Fluids

34 pages, 25005 KiB

Open AccessArticle

Indoor Transmission of Respiratory Droplets Under Different Ventilation Systems Using the Eulerian Approach for the Dispersed Phase

by Yi Feng, Dongyue Li, Daniele Marchisio, Marco Vanni and Antonio Buffo

Fluids 2025, 10(7), 185; https://doi.org/10.3390/fluids10070185 (registering DOI) - 14 Jul 2025

Abstract

Infectious diseases can spread through virus-laden respiratory droplets exhaled into the air. Ventilation systems are crucial in indoor settings as they can dilute or eliminate these droplets, underscoring the importance of understanding their efficacy in the management of indoor infections. Within the field [...] Read more.

Infectious diseases can spread through virus-laden respiratory droplets exhaled into the air. Ventilation systems are crucial in indoor settings as they can dilute or eliminate these droplets, underscoring the importance of understanding their efficacy in the management of indoor infections. Within the field of fluid dynamics methods, the dispersed droplets may be approached through either a Lagrangian framework or an Eulerian framework. In this study, various Eulerian methodologies are systematically compared against the Eulerian–Lagrangian (E-L) approach across three different scenarios: the pseudo-single-phase model (PSPM) for assessing the transport of gaseous pollutants in an office with displacement ventilation (DV), stratum ventilation (SV), and mixing ventilation (MV); the two-fluid model (TFM) for evaluating the transport of non-evaporating particles within an office with DV and MV; and the two-fluid model-population balance equation (TFM-PBE) approach for analyzing the transport of evaporating droplets in a ward with MV. The Eulerian and Lagrangian approaches present similar agreement with the experimental data, indicating that the two approaches are comparable in accuracy. The computational cost of the E-L approach is closely related to the number of tracked droplets; therefore, the Eulerian approach is recommended when the number of droplets required by the simulation is large. Finally, the performances of DV, SV, and MV are presented and discussed. DV creates a stratified environment due to buoyant flows, which transport respiratory droplets upward. MV provides a well-mixed environment, resulting in a uniform dispersion of droplets. SV supplies fresh air directly to the breathing zone, thereby effectively reducing infection risk. Consequently, DV and SV are preferred to reduce indoor infection. Full article

(This article belongs to the Special Issue Respiratory Flows)

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21 pages, 4101 KiB

Open AccessArticle

A Physics-Informed Neural Network Solution for Rheological Modeling of Cement Slurries

by Huaixiao Yan, Jiannan Ding and Chengcheng Tao

Fluids 2025, 10(7), 184; https://doi.org/10.3390/fluids10070184 (registering DOI) - 13 Jul 2025

Abstract

Understanding the rheological properties of fresh cement slurries is essential to maintain optimal pumpability, achieve dependable zonal isolation, and preserve long-term well integrity in oil and gas cementing operations and the 3D printing cement and concrete industry. However, accurately and efficiently modeling the [...] Read more.

Understanding the rheological properties of fresh cement slurries is essential to maintain optimal pumpability, achieve dependable zonal isolation, and preserve long-term well integrity in oil and gas cementing operations and the 3D printing cement and concrete industry. However, accurately and efficiently modeling the rheological behavior of cement slurries remains challenging due to the complex fluid properties of fresh cement slurries, which exhibit non-Newtonian and thixotropic behavior. Traditional numerical solvers typically require mesh generation and intensive computation, making them less practical for data-scarce, high-dimensional problems. In this study, a physics-informed neural network (PINN)-based framework is developed to solve the governing equations of steady-state cement slurry flow in a tilted channel. The slurry is modeled as a non-Newtonian fluid with viscosity dependent on both the shear rate and particle volume fraction. The PINN-based approach incorporates physical laws into the loss function, offering mesh-free solutions with strong generalization ability. The results show that PINNs accurately capture the trend of velocity and volume fraction profiles under varying material and flow parameters. Compared to conventional solvers, the PINN solution offers a more efficient and flexible alternative for modeling complex rheological behavior in data-limited scenarios. These findings demonstrate the potential of PINNs as a robust tool for cement slurry rheological modeling, particularly in scenarios where traditional solvers are impractical. Future work will focus on enhancing model precision through hybrid learning strategies that incorporate labeled data, potentially enabling real-time predictive modeling for field applications. Full article

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

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19 pages, 7940 KiB

Open AccessArticle

High-Salinity Fluid Downslope Flow on Regolith Layer Examined by Laboratory Experiment: Implications for Recurring Slope Lineae on Martian Surfaces

by Yoshiki Tabuchi, Arata Kioka, Takeshi Tsuji and Yasuhiro Yamada

Fluids 2025, 10(7), 183; https://doi.org/10.3390/fluids10070183 (registering DOI) - 12 Jul 2025

Viewed by 15

Abstract

Numerous dark linear recurrent features called Recurring Slope Lineae (RSL) are observed on Martian surfaces, hypothesized as footprints of high-salinity liquid flow. This paper experimentally examined this “wet hypothesis” by analyzing the aspect ratios (length/width) of the flow traces on the granular material [...] Read more.

Numerous dark linear recurrent features called Recurring Slope Lineae (RSL) are observed on Martian surfaces, hypothesized as footprints of high-salinity liquid flow. This paper experimentally examined this “wet hypothesis” by analyzing the aspect ratios (length/width) of the flow traces on the granular material column to investigate how they vary with the granular material column, liquid and its flow rate, and inclination. While pure water produced low aspect ratios (<1.0) on the Martian regolith simulant column, high-salinity fluid (CaCl₂(aq)) traces exhibited significantly higher aspect ratios (>4.0), suggesting that pure water alone is insufficient to explain RSL formulation. Furthermore, the aspect ratios of high-salinity fluid traces on Martian regolith simulants were among the highest observed across all studied granular materials with similar particle sizes, aligning closely with actual RSL observed on Martian slopes. The results further suggest that variable ARs of actual RSL at the given slope can partly be explained by variable flow rates of high-salinity flow as well as salinity (i.e., viscosity) of flow. The results can be attributed to the unique granular properties of Martian regolith, characterized by the lowest permeability and Beavers–Joseph slip coefficient among the studied granular materials. This distinctive microstructure surface promotes surface flow over Darcy flow within the regolith column, leading to a narrow and long-distance feature with high aspect ratios observed in Martian RSL. Thus, our findings support that high-salinity flows are the primary driver behind RSL formation on Mars. Our study suggests the presence of salts on the Martian surface and paves the way for further investigation into RSL formulation processes. Full article

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

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29 pages, 15556 KiB

Open AccessArticle

Vortex-Induced Vibration Predictions of a Circular Cylinder Using an Efficient Pseudo-Time Code-Coupling Approach

by Hang Li and Kivanc Ekici

Fluids 2025, 10(7), 182; https://doi.org/10.3390/fluids10070182 (registering DOI) - 11 Jul 2025

Viewed by 14

Abstract

Presented in this work is a harmonic balance (HB)-based pseudo-time code-coupling approach applied to a one-degree-of-freedom vortex-induced vibration (VIV) problem of a circular cylinder in a low-Reynolds-number laminar flow regime. Unlike physical time coupling used in traditional time-accurate methods, this novel approach updates [...] Read more.

Presented in this work is a harmonic balance (HB)-based pseudo-time code-coupling approach applied to a one-degree-of-freedom vortex-induced vibration (VIV) problem of a circular cylinder in a low-Reynolds-number laminar flow regime. Unlike physical time coupling used in traditional time-accurate methods, this novel approach updates both of the fluid and structure fields by integrating respective HB forms of governing equations in pseudo-time, and then couples the two fields in pseudo-time using a partitioned approach. A separate procedure is adopted to determine the VIV frequency at every code-coupling iteration, which enables the simultaneous convergence of variables of both fields in a single run of the solver. For the cases considered here, lock-in vibrations are predicted over a range of Reynolds numbers, inside and outside the resonant range. The results are verified by a time-accurate method and also validated against earlier experimental data, demonstrating the efficiency and robustness of the pseudo-time code-coupling approach. Full article

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

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27 pages, 7643 KiB

Open AccessArticle

Enhancing Thermal Comfort in Buildings: A Computational Fluid Dynamics Study of Multi-Layer Encapsulated Phase Change Materials–Integrated Bricks for Energy Management

by Farzad Ghafoorian, Mehdi Mehrpooya, Seyed Reza Mirmotahari and Mahmood Shafiee

Fluids 2025, 10(7), 181; https://doi.org/10.3390/fluids10070181 - 10 Jul 2025

Viewed by 96

Abstract

Thermal energy storage plays a vital role in enhancing the efficiency of energy systems, particularly in building applications. Phase change materials (PCMs) have gained significant attention as a passive solution for energy management within building envelopes. This study examines the thermal performance of [...] Read more.

Thermal energy storage plays a vital role in enhancing the efficiency of energy systems, particularly in building applications. Phase change materials (PCMs) have gained significant attention as a passive solution for energy management within building envelopes. This study examines the thermal performance of encapsulated PCMs integrated into bricks as a passive cooling method, taking into account the outdoor climate conditions to enhance indoor thermal comfort throughout summer and winter seasons. A computational fluid dynamics (CFDs) analysis is performed to compare three configurations: a conventional brick, a brick with a single PCM layer, and a brick with three PCM layers. Results indicate that the three-layer PCM configuration provides the most effective thermal regulation, reducing peak indoor temperature fluctuations by up to 4 °C in summer and stabilizing indoor temperature during winter. Also, the second and third PCM layers exhibit minimal latent heat absorption, with their liquid fractions indicating that melting does not occur. As a result, these layers primarily serve as thermal insulation—limiting heat ingress in summer and reducing heat loss in winter. During summer, the absence of the first PCM layer in the single-layer configuration leads to faster thermal penetration, causing the brick to reach peak temperatures approximately two hours earlier in the afternoon and increasing the temperature by about 5 °C. Full article

(This article belongs to the Special Issue Heat Transfer in the Industry)

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16 pages, 5864 KiB

Open AccessArticle

Numerical Study on the Shear Stress Field Development on Dam Break Flows of Viscoplastic Fluids

by Roberta Brondani Minussi, Marcus Vinícius Canhoto Alves and Geraldo de Freitas Maciel

Fluids 2025, 10(7), 180; https://doi.org/10.3390/fluids10070180 - 10 Jul 2025

Viewed by 142

Abstract

The dam break flow problem consists of the phenomena where a fluid is suddenly released and is often used as a test case for multiphase flows numerical models or to analyze the underlying physics of complex free surface flows of both Newtonian and [...] Read more.

The dam break flow problem consists of the phenomena where a fluid is suddenly released and is often used as a test case for multiphase flows numerical models or to analyze the underlying physics of complex free surface flows of both Newtonian and non-Newtonian fluids. Dam break flows of viscoplastic fluids (i.e., fluids that present a yield stress) are especially interesting for two reasons: many geological and industrial fluids can be characterized as viscoplastic fluids, and the yield stress represents a difficulty for numerical solutions. The viscoplastic fluids are simulated using the Bingham and Herschel–Bulkley models, and the results are compared with the flow development of power-law and Newtonian fluids (i.e., with no yield stress). This paper focuses on the numerical modeling of viscoplastic two-dimensional dam-break flows on an inclined bed as a means to analyze the shear stress field development over time and the formation of plug and pseudo-plug zones. It is shown that, for the very beginning of flow, the yield stress fluids were characterized by three distinctive shear stress zones, an occurrence that could not be found on the fluid with no yield stress. Full article

(This article belongs to the Section Non-Newtonian and Complex Fluids)

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18 pages, 1371 KiB

Open AccessArticle

Reduced-Order Model for Catalytic Cracking of Bio-Oil

by Francisco José de Souza, Jonathan Utzig, Guilherme do Nascimento, Alicia Carvalho Ribeiro, Higor de Bitencourt Rodrigues and Henry França Meier

Fluids 2025, 10(7), 179; https://doi.org/10.3390/fluids10070179 - 7 Jul 2025

Viewed by 120

Abstract

This work presents a one-dimensional (1D) model for simulating the behavior of an FCC riser reactor processing bio-oil. The FCC riser is modeled as a plug-flow reactor, where the bio-oil feed undergoes vaporization followed by catalytic cracking reactions. The bio-oil droplets are represented [...] Read more.

This work presents a one-dimensional (1D) model for simulating the behavior of an FCC riser reactor processing bio-oil. The FCC riser is modeled as a plug-flow reactor, where the bio-oil feed undergoes vaporization followed by catalytic cracking reactions. The bio-oil droplets are represented using a Lagrangian framework, which accounts for their movement and evaporation within the gas-solid flow field, enabling the assessment of droplet size impact on reactor performance. The cracking reactions are modeled using a four-lumped kinetic scheme, representing the conversion of bio-oil into gasoline, kerosene, gas, and coke. The resulting set of ordinary differential equations is solved using a stiff, second- to third-order solver. The simulation results are validated against experimental data from a full-scale FCC unit, demonstrating good agreement in terms of product yields. The findings indicate that heat exchange by radiation is negligible and that the Buchanan correlation best represents the heat transfer between the droplets and the catalyst particles/gas phase. Another significant observation is that droplet size, across a wide range, does not significantly affect conversion rates due to the bio-oil’s high vaporization heat. The proposed reduced-order model provides valuable insights into optimizing FCC riser reactors for bio-oil processing while avoiding the high computational costs of 3D CFD simulations. The model can be applied across multiple applications, provided the chemical reaction mechanism is known. Compared to full models such as CFD, this approach can reduce computational costs by thousands of computing hours. Full article

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

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21 pages, 3863 KiB

Open AccessArticle

Zeta Potential as a Key Indicator of Network Structure and Rheological Behavior in Smectite Clay Dispersions

by Hiroshi Kimura, Haruka Tanabe and Susumu Shinoki

Fluids 2025, 10(7), 178; https://doi.org/10.3390/fluids10070178 - 6 Jul 2025

Viewed by 128

Abstract

Smectite clay minerals are known to readily form thixotropic physical gels in aqueous media, even at low volume fractions. Although the rheological properties of these gels are closely related to the microstructure of the network, the influence of the clay’s physicochemical characteristics remains [...] Read more.

Smectite clay minerals are known to readily form thixotropic physical gels in aqueous media, even at low volume fractions. Although the rheological properties of these gels are closely related to the microstructure of the network, the influence of the clay’s physicochemical characteristics remains insufficiently understood. In this study, we systematically investigated the relationships between particle size, cation exchange capacity, and zeta potential, and the rheological behavior of aqueous dispersions of four synthetic smectites. After thorough deionization, dispersions were prepared at controlled NaCl concentrations. We found that the zeta potential strongly correlates with the fineness of the network structure and governs macroscopic rheological responses such as viscosity, yield stress, and gelation behavior. Even under identical conditions, gel transparency and structural coarseness varied significantly among clay types. Furthermore, the storage modulus was influenced not only by network density but also by the intrinsic stiffness of the clay branches. These findings demonstrate that zeta potential serves as a unified indicator of structure and function in smectite dispersions and offer useful insights for gel design in colloidal and soft matter systems. Full article

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23 pages, 3203 KiB

Open AccessArticle

Experimental Investigation of the Entrainment Mechanism in Circular and Lobed Hemispherical Jets

by Saad Aldossary, Mouhammad El Hassan, Nikolay Bukharin, Kamel Abed-Meraim and Anas Sakout

Fluids 2025, 10(7), 177; https://doi.org/10.3390/fluids10070177 - 6 Jul 2025

Viewed by 188

Abstract

Better mixing in the near-field region of jets with their surrounding fluid is of high interest for several industrial applications. Passive control that involves jet geometry modifications as compared to the traditional circular design is used in the present work. An analysis of [...] Read more.

Better mixing in the near-field region of jets with their surrounding fluid is of high interest for several industrial applications. Passive control that involves jet geometry modifications as compared to the traditional circular design is used in the present work. An analysis of the entrainment mechanism in the near jet-exit field is proposed for innovative hemispherical nozzles (circular and six-lobed). High-speed Time-Resolved Particle Image Velocimetry (TR-PIV) measurements are used to experimentally characterize the entrainment mechanism in these jets. The distributions of mean entrainment rates, shear layer growth, and momentum flux are investigated along the longitudinal direction within the near-field region of both circular and lobed hemispherical jets. Significant entrainment enhancement is found using the hemispherical geometry as a passive-control method. By comparing both investigated hemispherical nozzle geometries, it has been demonstrated that the lobed nozzle provides higher mixing rates compared to the circular jet. This enhancement in mixing can be attributed to the stronger streamwise vortex structures generated by the lobed nozzle geometry, which promote increased entrainment of the surrounding fluid. Full article

(This article belongs to the Section Heat and Mass Transfer)

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15 pages, 1351 KiB

Open AccessArticle

An Overlapping IBM-PISO Algorithm with an FFT-Based Poisson Solver for Parallel Incompressible Flow Simulations

by Jiacheng Lian, Qinghe Yao and Zichao Jiang

Fluids 2025, 10(7), 176; https://doi.org/10.3390/fluids10070176 - 4 Jul 2025

Viewed by 241

Abstract

This study addresses computational challenges in the immersed boundary method (IBM) with the pressure implicit with split operator (PISO) algorithm for simulating incompressible flows. We introduce a novel time-step splitting method to implement communication overlapping optimization, aiming to reduce costs dominated by the [...] Read more.

This study addresses computational challenges in the immersed boundary method (IBM) with the pressure implicit with split operator (PISO) algorithm for simulating incompressible flows. We introduce a novel time-step splitting method to implement communication overlapping optimization, aiming to reduce costs dominated by the pressure Poisson solver. Using a fast Fourier transform (FFT)-based approach, the Poisson equation is solved efficiently with

O (N log N)

complexity. Our method interleaves IBM force calculations with Poisson phases, employing asynchronous communication to overlap computation with global data exchanges. This reduces communication overhead, enhancing scalability. Validation through benchmark simulations, including flow around a cylinder and particle-laden flows, shows improved efficiency and accuracy comparable with traditional methods. Implemented in a custom C++ solver using the FFTW library, tests indicate substantial acceleration, with results showing a 40% speed-up and less than 3% deviation in drag and lift coefficients. This research provides an efficient and promising simulation tool for complex flow. Full article

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

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21 pages, 3755 KiB

Open AccessArticle

Effect of Pore-Scale Anisotropic and Heterogeneous Structure on Rarefied Gas Flow in Three-Dimensional Porous Media

by Wenqiang Guo, Jinshan Zhao, Gang Wang, Ming Fang and Ke Zhu

Fluids 2025, 10(7), 175; https://doi.org/10.3390/fluids10070175 - 3 Jul 2025

Viewed by 220

Abstract

Porous media have great application prospects, such as transpiration cooling for the aerospace industry. The main challenge for the prediction of gas permeability includes the geometrical complexity and high Knudsen number of gas flow at the nano-scale to micro-scale, leading to failure of [...] Read more.

Porous media have great application prospects, such as transpiration cooling for the aerospace industry. The main challenge for the prediction of gas permeability includes the geometrical complexity and high Knudsen number of gas flow at the nano-scale to micro-scale, leading to failure of the conventional Darcy’s law. To address these issues, the Quartet Structure Generation Set (QSGS) method is improved to construct anisotropic and heterogeneous three-dimensional porous media, and the lattice Boltzmann method (LBM) with the multiple relaxation time (MRT) collision operator is adopted. Using MRT-LBM, the pressure boundary conditions at the inlet and outlet are firstly dealt with using the moment-based boundary conditions, demonstrating good agreement with the analytical solutions in two benchmark tests of three-dimensional Poiseuille flow and flow through a body-centered cubic array of spheres. Combined with the Bosanquet-type effective viscosity model and Maxwellian diffuse reflection boundary condition, the gas flow at high Knudsen (Kn) numbers in three-dimensional porous media is simulated to study the relationship between pore-scale anisotropy, heterogeneity and Kn, and permeability and micro-scale slip effects in porous media. The slip factor is positively correlated with the anisotropic factor, which means that the high Kn effect is stronger in anisotropic structures. There is no obvious correlation between the slip factor and heterogeneity factor. Full article

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

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15 pages, 5152 KiB

Open AccessArticle

Hydraulic Performance and Flow Characteristics of a High-Speed Centrifugal Pump Based on Multi-Objective Optimization

by Yifu Hou and Rong Xue

Fluids 2025, 10(7), 174; https://doi.org/10.3390/fluids10070174 - 2 Jul 2025

Viewed by 192

Abstract

Pump-driven liquid cooling systems are widely utilized in unmanned aerial vehicle (UAV) electronic thermal management. As a critical power component, the miniaturization and lightweight design of the pump are essential. Increasing the operating speed of the pump allows for a reduction in impeller [...] Read more.

Pump-driven liquid cooling systems are widely utilized in unmanned aerial vehicle (UAV) electronic thermal management. As a critical power component, the miniaturization and lightweight design of the pump are essential. Increasing the operating speed of the pump allows for a reduction in impeller size while maintaining hydraulic performance, thereby significantly decreasing the overall volume and mass. However, high-speed operation introduces considerable internal flow losses, placing stricter demands on the geometric design and flow-field compatibility of the impeller. In this study, a miniature high-speed centrifugal pump (MHCP) was investigated, and a multi-objective optimization of the impeller was carried out using response surface methodology (RSM) to improve internal flow characteristics and overall hydraulic performance. Numerical simulations demonstrated strong predictive capability, and experimental results validated the model’s accuracy. At the design condition (10,000 rpm, 4.8 m³/h), the pump achieved a head of 46.1 m and an efficiency of 49.7%, corresponding to its best efficiency point (BEP). Sensitivity analysis revealed that impeller outlet diameter and blade outlet angle were the most influential parameters affecting pump performance. Following the optimization, the pump head increased by 3.7 m, and the hydraulic efficiency improved by 4.8%. In addition, the pressure distribution and streamlines within the impeller exhibited better uniformity, while the turbulent kinetic energy near the blade suction surface and at the impeller outlet was markedly decreased. This work provides theoretical support and design guidance for the efficient application of MHCPs in UAV thermal management systems. Full article

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18 pages, 1779 KiB

Open AccessArticle

Hybrid Estimation of Inflow Multiphase Production Rates Using a Dynamic Wellbore Flow Model

by Anton Gryzlov, Eugene Magadeev, Andrey Kovalskii and Muhammad Arsalan

Fluids 2025, 10(7), 173; https://doi.org/10.3390/fluids10070173 - 30 Jun 2025

Viewed by 166

Abstract

This paper considers the problem of estimating the quantitative parameters of a two-phase fluid flow in a well based on the dynamic physical flow model. This is a challenging problem in the oil and gas industry, where the knowledge of multiphase production rates [...] Read more.

This paper considers the problem of estimating the quantitative parameters of a two-phase fluid flow in a well based on the dynamic physical flow model. This is a challenging problem in the oil and gas industry, where the knowledge of multiphase production rates plays an important role during reservoir characterization, production optimization and reservoir management. As the direct measurement of these rates is not easily available, they can be inferred from conventional sensors (e.g., pressure gauges) in combination with a dynamic multiphase flow model. The methodology proposed in this work uses inverse modeling concepts to estimate flow rates that are not measured directly. The mismatch between the available data and model prediction is numerically minimized, leading to the optimal set of dynamic flow variables characterizing the flow. Two different scenarios are considered: firstly, when the well has only a flow meter located at the wellhead (minimum amount of available information), and when the well has distributed pressure sensors in addition to the topside flow meter (maximum amount of information). The feasibility of the proposed concept is assessed via several simulation-based case studies. Full article

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

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16 pages, 2361 KiB

Open AccessArticle

Numerical Investigation of a Gas Bubble in Complex Geometries for Industrial Process Equipment Design

by Daniel B. V. Santos, Antônio E. M. Santos, Enio P. Bandarra Filho and Gustavo R. Anjos

Fluids 2025, 10(7), 172; https://doi.org/10.3390/fluids10070172 - 30 Jun 2025

Viewed by 177

Abstract

This study investigates three-dimensional two-phase flows in complex geometries found in industrial process equipment design using finite-element numerical simulations. The governing equations are formulated in three-dimensional Cartesian coordinates and solved on unstructured meshes employing the Taylor–Hood “Mini” element, selected for its numerical stability [...] Read more.

This study investigates three-dimensional two-phase flows in complex geometries found in industrial process equipment design using finite-element numerical simulations. The governing equations are formulated in three-dimensional Cartesian coordinates and solved on unstructured meshes employing the Taylor–Hood “Mini” element, selected for its numerical stability and convergence properties. The convective term in the momentum equation is discretized using a first-order semi-Lagrangian scheme. The two fluid phases are separated by an interface mesh composed of triangular surface elements, which is independent of the primary volumetric fluid mesh. Surface tension effects are incorporated as a source term using the continuum surface force (CSF) model, with the curvature computed via the Laplace–Beltrami operator. At each time step, the positions of the interface mesh nodes are updated according to the local fluid velocity field. The results show that the methodology is stable and can be used to accurately model two-phase flows in complex geometries found in several engineering solutions. Full article

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

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30 pages, 12972 KiB

Open AccessArticle

Simulation and Optimization of Conveying Parameters for Vertical Screw Conveyor Based on CFD + DEM

by Xiao Mei, Xiaoyu Fang, Liyang Zhang, Yandi Wang and Yuan Tian

Fluids 2025, 10(7), 171; https://doi.org/10.3390/fluids10070171 - 30 Jun 2025

Cited by 1 | Viewed by 259

Abstract

This study investigates the interaction between airflow and low-density bulk particles within vertical screw conveyors and examines its impact on conveying performance. A combined simulation approach integrating the Discrete Element Method and Computational Fluid Dynamics was employed to model both single-phase particle flow [...] Read more.

This study investigates the interaction between airflow and low-density bulk particles within vertical screw conveyors and examines its impact on conveying performance. A combined simulation approach integrating the Discrete Element Method and Computational Fluid Dynamics was employed to model both single-phase particle flow and gas–solid two-phase flow. A periodic model was developed based on the structural characteristics of the conveyor. Particle motion dynamics under both single-phase and coupled two-phase conditions were analyzed using EDEM and coupled Fluent-EDEM simulations. The effects of key operational parameters, including screw speed, filling rate, and helix angle, on mass flow rate were systematically evaluated. A comprehensive performance index was established to quantify conveying efficiency, and its validity was confirmed through analysis of variance on the regression model. Finally, the response surface methodology was applied to optimize parameters and determine the optimal combination of screw speed and filling rate to enhance mass flow efficiency. The results indicate that the gas–solid two-phase flow model provides a more accurate representation of real-world conveying dynamics. Future research may extend the model to accommodate more complex material conditions. Full article

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

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17 pages, 5117 KiB

Open AccessReview

Statistical Physics Perspective on Droplet Spreading in Reactive Wetting Interfaces

by Haim Taitelbaum

Fluids 2025, 10(7), 170; https://doi.org/10.3390/fluids10070170 - 29 Jun 2025

Viewed by 202

Abstract

Droplet spreading is a fascinating phenomenon. Especially when the droplet spreads, reacts, and dissolves on and into metal substrates. This reactive wetting mainly occurs at high temperatures, with a vast number of applications in industry and material science. It is common to monitor [...] Read more.

Droplet spreading is a fascinating phenomenon. Especially when the droplet spreads, reacts, and dissolves on and into metal substrates. This reactive wetting mainly occurs at high temperatures, with a vast number of applications in industry and material science. It is common to monitor and study the process using a side-view projection of the droplet, focusing on the dynamics and shape of its contact line. However, when the spreading is monitored top-view, rich and non-trivial spatio-temporal patterns are revealed during different stages of the process. These patterns call for a different type of study of the perimeter of the entire droplet. Statistical physics is the natural candidate to perform such tasks, using tools developed for the study of kinetic roughening of advancing interfaces. In this review, we demonstrate the use of these tools, the growth, roughness, and persistence exponents, to study the spreading of mercury droplets on metal-on-glass at room temperature, which by itself is a unique experimental system at this range of temperatures. The universality of the results is discussed in comparison with similar patterns of reactive wetting at high temperatures. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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13 pages, 2053 KiB

Open AccessArticle

Rheological Features of Aqueous Polymer Solutions Tailored by Hydrodynamic Cavitation

by Santiago Nicolás Fleite, María del Pilar Balbi, María Alejandra Ayude and Miryan Cassanello

Fluids 2025, 10(7), 169; https://doi.org/10.3390/fluids10070169 - 29 Jun 2025

Viewed by 189

Abstract

Hydrodynamic cavitation (HC) has emerged as a versatile method for modifying the rheological properties of polymer solutions, offering advantages such as scalability and operational simplicity. This work investigates the effect of HC on aqueous polyacrylamide (PAM) solutions, focusing on viscosity and viscoelasticity changes [...] Read more.

Hydrodynamic cavitation (HC) has emerged as a versatile method for modifying the rheological properties of polymer solutions, offering advantages such as scalability and operational simplicity. This work investigates the effect of HC on aqueous polyacrylamide (PAM) solutions, focusing on viscosity and viscoelasticity changes as a function of the number of passes through a vortex-type HC device and the presence of dissolved salts (CaCl₂ or KCl). Viscosity measurements were modeled using the power law equation, while oscillatory tests were used to determine storage and loss moduli. The results show that HC substantially reduced viscosity and elastic behavior, with the degree of modification strongly influenced by the number of passes. A critical molecular size limit was suggested, below which further degradation becomes limited. Salt addition enhanced depolymerization, likely due to charge screening, hydrodynamic radius reduction, and the increased solubility and mobility of polymer chains within cavitation bubbles. HC eliminated elasticity in all cases, yielding solutions with near-Newtonian behavior. The transformation is attributed to molecular weight reduction and changes in molecular size distribution. These findings support the use of HC as a practical approach to tailor the flow properties of PAM solutions, while highlighting intrinsic limitations imposed by cavitation dynamics and polymer chain dimensions. Full article

(This article belongs to the Special Issue Cavitation and Bubble Dynamics)

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23 pages, 12509 KiB

Open AccessArticle

Tuned Generalised k-ω (GEKO) Turbulence Model Parameters for Predicting Transitional Flow Through Stenosis Geometries of Various Degrees

by Jake Emmerling, Sara Vahaji, David A. V. Morton, Svetlana Stevanovic, David F. Fletcher and Kiao Inthavong

Fluids 2025, 10(7), 168; https://doi.org/10.3390/fluids10070168 - 28 Jun 2025

Viewed by 309

Abstract

Stenosis geometries are constrictions of a biological tube that can be found in many forms in the human body. Capturing the flow field in such geometries is important. For this purpose, simulations were performed using the generalised k-

ω

(GEKO) turbulence model [...] Read more.

Stenosis geometries are constrictions of a biological tube that can be found in many forms in the human body. Capturing the flow field in such geometries is important. For this purpose, simulations were performed using the generalised k-

ω

(GEKO) turbulence model to study flow through stenosis geometries with throat constrictions of 75, 50 and 25% area reduction. Laminar flow conditions of Re = 2000 and 1000 were applied and the results were compared with experimental data. The effect of four GEKO parameters (

C_{S E P}

,

C_{N W}

,

C_{J E T}

and

C_{M I X}

) on flow in the post-stenotic region was investigated by simulating a wide range of parameter values. Results showed that the

C_{M I X}

parameter, combined with a modified GEKO blending function, had the greatest effect on axial velocity, velocity fluctuations and the location of the jet breakdown region. A

C_{M I X}

value of 0.4 closely matched the experimental results for a 75% area reduction stenosis at

R e = 2000

and showed significant improvements over existing Reynolds-averaged Navier–Stokes models. The GEKO model was also able to closely match the axial velocity results predicted by previously published large-eddy simulation models under the same flow conditions. Furthermore, the GEKO model was applied to a realistic oral-to-tracheal airway model for a Reynolds number of 2000 and produced results consistent with the idealised stenotic tube. Full article

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

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22 pages, 4216 KiB

Open AccessArticle

Quantitative Assessment of Red Blood Cell Disaggregation in Chronic Lymphocytic Leukemia via Software Image Flow Analysis

by Anika Alexandrova-Watanabe, Emilia Abadjieva, Miroslava Ivanova, Lidia Gartcheva, Ariana Langari, Margarita Guenova, Tihomir Tiankov, Elena V. Nikolova, Sashka Krumova and Svetla Todinova

Fluids 2025, 10(7), 167; https://doi.org/10.3390/fluids10070167 - 27 Jun 2025

Viewed by 286

Abstract

Red blood cell (RBC) aggregation and disaggregation are key factors in microcirculatory flow, and their disturbance can lead to alterations in the rheological properties of blood in disorders such as chronic lymphocytic leukemia (CLL). This study aimed to determine the critical shear rate [...] Read more.

Red blood cell (RBC) aggregation and disaggregation are key factors in microcirculatory flow, and their disturbance can lead to alterations in the rheological properties of blood in disorders such as chronic lymphocytic leukemia (CLL). This study aimed to determine the critical shear rate required to fully disaggregate RBC aggregates using samples from healthy individuals, providing a reference point for evaluating pathological changes. Using a microfluidic system and software-image-based flow analysis, RBC disaggregation was assessed by the Aggregation-Area Indicator at a high shear rate (AAI_H) changes and the number of undestroyed aggregates. The defined critical shear rate at 446 s⁻¹ was applied to assess RBC disaggregation in CLL patients, both untreated and treated with Obinutuzumab/Venetoclax or Ibrutinib. CLL samples exhibited significantly elevated AAI_H values compared to controls, indicating a greater resistance to shear-induced dispersion. Although both treatments reduced the number of stable aggregates, neither therapy fully normalized RBC disaggregation to the levels observed in healthy controls. Moreover, there was a notable heterogeneity among Ibrutinib-treated patients, revealing different therapeutic effects on RBC rheology. These findings suggest alterations in the RBC rheology in CLL despite therapy and support the use of a shear-dependent disaggregation analysis as a complementary tool for understanding and monitoring CLL-related hematologic abnormalities. Full article

(This article belongs to the Section Non-Newtonian and Complex Fluids)

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17 pages, 3195 KiB

Open AccessReview

A Comprehensive Review of Mock Circulation Loop Systems for Experimental Hemodynamics of Cardiovascular Diseases

by Weichen Hong, Vijay Tewari, Jun Chen, Alan P. Sawchuk and Huidan Yu

Fluids 2025, 10(7), 166; https://doi.org/10.3390/fluids10070166 - 27 Jun 2025

Viewed by 312

Abstract

Cardiovascular diseases remain the leading cause of morbidity and mortality worldwide, underscoring the need for continuous innovation in diagnostics and treatment. Mock circulation loops (MCLs) systems have recently emerged as new research platforms capable of replicating the hemodynamics of the human cardiovascular system. [...] Read more.

Cardiovascular diseases remain the leading cause of morbidity and mortality worldwide, underscoring the need for continuous innovation in diagnostics and treatment. Mock circulation loops (MCLs) systems have recently emerged as new research platforms capable of replicating the hemodynamics of the human cardiovascular system. This review explores the expanding applications of MCLs to cardiovascular diseases beyond their traditional role in testing ventricular assist devices and heart failure management. We focus on their versatility in simulating various cardiovascular conditions, particularly arterial diseases such as atherosclerosis, stenosis, and aneurysms. This review traces the evolution of MCLs and their integration with computational simulations and real-time data acquisition systems. MCLs provide detailed insights into hemodynamic responses under diverse conditions, enhancing the precision and safety of cardiovascular interventions. This comprehensive review emphasizes the critical role of MCLs in advancing cardiovascular research, refining clinical interventions, and improving patient outcomes. Full article

(This article belongs to the Special Issue Mock Circulation Loops for Cardiovascular Research)

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31 pages, 9230 KiB

Open AccessArticle

Particle Image Velocimetry Analysis of Bedload Sampling in a Sand-Bed River

by Rodrigo B. Pereira, Glauber A. Carvalho, Tobias Bleninger, Pedro A. P. Zamboni, Liege Wosiacki, Fábio V. Gonçalves and Johannes Gérson Janzen

Fluids 2025, 10(7), 165; https://doi.org/10.3390/fluids10070165 - 27 Jun 2025

Viewed by 359

Abstract

Both the excess and alteration of bed sediments in river systems can cause socioeconomic and environmental damage; thus, the quantification of bedload transport is an important tool to assess the health of rivers and help in decision-making imposed by the agencies responsible for [...] Read more.

Both the excess and alteration of bed sediments in river systems can cause socioeconomic and environmental damage; thus, the quantification of bedload transport is an important tool to assess the health of rivers and help in decision-making imposed by the agencies responsible for water resource management. This work aims to evaluate the efficiency of pressure-difference samplers (Helley–Smith) qualitatively and quantitatively when used in environments with sandy characteristics. The experiments were carried out in a stream with full transparency and two pressure-difference samplers with nozzle dimensions of 7.20 × 7.20 cm and 8.89 × 7.50 cm. The Particle Image Velocimetry technique was used to analyze the sampler efficiency simultaneously with an Acoustic Doppler Current Profiler. Qualitative results showed that the way the equipment is allocated at the bottom of the river can generate overestimated or underestimated sediment transport measurements. Additionally, evaluating it quantitatively, we see that the collection efficiency of the equipment varied between 15.45% and 534.78% when compared to the results obtained by the Particle Image Velocimetry technique. Full article

(This article belongs to the Special Issue Environmental Hydraulics, Turbulence and Sediment Transport, 3rd Edition)

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6 pages, 165 KiB

Open AccessEditorial

Industrial CFD and Fluid Modeling in Engineering—2nd Edition

by Francesco De Vanna

Fluids 2025, 10(7), 164; https://doi.org/10.3390/fluids10070164 - 27 Jun 2025

Viewed by 207

Abstract

Following the success of the first edition of “Industrial CFD and Fluid Modeling in Engineering”, this second edition continues to showcase cutting-edge research that pushes the boundaries of computational fluid dynamics (CFD) in addressing complex industrial flow problems [...] Full article

(This article belongs to the Special Issue Industrial CFD and Fluid Modelling in Engineering, 2nd Edition)

33 pages, 14482 KiB

Open AccessArticle

AI-Driven Surrogate Model for Room Ventilation

by Jaume Luis-Gómez, Francisco Martínez, Alejandro González-Barberá, Javier Mascarós, Guillem Monrós-Andreu, Sergio Chiva, Elisa Borrás and Raúl Martínez-Cuenca

Fluids 2025, 10(7), 163; https://doi.org/10.3390/fluids10070163 - 26 Jun 2025

Viewed by 227

Abstract

The control of ventilation systems is often performed by automatic algorithms which often do not consider the future evolution of the system in its control politics. Digital twins allow system forecasting for a more sophisticated control. This paper explores a novel methodology to [...] Read more.

The control of ventilation systems is often performed by automatic algorithms which often do not consider the future evolution of the system in its control politics. Digital twins allow system forecasting for a more sophisticated control. This paper explores a novel methodology to create a Machine Learning (ML) model for the predictive control of a ventilation system combining Computational Fluid Dynamics (CFD) with Artificial Intelligence (AI). This predictive model was created to forecast the temperature and humidity evolution of a ventilated room to be implemented in a digital twin for better unsupervised control strategies. To replicate the full range of annual conditions, a series of CFD simulations were configured and executed based on seasonal data collected by sensors positioned inside and outside the room. These simulations generated a dataset used to develop the predictive model, which was based on a Deep Neural Network (DNN) with fully connected layers. The model’s performance was evaluated, yielding final average absolute errors of 0.34 degrees Kelvin for temperature and 2.2 percentage points for relative humidity. The presented results highlight the potential of this methodology to create AI-driven digital twins for the control of room ventilation. Full article

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

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34 pages, 3719 KiB

Open AccessArticle

Experimental and Numerical Study of Film Boiling Around a Small Nickel Sphere

by Charles Brissot, Léa Cailly-Brandstäter, Romain Castellani, Elie Hachem and Rudy Valette

Fluids 2025, 10(7), 162; https://doi.org/10.3390/fluids10070162 - 24 Jun 2025

Viewed by 165

Abstract

This work—mixing an original experimental approach, as well as numerical simulations—proposes to study film boiling modes around a small nickel sphere. While dealing with a simple looking phenomenon that is found in many industrial processes and has been solved for basic quenching regimes, [...] Read more.

This work—mixing an original experimental approach, as well as numerical simulations—proposes to study film boiling modes around a small nickel sphere. While dealing with a simple looking phenomenon that is found in many industrial processes and has been solved for basic quenching regimes, we focus on describing precisely how vapor formation and film thicknesses, as well as vapor bubble evacuation, affect cooling kinetics. As instrumenting small spheres may lead to experimental inaccuracies, we optically captured, using a high-speed camera, the vapor film thickness at mid height, the vapor bubble volume, and the bubble detachment frequency, along with the heat flux. More precisely, an estimation of the instant sphere temperature, in different conditions, was obtained through cooling time measurement before the end of the film boiling mode, subsequently facilitating heat flux evaluation. We encountered a nearly linear decrease in both the vapor film thickness and vapor bubble volume as the sphere temperature decreased. Notably, the detachment frequency remained constant across the whole temperature range. The estimation of the heat fluxes confirmed the prevalence of conduction as the primary heat transfer mode; a major portion of the energy was spent increasing the liquid temperature. The results were then compared to finite element simulations using an in-house multiphysics solver, including thermic phase changes (liquid to vapor) and their hydrodynamics, and we also captured the interfaces. While presenting a challenge due to the contrast in densities and viscosities between phases, the importance of the small circulations along them, which improve the heat removal in the liquid phase, was highlighted; we also assessed the suitability of the model and the numerical code for the simulation of such quenching cases when subcooling in the vicinity of a saturation temperature. Full article

(This article belongs to the Section Heat and Mass Transfer)

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23 pages, 4919 KiB

Open AccessArticle

Hybrid Symbolic Regression and Machine Learning Approaches for Modeling Gas Lift Well Performance

by Samuel Nashed and Rouzbeh Moghanloo

Fluids 2025, 10(7), 161; https://doi.org/10.3390/fluids10070161 - 21 Jun 2025

Viewed by 287

Abstract

Proper determination of the bottomhole pressure in a gas lift well is essential to enhance production, tackle operating concerns, and use the least amount of gas. Mechanistic models, empirical correlation, and hybrid models are usually limited by the requirements for calibration, large amounts [...] Read more.

Proper determination of the bottomhole pressure in a gas lift well is essential to enhance production, tackle operating concerns, and use the least amount of gas. Mechanistic models, empirical correlation, and hybrid models are usually limited by the requirements for calibration, large amounts of inputs, or limited scope of work. Through this study, sixteen well-tested machine learning (ML) models, such as genetic programming-based symbolic regression and neural networks, are developed and studied to accurately predict flowing BHP at the perforation depth, using a dataset from 304 gas lift wells. The dataset covers a variety of parameters related to reservoirs, completions, and operations. After careful preprocessing and analysis of features, the models were prepared and tested with cross-validation, random sampling, and blind testing. Among all approaches, using the L-BFGS optimizer on the neural network gave the best predictions, with an R² of 0.97, low errors, and better accuracy than other ML methods. Upon using SHAP analysis, it was found that the injection point depth, tubing depth, and fluid flow rate are the main determining factors. Further using the model on 30 unseen additional wells confirmed its reliability and real-world utility. This study reveals that ML prediction for BHP is an effective alternative for traditional models and pressure gauges, as it is simpler, quicker, more accurate, and more economical. Full article

(This article belongs to the Special Issue Advances in Multiphase Flow Simulation with Machine Learning)

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