Fluids

25 pages, 4477 KiB

Open AccessArticle

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 (registering DOI) - 2 Feb 2025

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)

17 pages, 2827 KiB

Open AccessArticle

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 (registering DOI) - 1 Feb 2025

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)

19 pages, 1102 KiB

Open AccessArticle

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 (registering DOI) - 31 Jan 2025

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)

10 pages, 4801 KiB

Open AccessArticle

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 (registering DOI) - 31 Jan 2025

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

Open AccessArticle

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

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, 1179 KiB

Open AccessArticle

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

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)

24 pages, 12562 KiB

Open AccessArticle

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

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

20 pages, 1027 KiB

Open AccessArticle

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

Abstract

Vaccination cold chains depend critically on maintaining temperatures within the 2–8

^{\circ}

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 [...] Read more.

Vaccination cold chains depend critically on maintaining temperatures within the 2–8

^{\circ}

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

^{\circ}

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)

41 pages, 26963 KiB

Open AccessArticle

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

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)

20 pages, 2071 KiB

Open AccessArticle

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

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^{\circ}

, 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

Open AccessReview

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

Abstract

This review summarizes the most important measurement techniques for determination of the volumetric liquid-phase mass transfer coefficient k_La. In addition, the main empirical correlations (with their applicability ranges) for k_La 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 k_La. In addition, the main empirical correlations (with their applicability ranges) for k_La 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 k_La value. The semi-empirical correlations for k_La 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 k_La are discussed. Full article

(This article belongs to the Special Issue Mass Transfer in Multiphase Reactors)

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18 pages, 10143 KiB

Open AccessArticle

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

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

Open AccessArticle

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

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, 1612 KiB

Open AccessArticle

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

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)

12 pages, 3312 KiB

Open AccessArticle

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

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, 438 KiB

Open AccessArticle

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

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)

6 pages, 170 KiB

Open AccessEditorial

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

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)

74 pages, 7040 KiB

Open AccessArticle

The Lattice Boltzmann Method with Deformable Boundary for Colonic Flow Due to Segmental Circular Contractions

by Irina Ginzburg

Fluids 2025, 10(2), 22; https://doi.org/10.3390/fluids10020022 - 21 Jan 2025

Abstract

We extend the 3D Lattice Boltzmann method with a deformable boundary (LBM-DB) for the computations of the full-volume colonic flow of the Newtonian fluid driven by the peristaltic segmented circular contractions which obey the three-step “intestinal law”: (i) deflation, (ii) inflation, and (iii) [...] Read more.

We extend the 3D Lattice Boltzmann method with a deformable boundary (LBM-DB) for the computations of the full-volume colonic flow of the Newtonian fluid driven by the peristaltic segmented circular contractions which obey the three-step “intestinal law”: (i) deflation, (ii) inflation, and (iii) elastic relaxation. The key point is that the LBM-DB accurately prescribes a curved deforming surface on the regular computational grid through precise and compact Dirichlet velocity schemes, without the need to recover for an adaptive boundary mesh or surface remesh, and without constraint of fluid volume conservation. The population “refill” of “fresh” fluid nodes, including sharp corners, is reformulated with the improved reconstruction algorithms by combining bulk and advanced boundary LBM steps with a local sub-iterative collision update. The efficient parallel LBM-DB simulations in silico then extend the physical experiments performed in vitro on the Dynamic Colon Model (DCM, 2020) to highly occlusive contractile waves. The motility scenarios are modeled both in a cylindrical tube and in a new geometry of “parabolic” transverse shape, which mimics the dynamics of realistic triangular lumen aperture. We examine the role of cross-sectional shape, motility pattern, occlusion scenario, peristaltic wave speed, elasticity effect, kinematic viscosity, inlet/outlet conditions and numerical compressibility on the temporal localization of pressure and velocity oscillations, and especially the ratio of retrograde vs antegrade velocity amplitudes, in relation to the major contractile events. The developed numerical approach could contribute to a better understanding of the intestinal physiology and pathology due to a possibility of its straightforward extension to the non-Newtonian chyme rheology and anatomical geometry. Full article

(This article belongs to the Special Issue Lattice Boltzmann Methods: Fundamentals and Applications)

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30 pages, 1045 KiB

Open AccessArticle

Pressure Behavior in a Linear Porous Media for Partially Miscible Displacement of Oil by Gas

by Luara K. S. Sousa, Wagner Q. Barros, Adolfo P. Pires and Alvaro M. M. Peres

Fluids 2025, 10(2), 21; https://doi.org/10.3390/fluids10020021 - 21 Jan 2025

Abstract

Miscible gas flooding improves oil displacement through mass exchange between oil and gas phases. It is one of the most efficient enhanced oil recovery methods for intermediate density oil reservoirs. In this work, analytical solutions for saturation, concentration and pressure are derived for [...] Read more.

Miscible gas flooding improves oil displacement through mass exchange between oil and gas phases. It is one of the most efficient enhanced oil recovery methods for intermediate density oil reservoirs. In this work, analytical solutions for saturation, concentration and pressure are derived for oil displacement by a partially miscible gas injection at a constant rate. The mathematical model considers two-phase, three-component fluid flow in a one-dimensional homogeneous reservoir initially saturated by a single oil phase. Phase saturations and component concentrations are described by a

2 \times 2

hyperbolic system of partial differential equations, which is solved by the method of characteristics. Once this Goursat–Riemann problem is solved, the pressure drop between two points in the porous media is obtained by the integration of Darcy’s law. The solution of this problem may present three different fluid regions depending on the rock–fluid parameters: a single-phase gas region near the injection point, followed by a two-phase region where mass transfer takes place and a single-phase oil region. We considered the single-phase gas and the two-phase gas/oil regions as incompressible, while the single-phase oil region may be incompressible or slightly compressible. The solutions derived in this work are applied for a specific set of rock and fluid properties. For this data set, the two-phase region displays rarefaction waves, shock waves and constant states. The pressure behavior depends on the physical model (incompressible, compressible and finite or infinite porous media). In all cases, the injection pressure is the result of the sum of two terms: one represents the effect of the mobility contrast between phases and the other represents the single-phase oil solution. The solutions obtained in this work are compared to an equivalent immiscible solution, which shows that the miscible displacement is more efficient. Full article

(This article belongs to the Special Issue Multiphase Flow for Industry Applications)

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