic flow around an aerodynamic body. The task is to select and implement in an experiment the parameters of a supersonic flow and to study the flow pattern near [...] Read more.
This work studies the influence of a growing boundary layer on the process of supersonic flow around an aerodynamic body. The task is to select and implement in an experiment the parameters of a supersonic flow and to study the flow pattern near the surface of an aerodynamic body at different viscosity values for the incoming flow. Visualization of the shock wave configuration in front of the body and studying the change in the pressure field in the flow region under these conditions is the main goal of this work. The experiment was carried out on an experimental stand created on the basis of a shock tube. The aerodynamic body under study (a semi-cylinder pointed along a circle or an ellipse) was placed in a supersonic nozzle. The model was clamped by lateral transparent walls, which were simultaneously a source of boundary layer growth and the viewing windows for visualizing the flow. For selected modes with Reynolds numbers from 8200 to 45,000, schlieren flow patterns and pressure distribution fields near the surface of the streamlined models and the plate of the growing boundary layer were obtained. The data show a complex, unsteady flow pattern realized near the model which was caused by the viscous-inviscid interaction of the boundary layer with the bow shock wave near the wall. Full article
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24 pages, 21088 KiB  
Article
Transonic Aerodynamic Performance Analysis of a CRM Joined-Wing Configuration
by Paul Hanman, Yufeng Yao and Abdessalem Bouferrouk
Fluids 2025, 10(2), 27; https://doi.org/10.3390/fluids10020027 - 25 Jan 2025
Viewed by 731
Abstract
This study examines the aerodynamic performance of a joined-wing (JW) aircraft design based on the NASA Common Research Model (CRM), aiming to assess its potential for efficient commercial transport or cargo aircraft at transonic speed (Mach 0.85). The CRM wing, optimised for transonic [...] Read more.
This study examines the aerodynamic performance of a joined-wing (JW) aircraft design based on the NASA Common Research Model (CRM), aiming to assess its potential for efficient commercial transport or cargo aircraft at transonic speed (Mach 0.85). The CRM wing, optimised for transonic flight, was transformed into a JW design featuring a high-aspect-ratio main wing. An initial parametric study using the vortex lattice minimum drag panel method identified viable designs. The selected JW configuration, comprising front and rear wings joined by a vertical fin, was analysed using ANSYS Fluent to understand flow interactions and aerodynamic performance. At an angle of attack (AoA) of −1°, the JW design achieved a peak lift-to-drag ratio (L/D) of 17.45, close to the CRM’s peak L/D of 19.64 at 2°, demonstrating competitive efficiency. The JW’s L/D exceeded the CRM’s between AoA −3° and 0.8°, but the CRM performed better above 0.8°, with differences decreasing at a higher AoA. Based on induced drag alone, the JW outperformed the CRM across AoA −3° to 8°, but flow complications restricted its L/D advantage to a small, low AoA range. A strong shock on the vertical fin’s inboard side due to high incoming flow speed delayed shock formation on the main wing near the joint. Optimising the vertical fin shape slightly improved L/D, suggesting potential for further enhancements or that other design factors significantly affect JW performance. This study provides insights into JW aerodynamics at transonic speeds, revealing its potential benefits and challenges compared to the CRM design. Full article
(This article belongs to the Special Issue Drag Reduction in Turbulent Flows, 2nd Edition)
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18 pages, 5358 KiB  
Article
Liquid–Liquid Flow and Mass Transfer Enhancement in Tube-in-Tube Millireactors with Structured Inserts and Advanced Inlet Designs
by Feng Zhu, Xingxing Pan, Xichun Cao, Yandan Chen, Rijie Wang, Jiande Lin and Hanyang Liu
Fluids 2025, 10(2), 26; https://doi.org/10.3390/fluids10020026 - 24 Jan 2025
Viewed by 556
Abstract
Liquid–liquid mass transfer is crucial in chemical processes like extraction and desulfurization. Traditional tube-in-tube millireactors often overlook internal flow dynamics, focusing instead on entry modifications. This study explores mass transfer enhancement through structured inserts (twisted tapes, multi-blades) and inlet designs (multi-hole injectors, T-mixers). [...] Read more.
Liquid–liquid mass transfer is crucial in chemical processes like extraction and desulfurization. Traditional tube-in-tube millireactors often overlook internal flow dynamics, focusing instead on entry modifications. This study explores mass transfer enhancement through structured inserts (twisted tapes, multi-blades) and inlet designs (multi-hole injectors, T-mixers). Using high-speed imaging and water–succinic acid–butanol experiments, flow patterns and mass transfer rates were analyzed. Results show annular and dispersion flows dominate under tested conditions with structured inserts lowering the threshold for dispersion flow. Multi-hole injectors improved mass transfer by over 40% compared to T-mixers in plain tubes, while C-tape inserts achieved the highest volumetric mass transfer coefficient (2.43 s−1) due to increased interfacial area and droplet breakup from energy dissipation. This approach offers scalable solutions to enhance tube-in-tube millireactor performance for industrial applications. Full article
(This article belongs to the Special Issue Mass Transfer in Multiphase Reactors)
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12 pages, 3312 KiB  
Article
Experimental Optimization of a Venturi-Type Fine Bubble Generation System Based on Gas Absorption Rate
by Gabriel Toma and Jesús Rafael Alcántara Avila
Fluids 2025, 10(2), 25; https://doi.org/10.3390/fluids10020025 - 24 Jan 2025
Viewed by 719
Abstract
Fine bubbles (FBs) are defined by the ISO/TC 281 as gas bubbles with a diameter of less than 100 μm, and they have interesting properties such as high surface-to-volume ratio, low buoyancy, long residence time, electric charge, and self-pressurization effect. Typically, FBs are [...] Read more.
Fine bubbles (FBs) are defined by the ISO/TC 281 as gas bubbles with a diameter of less than 100 μm, and they have interesting properties such as high surface-to-volume ratio, low buoyancy, long residence time, electric charge, and self-pressurization effect. Typically, FBs are characterized in terms of size distribution, concentration, and zeta potential through specialized microscopic and nanoscopic measuring devices. This work proposes a multi-objective optimization problem to find the optimal conditions to generate FBs from experimental macroscopic measurements in terms of dissolved oxygen (DO). Then, detailed microscopic measurements in terms of size distribution and zeta potential are conducted. Additionally, two venturi-type Fine Bubble Generators (FBGs) were 3D-printed in-house to evaluate the relationship between the internal structure and the generation of FBs. The best FBGs have an obstacle in the diverging section that promotes FB generation under the evaluated experimental conditions. Under the best operating conditions, FBs were stable over 7 days with a size distribution between 60 and 90 nm and with an average of −21 mV. Full article
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28 pages, 451 KiB  
Article
Magnetohyrodynamic Turbulence in a Spherical Shell: Galerkin Models, Boundary Conditions, and the Dynamo Problem
by John V. Shebalin
Fluids 2025, 10(2), 24; https://doi.org/10.3390/fluids10020024 - 23 Jan 2025
Viewed by 680
Abstract
The ‘dynamo problem’ requires that the origin of the primarily dipole geomagnetic field be found. The source of the geomagnetic field lies within the outer core of the Earth, which contains a turbulent magnetofluid whose motion is described by the equations of magnetohydrodynamics [...] Read more.
The ‘dynamo problem’ requires that the origin of the primarily dipole geomagnetic field be found. The source of the geomagnetic field lies within the outer core of the Earth, which contains a turbulent magnetofluid whose motion is described by the equations of magnetohydrodynamics (MHD). A mathematical model can be based on the approximate but essential features of the problem, i.e., a rotating spherical shell containing an incompressible turbulent magnetofluid that is either ideal or real but maintained in an equilibrium state. Galerkin methods use orthogonal function expansions to represent dynamical fields, with each orthogonal function individually satisfying imposed boundary conditions. These Galerkin methods transform the problem from a few partial differential equations in physical space into a huge number of coupled, non-linear ordinary differential equations in the phase space of expansion coefficients, creating a dynamical system. In the ideal case, using Dirichlet boundary conditions, equilibrium statistical mechanics has provided a solution to the problem. As has been presented elsewhere, the solution also has relevance to the non-ideal case. Here, we examine and compare Galerkin methods imposing Neumann or mixed boundary conditions, in addition to Dirichlet conditions. Any of these Galerkin methods produce a dynamical system representing MHD turbulence and the application of equilibrium statistical mechanics in the ideal case gives solutions of the dynamo problem that differ only slightly in their individual sets of wavenumbers. One set of boundary conditions, Neumann on the outer and Dirichlet on the inner surface, might seem appropriate for modeling the outer core as it allows for a non-zero radial component of the internal, turbulent magnetic field to emerge and form the geomagnetic field. However, this does not provide the necessary transition of a turbulent MHD energy spectrum to match that of the surface geomagnetic field. Instead, we conclude that the model with Dirichlet conditions on both the outer and the inner surfaces is the most appropriate because it provides for a correct transition of the magnetic field, through an electrically conducting mantle, from the Earth’s outer core to its surface, solving the dynamo problem. In addition, we consider how a Galerkin model velocity field can satisfy no-slip conditions on solid boundaries and conclude that some slight, kinetically driven compressibility must exist, and we show how this can be accomplished. Full article
(This article belongs to the Section Geophysical and Environmental Fluid Mechanics)
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6 pages, 170 KiB  
Editorial
Recent Developments and Future Directions in Flow Visualization: Experiments and Techniques
by Mingming Ge, Guangjian Zhang and Xinlei Zhang
Fluids 2025, 10(2), 23; https://doi.org/10.3390/fluids10020023 - 22 Jan 2025
Viewed by 949
Abstract
Flow visualization has long been a critical tool for understanding complex fluid dynamics in both natural and engineered systems [...] Full article
(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)
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