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Volume 10
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Fluids, Volume 10, Issue 5 (May 2025) – 33 articles

Cover Story (view full-size image): This article revisits the foundational concept of the Reynolds number—a dimensionless parameter that quantifies the ratio between inertial and viscous forces—which O. Reynolds initially proposed to distinguish between laminar and turbulent flow regimes. Beyond its classical context, we explore how its interpretation and applicability have expanded—from microscopic systems to astrophysical scales. We briefly present its historical development and highlight the conceptual flexibility that makes the Reynolds number a powerful tool for modeling complex transport phenomena. We consider the insights of O. Reynolds and other pioneers, whose recognition of this force ratio laid the groundwork for modern fluid dynamics. View this paper
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22 pages, 2300 KiB  
Article
The Impact of Nonlinear Flow Regime on the Flow Rate in Fractal Fractures
by Jianting Zhu
Fluids 2025, 10(5), 139; https://doi.org/10.3390/fluids10050139 - 21 May 2025
Viewed by 94
Abstract
Geometric properties of fractures, such as aperture and width, among others, significantly affect the fluid flow behaviors in fractured media. Previous studies have shown that fractures exhibit fractal properties. In this study, we examine the impact of nonlinear flow regimes and aperture and [...] Read more.
Geometric properties of fractures, such as aperture and width, among others, significantly affect the fluid flow behaviors in fractured media. Previous studies have shown that fractures exhibit fractal properties. In this study, we examine the impact of nonlinear flow regimes and aperture and width fractal distributions on the flow behavior through fractal fractures. Both the aperture and width are treated independently following fractal distribution, but with distinct fractal dimensions. We explicitly examine the flow features without using Darcy’s law concept, which relies on the linear flow assumption with an effective permeability of fractal fractures. We directly consider the flow rate in a fracture with average aperture, average flow rate, and flow rate of linear flow in all the fractures, and nonlinear flow rate in all the fractures, and more realistically, the average flow rate when linear and nonlinear flows may coexist in different fractures and their differences. The results demonstrate that the nonlinear flow regime significantly reduces the flow rate through the fractal fractures, which could be quantified by the ratio of critical aperture to the minimum aperture in the fractal fractures. A large ratio of the maximum over the minimum apertures results in a large average flow rate in the fractal fractures. The increase in the minimum aperture also enhances the average flow rate. When the minimum aperture is close to the critical aperture, however, the flow rate in the fractal fractures starts to turn into nonlinear flow in all the fractures, and the average flow rate decreases. The nonlinear effect is amplified in fractal fractures compared to that in a single fracture. A larger fractal dimension of aperture leads to a lower average flow rate in the fractal fractures, as the average aperture decreases with the fractal dimension. However, the fraction of flow rate from the linear flow portion in the fractal fractures over the pure linear flow in all the fractures increases with the fractal dimension. Full article
(This article belongs to the Section Geophysical and Environmental Fluid Mechanics)
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17 pages, 2007 KiB  
Article
Low-Frequency Acoustic Emissions During Granular Discharge in Inclined Silos
by Josué Roberto Hernández-Juárez, Abel López-Villa, Abraham Medina and Daniel Armando Serrano Huerta
Fluids 2025, 10(5), 138; https://doi.org/10.3390/fluids10050138 - 20 May 2025
Viewed by 108
Abstract
In this work, experimental results on the generation of acoustic contributions during the discharge process of different granular materials from both vertical and inclined silos are presented. The experiments show that the generation of acoustic emissions associated with the “silo music” phenomenon occurs [...] Read more.
In this work, experimental results on the generation of acoustic contributions during the discharge process of different granular materials from both vertical and inclined silos are presented. The experiments show that the generation of acoustic emissions associated with the “silo music” phenomenon occurs not only in vertical silos but also in inclined ones. The acoustic signals produced during the silo discharge process are recorded and analysed in both time and frequency domains. The frequency analysis focuses on low frequencies near the lower auditory threshold of 20 Hz, demonstrating that the spectral components of the acoustic signals are related to the mass flow rate and the discharge velocity of the granular material. Full article
(This article belongs to the Collection Advances in Flow of Multiphase Fluids and Granular Materials)
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8 pages, 174 KiB  
Editorial
Recent Advances in Fluid Mechanics: Feature Papers, 2024
by Giuliano De Stefano and D. Andrew S. Rees
Fluids 2025, 10(5), 137; https://doi.org/10.3390/fluids10050137 - 20 May 2025
Viewed by 212
Abstract
The present Special Issue consists of a collection of feature articles by distinct investigators and research groups discussing new findings or cutting-edge developments concerning different aspects of fluid mechanics [...] Full article
(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2024)
30 pages, 7346 KiB  
Article
Numerical Analysis of Submerged Horizontal Plate Wave Energy Converter Device Considering Float Effects
by Rodrigo Costa Batista, Marla Rodrigues de Oliveira, Elizaldo Domingues dos Santos, Luiz Alberto Oliveira Rocha, Liércio André Isoldi and Mateus das Neves Gomes
Fluids 2025, 10(5), 136; https://doi.org/10.3390/fluids10050136 - 19 May 2025
Viewed by 248
Abstract
This study proposes a three-dimensional numerical wave tank (NWT) to calculate wave propagation and hydrodynamic forces based on the Navier–Stokes equation, using commercial Computational Fluid Dynamic (CFD) software ANSYS Fluent. The VOF Method is utilized to identify the free surface. The CFD model [...] Read more.
This study proposes a three-dimensional numerical wave tank (NWT) to calculate wave propagation and hydrodynamic forces based on the Navier–Stokes equation, using commercial Computational Fluid Dynamic (CFD) software ANSYS Fluent. The VOF Method is utilized to identify the free surface. The CFD model employed for generating waves in the NWT is initially verified using analytical theory to evaluate the accuracy of the results. In addition, the User-Defined Function (UDF) in ANSYS Fluent is implemented to ensure the model performs under the oscillatory conditions of the Submerged Horizontal Plate (SHP) Wave Energy Converter (WEC) device, which is localized at the center of the NWT. Finally, the influence of SHP oscillation on the device’s average efficiency was analyzed by comparing seven cases with different geometric configurations, considering both the oscillating and non-oscillating conditions of the SHP under the incidence of different waves. The results indicated that the geometric configuration and wave conditions of Case 4 achieved the best performance, reaching an average efficiency of 35.68%. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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15 pages, 6185 KiB  
Article
Investigating Moisture-Induced Particle Behavior in a Horizontal Shaft Mixer
by Minkyung Sim and Kwang Kim
Fluids 2025, 10(5), 135; https://doi.org/10.3390/fluids10050135 - 19 May 2025
Viewed by 175
Abstract
Grains stored in silos and pellets for injection molding deteriorate in quality due to increased moisture in the particles when exposed to air for a long period of time, so it is necessary to reduce the moisture in the particles through the mixing [...] Read more.
Grains stored in silos and pellets for injection molding deteriorate in quality due to increased moisture in the particles when exposed to air for a long period of time, so it is necessary to reduce the moisture in the particles through the mixing process. However, few studies have conducted parallel experiments and simulations to understand the behavior of particles depending on their moisture content. In this study, mixing experiments were conducted using superabsorbent polymer (SAP) beads that expand depending on the moisture content, and the interparticle friction coefficient and interface friction coefficient required for simulation were derived. As a result, it was found that moisture generates an adhesive force that causes interparticle cohesion, and as the moisture content increases further, the particles adhere to the vessel wall due to the adhesive force. In addition, particles with high moisture content (e.g., 90%) showed faster mixing behavior similar to dry particles, as indicated by the Lacey Mixing Index (LMI), while low moisture particles (e.g., 60%) showed the slowest mixing. It is expected that the mixing characteristics of particles depending on the moisture content can be understood and will be useful for the design of horizontal shaft mixers. Full article
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46 pages, 18159 KiB  
Review
Recent Developments in the Immersed Boundary Method for Complex Fluid–Structure Interactions: A Review
by Omkar Powar, Pedapudi Anantha Hari Arun, Anwak Manoj Kumar, Mithun Kanchan, B. M. Karthik, Poornesh Mangalore and Mohith Santhya
Fluids 2025, 10(5), 134; https://doi.org/10.3390/fluids10050134 - 16 May 2025
Viewed by 343
Abstract
The “immersed boundary method (IBM)” is considered to be the most efficacious and versatile technique to solve flow problems associated with intricate geometries. The first part of this review examines recent advancements in IBM, essential for the simulation of “fluid–structure interactions (FSIs)” in [...] Read more.
The “immersed boundary method (IBM)” is considered to be the most efficacious and versatile technique to solve flow problems associated with intricate geometries. The first part of this review examines recent advancements in IBM, essential for the simulation of “fluid–structure interactions (FSIs)” in sophisticated systems. This review highlights significant developments in turbulence modeling, adaptive mesh refinement, and complex geometric simulations, demonstrating IB methods’ capacity to seamlessly integrate arbitrary geometries into structured computational grids while preserving computational efficiency. Various IB techniques are analyzed for enforcing boundary conditions on dynamic immersed boundaries, with notable breakthroughs in managing velocity discontinuities, spurious oscillations, and large-scale deformations. Recent findings illustrate the versatility of IB methods, with applications encompassing biological fluid dynamics, turbulent multiphase flows, and cavitating flows. These innovations not only enhance computational performance but also address evolving challenges across engineering and scientific fields, establishing IB methods as a robust tool for resolving complex, multidisciplinary problems with high accuracy and efficiency. Full article
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40 pages, 4791 KiB  
Article
Modified Semi-Lagrangian Godunov-Type Method Without Numerical Viscosity for Shocks
by Valeriy Nikonov
Fluids 2025, 10(5), 133; https://doi.org/10.3390/fluids10050133 - 16 May 2025
Viewed by 121
Abstract
Most high-order Euler-type methods have been proposed to solve one-dimensional scalar hyperbolic conservational law. These methods resolve smooth variations in flow parameters accurately and simultaneously identify the discontinuities. A disadvantage of Euler-type methods is the parameter change stretching in the shock over a [...] Read more.
Most high-order Euler-type methods have been proposed to solve one-dimensional scalar hyperbolic conservational law. These methods resolve smooth variations in flow parameters accurately and simultaneously identify the discontinuities. A disadvantage of Euler-type methods is the parameter change stretching in the shock over a few mesh cells. In reality, in the shock, the flow properties change abruptly at once for the computational mesh. In our considerations, the mean free path of a flow particle is much smaller than the mesh cell size. This paper describes a modification of the semi-Lagrangian Godunov-type method, which was proposed by the author in the previously published paper. The modified method also does not have numerical viscosity for shocks. In the previous article, a linear law for the distribution of flow parameters was employed for a rarefaction wave when modeling the Shu-Osher problem with the aim of reducing parasitic oscillations. Additionally, the nonlinear law derived from the Riemann invariants was used for the remaining test problems. This article proposes an advanced method, namely, a unified formula for the density distribution of rarefaction waves and modification of the scheme for modeling moderately strong shock waves. The obtained results of numerical analysis, including the standard problem of Sod, the Riemann problem of Lax, the Shu–Osher shock-tube problem and a few author’s test cases are compared with the exact solution, the data of the previous method and the Total Variation Deminishing (TVD) scheme results. This article delineates the further advancement of the numerical scheme of the proposed method, specifically presenting a unified mathematical formulation for an expanded set of test problems. Full article
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38 pages, 22598 KiB  
Article
Assessing the Effect of Air Ventilation on the Dispersion of Exhaled Aerosol Particles in a Lecture Hall: Simulation Strategy and Streamlined Workflow
by Arnav Ajmani, Lars Kirchhof, Alireza Rouhi and Carsten Mehring
Fluids 2025, 10(5), 132; https://doi.org/10.3390/fluids10050132 - 15 May 2025
Viewed by 143
Abstract
An efficient solution strategy based on fluid network modeling, computational fluid dynamics (CFD) and discrete particle modeling (DPM) is presented in order to predict and improve air quality, specifically regarding breathing aerosol concentration, in a person-occupied mechanically ventilated room. The efficiency of the [...] Read more.
An efficient solution strategy based on fluid network modeling, computational fluid dynamics (CFD) and discrete particle modeling (DPM) is presented in order to predict and improve air quality, specifically regarding breathing aerosol concentration, in a person-occupied mechanically ventilated room. The efficiency of the proposed workflow is evaluated for the specific case of a lecture hall. It is found that the actual vent system is imbalanced and inefficient in managing the aerosol concentration within the room. Despite a high volumetric exchange rate, aerosol residence times and local aerosol concentrations remain high over an extended period of time, without additional efforts to alter air flow circulation throughout the room. The proposed strategy illustrates how such changes can be efficiently implemented in the basic 1D/3D co-simulation workflow. Analysis of the lecture hall and vent system shows that the execution time for the overall process workflow can be optimized by the following: (1) CAD geometry generation of the room via 3D laser scanning, (2) mesh generation based on the anticipated air discharge behavior from the vent system and (3) by employing HPC resources. Additional simplifications such as the decoupling of vent air flow and room aerodynamics, as observed for the investigated test case, one-way coupling between air flow and aerosol dispersion at low aerosol concentrations and the successive solution of flow field equations can further reduce the problem’s complexity and processing times. Full article
(This article belongs to the Special Issue Industrial CFD and Fluid Modelling in Engineering, 2nd Edition)
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22 pages, 11898 KiB  
Article
Impact of Viscous Droplets on Dry and Wet Substrates for Spray Painting Processes
by Qiaoyan Ye, Oliver Tiedje, Bo Shen and Joachim Domnick
Fluids 2025, 10(5), 131; https://doi.org/10.3390/fluids10050131 - 15 May 2025
Viewed by 210
Abstract
This paper presents numerical studies of the viscous droplet impact on dry and wetted solid walls for spray painting applications, focusing on air entrapment, film structure, and flake (flat pigment) orientation. The results were compared with experimental observations using various high-speed camera arrangements. [...] Read more.
This paper presents numerical studies of the viscous droplet impact on dry and wetted solid walls for spray painting applications, focusing on air entrapment, film structure, and flake (flat pigment) orientation. The results were compared with experimental observations using various high-speed camera arrangements. For paint droplet impact on dry substrates, a dynamic contact angle model was developed and used in numerical simulations. This contact angle model was verified with experimental observations. For the droplet impact on wet surfaces, characteristic crater sizes (diameter and depth) were defined considering also the effect of the film thickness. A strong correlation with the droplet impact Reynolds number was observed. In addition, a user-defined 6DOF (6-degrees-of-freedom) solver was implemented in a CFD program to perform calculations of rigid body motions within the impacting droplet, technically relevant for the resulting effect of flakes in metallic effect paints. The developed models were applied in parameter studies to further clarify the existing dependencies on application and fluid parameters more quantitatively. The simulation results are helpful to understand and to improve painting processes with respect to the final quality parameters. Full article
(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)
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16 pages, 2119 KiB  
Article
Steady Particulate Taylor-Couette Flow with Particle Migration
by C. Q. Ru
Fluids 2025, 10(5), 130; https://doi.org/10.3390/fluids10050130 - 14 May 2025
Viewed by 261
Abstract
Steady Taylor-Couette flow of a particle-fluid suspension with non-neutrally buoyant particles between two coaxial rotating cylinders is studied with a novel two-fluid model. It is shown that steady particle distribution in particulate Taylor-Couette flow can exist in the case when the solid walls [...] Read more.
Steady Taylor-Couette flow of a particle-fluid suspension with non-neutrally buoyant particles between two coaxial rotating cylinders is studied with a novel two-fluid model. It is shown that steady particle distribution in particulate Taylor-Couette flow can exist in the case when the solid walls are permeable where the particles and the fluid can be sucked or injected with equal but opposite normal fluxes. With this assumption, an explicit formula is derived for the axisymmetric steady radial distribution of particles with particle migration in the dilute limit. Detailed results for several cases of major interest show that the local rate of particle migration depends largely on the local azimuthal speed, and the steady volume fraction of particles typically attains its maximum (or minimum) at the location of minimum (or maximum) local azimuthal speed. In particular, with a wider gap between two cylinders and a Stokes number of particles around the order of unity, the non-monotonic radial distribution of particle volume fraction with interior local maximum and/or minimum can occur when the inner cylinder rotates with the outer cylinder fixed or when the two cylinders counter-rotate with equal but opposite angular velocities. Full article
(This article belongs to the Section Flow of Multi-Phase Fluids and Granular Materials)
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41 pages, 15728 KiB  
Review
A Review of Mesh Adaptation Technology Applied to Computational Fluid Dynamics
by Guglielmo Vivarelli, Ning Qin and Shahrokh Shahpar
Fluids 2025, 10(5), 129; https://doi.org/10.3390/fluids10050129 - 13 May 2025
Viewed by 371
Abstract
Mesh adaptation techniques can significantly impact Computational Fluid Dynamics by improving solution accuracy and reducing computational costs. In this review, we begin by defining the concept of mesh adaptation, its core components and the terminology commonly used in the community. We then categorise [...] Read more.
Mesh adaptation techniques can significantly impact Computational Fluid Dynamics by improving solution accuracy and reducing computational costs. In this review, we begin by defining the concept of mesh adaptation, its core components and the terminology commonly used in the community. We then categorise and evaluate the main adaptation strategies, focusing both on error estimation and mesh modification techniques. In particular, we analyse the two most prominent families of error estimation: feature-based techniques, which target regions of high physical gradients and goal-oriented adjoint methods, which aim to reduce the error in a specific integral quantity of interest. Feature-based methods are advantageous due to their reduced computational cost: they do not require adjoint solvers, and they have a natural ability to introduce anisotropy. A substantial portion of the literature relies on second-order derivatives of scalar flow quantities to construct sensors that can be equidistributed to minimise discretisation error. However, when used carelessly, these methods can lead to over-refinement, and they are generally outperformed by adjoint-based techniques when improving specific target quantities. Goal-oriented methods typically achieve higher accuracy in fewer adaptation steps with coarser meshes. It will be seen that various approaches have been developed to incorporate anisotropy into adjoint-based adaptation, including hybrid error sensors that combine feature-based and goal-oriented indicators, sequential strategies and adjoint weighting of fluxes. After years of limited progress, recent work has demonstrated promising results, including certifiable solutions and applications to increasingly complex cases such as transonic compressor blades and film-cooled turbines. Despite these advances, several critical challenges remain: efficient parallelisation, robust geometry integration, application to unsteady flows and deployment in high-order discretisation frameworks. Finally, examples of the potential role of artificial intelligence in guiding or accelerating mesh adaptation are also discussed. Full article
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25 pages, 1437 KiB  
Review
Review of the Color Gradient Lattice Boltzmann Method for Simulating Multi-Phase Flow in Porous Media: Viscosity, Gradient Calculation, and Fluid Acceleration
by Fizza Zahid and Jeffrey A. Cunningham
Fluids 2025, 10(5), 128; https://doi.org/10.3390/fluids10050128 - 13 May 2025
Viewed by 298
Abstract
The lattice Boltzmann method (LBM) is widely applied to model the pore-scale two-phase flow of immiscible fluids through porous media, and one common variant of the LBM is the color gradient method (CGM). However, in the literature, many competing algorithms have been proposed [...] Read more.
The lattice Boltzmann method (LBM) is widely applied to model the pore-scale two-phase flow of immiscible fluids through porous media, and one common variant of the LBM is the color gradient method (CGM). However, in the literature, many competing algorithms have been proposed for accomplishing different steps in the CGM. Therefore, this paper is the first in a series that aims to critically review and evaluate different algorithms and methodologies that have been proposed for use in the CGM. Specifically, in this paper, we (1) provide a brief introduction to the LBM and CGM that enables and facilitates consideration of more sophisticated topics subsequently; (2) compare three methods for modeling the behavior of fluids of moderately different viscosities; (3) compare two methods for calculating the color gradient; and (4) compare two methods for modeling external forces or accelerations acting upon the fluids of interest. These topics are selected for the first paper in the series because proper selection of these algorithms is necessary and sufficient to perform two common “benchmark” simulations, namely bubble tests and layered Poiseuille flow. Future papers in the series will build upon these topics, considering more challenging conditions or phenomena. By systematically reviewing key aspects, features, capabilities, and limitations of the CGM, this series of papers will extend our collective ability to apply the method to a variety of important fluid flow problems in geosciences and engineering. Full article
(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2024)
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10 pages, 3383 KiB  
Article
Droplets at Liquid-Fluid Interfaces: Stages Leading to Coalescence
by Jose Davalos-Monteiro, Qi Liu and J. Carlos Santamarina
Fluids 2025, 10(5), 127; https://doi.org/10.3390/fluids10050127 - 12 May 2025
Viewed by 198
Abstract
Droplet coalescence at interfaces affects industrial and natural processes. Previous studies focused on droplet stability and thin film drainage. We use meticulous experiments to infer the evolution of coalescence for both ascending and descending droplets under different conditions. Images show the anticipatory deformation [...] Read more.
Droplet coalescence at interfaces affects industrial and natural processes. Previous studies focused on droplet stability and thin film drainage. We use meticulous experiments to infer the evolution of coalescence for both ascending and descending droplets under different conditions. Images show the anticipatory deformation of the interface and dimple formation during the approach phase. While the droplet rests at the interface, the two surfaces interact through the draining thin film, and the effective interfacial tension can be higher than twice the interfacial tension between the two fluids, suggesting not only concurrent action but also potential changes in interfacial tension in thin films. Following the film breakage, the unbalanced force propels the droplet into the continuous phase, i.e., the slingshot effect. Multiple droplets may coexist at the interface and collectively contribute to its deformation, which in turn pushes the droplets together. The various stages of droplet coalescence are influenced by the droplet and host fluid viscosities, densities, interfacial tension, size, and initial interface curvature. Full article
(This article belongs to the Section Flow of Multi-Phase Fluids and Granular Materials)
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12 pages, 5641 KiB  
Article
A Numerical Investigation of Sinusoidal Flow in Porous Media with a Simple Cubic Beam Structure at 1 Hz and 100 Hz Under Different Porosity Conditions
by Sin-Mao Chen, Boe-Shong Hong and Shiuh-Hwa Shyu
Fluids 2025, 10(5), 126; https://doi.org/10.3390/fluids10050126 - 12 May 2025
Viewed by 242
Abstract
This study aims to clarify how porosity and frequency interact to influence permeability and flow behavior in porous media subjected to sinusoidal pressure variations. Specifically, we investigate oscillatory flow at 1 Hz and 100 Hz under varying porosity conditions using a pore-scale Computational [...] Read more.
This study aims to clarify how porosity and frequency interact to influence permeability and flow behavior in porous media subjected to sinusoidal pressure variations. Specifically, we investigate oscillatory flow at 1 Hz and 100 Hz under varying porosity conditions using a pore-scale Computational Fluid Dynamics (CFD) model. The model is validated against the Johnson–Koplik–Dashen (JKD) model to ensure accuracy in capturing dynamic permeability. At 1 Hz, where the oscillation period greatly exceeds the system’s time constant τ, the flow reaches a quasi-steady state with dynamic permeability approximating static permeability. Increasing porosity enhances Darcy velocity, with minimal phase difference between velocity and pressure. At 100 Hz, flow behavior depends on the ratio of the oscillation period T to τ. For high porosity (φ=0.840, T≈τ), the flow does not fully develop before the pressure gradient reverses, leading to significant phase lag. For low porosity (φ=0.370, T≈12τ), the phase lag is smaller but remains non-zero due to the smooth temporal variation in pressure. This work contributes to the understanding of porous flow dynamics by revealing how porosity modulates both the amplitude and phase angle of dynamic permeability in frequency-dependent porous flows, providing a framework for predicting phase lag in frequency-sensitive applications. Full article
(This article belongs to the Collection Feature Paper for Mathematical and Computational Fluid Mechanics)
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94 pages, 11117 KiB  
Review
An Overview of Viscous and Highly Viscous Fluid Flows in Straight and Elbow Pipes: I—Single-Phase Flows
by Leonardo Di G. Sigalotti and Enrique Guzmán
Fluids 2025, 10(5), 125; https://doi.org/10.3390/fluids10050125 - 11 May 2025
Viewed by 365
Abstract
The flow of viscous and highly viscous fluids in straight and bent pipes and channels is a fundamental process in a wide variety of industrial applications and is, therefore, of great interest in science and engineering. Understanding the physics behind such flows has [...] Read more.
The flow of viscous and highly viscous fluids in straight and bent pipes and channels is a fundamental process in a wide variety of industrial applications and is, therefore, of great interest in science and engineering. Understanding the physics behind such flows has a direct impact on the design of efficient, safe and reliable systems. The type of fluid, which can be viscous or even highly viscous, and the pipe geometry can affect the flow dynamics, the pressure loss and the overall efficiency of the process. In this paper, we provide an extensive review of the state-of-the-art research concerning the flow of Newtonian and non-Newtonian, single-phase fluids in straight and bent pipes. Since a big amount of work in the literature is devoted to the study of Newtonian pipe flows, the paper starts with a brief outline of the nonlinear theory of viscous Newtonian fluid flow in pipes, including a survey of early and recent analytical solutions to the Navier–Stokes equations. The central part of the paper deals with an extensive overview of existing experimental and numerical research work on viscous Newtonian pipe flows. Separate sections are devoted to non-Newtonian fluid flows, the problem of entropy generation due to irreversible processes in the flow and hydromagnetic Newtonian and non-Newtonian pipe flow. The review closes with a brief survey of machine learning and artificial intelligence modeling applied to pipe flow along with future trends and challenges in pipe flow research. Full article
(This article belongs to the Special Issue Pipe Flow: Research and Applications, 2nd Edition)
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16 pages, 6209 KiB  
Article
Numerical Simulation of Blood Clot Extraction Process Using Aspiration-Based Mechanical Thrombectomy
by Sreenivas Venguru, Shyam Sunder Yadav, Tanmaya Mahapatra and Sanjay Kumar Kochar
Fluids 2025, 10(5), 124; https://doi.org/10.3390/fluids10050124 - 9 May 2025
Viewed by 232
Abstract
This paper simulates the blood clot extraction process inside an idealized cylindrical blood vessel model using the aspiration-based thrombectomy technique. A fully Eulerian technique is used within the finite volume method where incompressible Navier–Stokes equations are solved in the fluid region. In contrast, [...] Read more.
This paper simulates the blood clot extraction process inside an idealized cylindrical blood vessel model using the aspiration-based thrombectomy technique. A fully Eulerian technique is used within the finite volume method where incompressible Navier–Stokes equations are solved in the fluid region. In contrast, the Cauchy stress equation is solved in the clot region. Blood is assumed to be a Newtonian fluid, while the clot is either hyperelastic or viscoelastic material. In the hyperelastic formulation, the clot deformation is calculated based on the left Cauchy–Green deformation tensor, while the stresses are based on the linear Mooney–Rivlin model. In the viscoelastic formulation, the Oldroyd B model is used within the log conformation approach to calculate the viscoelastic stresses in the clot. The interface between the blood and the clot is tracked with the help of the geometric volume-of-fluid method. We focus on the role of flow variables like the pressure, velocity, and proximity between the clot and the catheter tip to successfully capture the clot under catheter suction. We observe that, once the clot is attracted to the catheter port due to pressure forces, the viscous stresses try to drag it inside the catheter. On the other hand, if the clot is not initially attracted, it is carried downstream by the viscous stresses. If the suction velocity is low (∼0.2 m/s), the clot cannot be sucked inside the catheter, even if it is touching the catheter. At a higher suction velocity of 0.4 m/s, the suction effect is strong enough to capture the clot despite the larger initial distance from the catheter. Hence, the pressure distribution and viscous stresses play essential roles in the suction or escape of the clot during the thrombectomy process. Also, the viscoelastic model predicts the rupture of the clot inside the catheter during suction. Full article
(This article belongs to the Special Issue Advances in Hemodynamics and Related Biological Flows)
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23 pages, 6282 KiB  
Article
Computational Modeling of Droplet-Based Printing Using Multiphase Volume of Fluid (VOF) Method: Prediction of Flow, Spread Behavior, and Printability
by Rauf Shah and Ram V. Mohan
Fluids 2025, 10(5), 123; https://doi.org/10.3390/fluids10050123 - 8 May 2025
Viewed by 257
Abstract
The evolution of droplets during the printing process is modeled using the volume of fluid (VOF) method, which involves solving the Navier–Stokes and continuity equations for incompressible flow with multiple immiscible phases on a finite volume grid. An indicator function tracks the interfaces [...] Read more.
The evolution of droplets during the printing process is modeled using the volume of fluid (VOF) method, which involves solving the Navier–Stokes and continuity equations for incompressible flow with multiple immiscible phases on a finite volume grid. An indicator function tracks the interfaces and calculates surface tension forces. A grid independence study confirmed the convergence and efficacy of the solutions. The computational model agreed well with experimental data, accurately capturing the impact, spreading, and recoiling of droplets on a solid surface. Additionally, the model validated the interaction of droplets with hydrophilic and hydrophobic surfaces for both constant and dynamic contact angles. Key non-dimensional numbers (Re, We, Oh) were considered to study the interplay of forces during droplet impact on a solid surface. The final print quality is influenced by droplet dynamics, governed by body forces (surface tension, gravity), contact angle, dissipative forces due to motion, and material properties. Computational studies provide insights into the overall process performance and final print quality under various process conditions and material properties. Full article
(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)
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21 pages, 11237 KiB  
Article
Investigation of Heat Transfer Enhancement Mechanisms in Elastic Tube Bundles Subjected to Exogenous Self-Excited Fluid Oscillation
by Jing Hu, Lei Guo and Shusheng Zhang
Fluids 2025, 10(5), 122; https://doi.org/10.3390/fluids10050122 - 8 May 2025
Viewed by 212
Abstract
Flow-induced vibration (FIV) characteristics are key factors in enhancing heat transfer. However, challenges such as insufficient heat transfer enhancement and the fatigue strength of the tube bundle persist in the context of improving the heat transfer in elastic tube bundle heat exchangers. This [...] Read more.
Flow-induced vibration (FIV) characteristics are key factors in enhancing heat transfer. However, challenges such as insufficient heat transfer enhancement and the fatigue strength of the tube bundle persist in the context of improving the heat transfer in elastic tube bundle heat exchangers. This study proposes a novel passive heat transfer enhancement paradigm for elastic tube bundles based on externally induced self-excited oscillations of fluid. By constructing a non-contact energy transfer system, the external oscillation energy is directed into the elastic tube bundle heat exchanger, achieving dynamic stress buffering and breaking through the steady-state flow heat transfer boundary layer. A three-dimensional fluid–structure interaction numerical model is established using Star CCM+2021.3 (16.06.008) to conduct a comparative analysis of the flow characteristics and heat transfer performance between the original structure without an oscillator and the improved structure equipped with a fluid oscillator. The results indicate that the improved structure, through the periodic unsteady jet induced by the fluid oscillator, significantly enhances the turbulence intensity of the shell-side fluid, with the turbulent kinetic energy increasing by over 50%. The radial flow area is notably expanded, thereby reducing the thermal resistance of the boundary layer. At cooling fluid velocities of 6 to 9 m/s, the heat transfer capability of the improved structure is enhanced by more than 50%. Compared with the original structure, the new structure, due to the loading of an external oscillation structure, causes the cold air to present a periodic up and down jet phenomenon. This jet phenomenon, on the one hand, increases the heat exchange area between the cold air and the outer surface of the tube bundle, thereby enhancing the heat exchange capacity. On the other hand, the large-area impact of the fluid reduces the thickness of the boundary layer, lowers the thermal resistance and thereby enhances the heat exchange capacity. Furthermore, this improved structure buffers the mechanical vibrations through self-excited oscillations of the fluid medium, ensuring that the stress levels in the tube bundle remain below the fatigue threshold, effectively mitigating the failure risks associated with traditional active vibration strategies. Full article
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26 pages, 10313 KiB  
Article
Design of a Portable Integrated Fluid–Structure Interaction-Based Piezoelectric Flag Energy-Harvesting System
by Haochen Wang, Xingrong Huang, Zhe Li and Le Fang
Fluids 2025, 10(5), 121; https://doi.org/10.3390/fluids10050121 - 8 May 2025
Viewed by 221
Abstract
Fluid–structure interaction-based energy-harvesting technology has gained significant attention due to its potential for energy conversion. However, most existing studies primarily focus on energy capture, resulting in incomplete systems with limited portability and a lack of integrated circuitry. To address these limitations, this study [...] Read more.
Fluid–structure interaction-based energy-harvesting technology has gained significant attention due to its potential for energy conversion. However, most existing studies primarily focus on energy capture, resulting in incomplete systems with limited portability and a lack of integrated circuitry. To address these limitations, this study presents a portable, integrated piezoelectric flag energy-harvesting system that achieves a complete closed-loop conversion from fluid kinetic energy, through structural strain energy, to electrical energy. The system utilizes an upstream bluff body to generate vortex-induced vibrations, a downstream support structure that maintains operational stability, and an internally integrated wiring channel that enables overall energy conversion. Charge–discharge experiments on the energy storage unit enable a comprehensive evaluation of system performance, marking the first efficiency measurement of a fully integrated energy-harvesting system. Experimental results demonstrate the first quantified map of losses across all conversion stages in a portable piezo-flag platform, highlighting the system’s potential for powering small-scale, low-power self-sustaining devices. This work establishes a reference framework and provides a novel technological pathway for advancing practical applications of fluid-induced energy harvesting, contributing to the development of autonomous power sources in various engineering fields. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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21 pages, 7419 KiB  
Article
On Numerical Simulations of Turbulent Flows over a Bluff Body with Aerodynamic Flow Control Based on Trapped Vortex Cells: Viscous Effects
by Dmitry A. Lysenko
Fluids 2025, 10(5), 120; https://doi.org/10.3390/fluids10050120 - 8 May 2025
Viewed by 211
Abstract
Turbulent flows over a semi-circular cylinder (a limiting case of a thick airfoil with a chord equal to the diameter base) are investigated using high-fidelity large-eddy simulations at a diameter-based Reynolds number, Re = 130,000, Mach number, M = 0.05, and a zero [...] Read more.
Turbulent flows over a semi-circular cylinder (a limiting case of a thick airfoil with a chord equal to the diameter base) are investigated using high-fidelity large-eddy simulations at a diameter-based Reynolds number, Re = 130,000, Mach number, M = 0.05, and a zero angle of attack. The aerodynamic flow control system, designed with two trapped vortex cells, achieves a complete non-separated flow over the bluff body, except for low-scale turbulence effects, reaching approximately 80% of the theoretical lift coefficient limit (2π for the half-circular airfoil). Viscous effects are analyzed using the conventional Reynolds-averaged Navier–Stokes approach for a broad range of Reynolds numbers, 75,000 ≤ Re ≤ 1,000,000. Numerical results demonstrate that the aerodynamic properties of the implemented concept are independent of the Reynolds number within this interval, highlighting its significant potential for further development. Full article
(This article belongs to the Collection Feature Paper for Mathematical and Computational Fluid Mechanics)
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21 pages, 8781 KiB  
Article
Optimizing the Mobile Pump and Its Equipment to Reduce the Risk of Pluvial Flooding
by Horas Yosua, Muhammad Syahril Badri Kusuma, Joko Nugroho, Eka Oktariyanto Nugroho and Deni Septiadi
Fluids 2025, 10(5), 119; https://doi.org/10.3390/fluids10050119 - 7 May 2025
Viewed by 163
Abstract
Pluvial flooding in South Jakarta poses significant economic disruptions, requiring efficient mitigation strategies. This study focuses on optimizing mobile pump deployment as a non-structural flood control measure. Despite the use of mobile pumps in flood response, there is limited research on their systematic [...] Read more.
Pluvial flooding in South Jakarta poses significant economic disruptions, requiring efficient mitigation strategies. This study focuses on optimizing mobile pump deployment as a non-structural flood control measure. Despite the use of mobile pumps in flood response, there is limited research on their systematic optimization for pluvial flood mitigation. This study presents a transferable framework for deploying mobile pumps to mitigate pluvial flood risks in urban areas, demonstrated through a case study in South Jakarta, Indonesia. The findings indicate that flood depths of 75 cm have a 20–50% probability of occurrence, and rainfall in South Jakarta follows a distinct hourly distribution, with 56.6% of the rainfall occurring in the first hour and 43.4% in the second. Radar imagery from the BMKG is used here as the main tool for real-time rainfall detection. The optimization framework considers channel capacity, flood frequency, impact severity, accessibility, and operational protocols. Among 29 flood-prone locations analyzed, 8 of them require mobile pump intervention. Seven locations benefit from integration with weather prediction tools and SCADA systems, while three require dedicated operational procedures (SOPs). Simulation results indicate that placing mobile pumps near the upstream section of the flooded area yields the most effective flood reduction. A minimum pump capacity of 0.5 m3/s is recommended for optimal performance. This study demonstrates that strategic mobile pump deployment, coupled with predictive tools, significantly reduces pluvial flood risks in South Jakarta and offers a transferable framework for other urban areas. Full article
(This article belongs to the Special Issue Environmental Hydraulics, Turbulence and Sediment Transport, 3rd Edition)
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29 pages, 13402 KiB  
Article
Modeling Microplastic Dispersion in the Salado Estuary Using Computational Fluid Dynamics
by Luis Velazquez-Araque, José Flor, Alfredo Méndez and Maritza Cárdenas-Calle
Fluids 2025, 10(5), 118; https://doi.org/10.3390/fluids10050118 - 6 May 2025
Viewed by 305
Abstract
Microplastics (MPs) have emerged as a major pollutant in aquatic ecosystems, primarily originating from industrial activities and plastic waste degradation. Understanding their transport dynamics is crucial for assessing environmental risks and developing mitigation strategies. This study employs Computational Fluid Dynamics (CFD) simulations to [...] Read more.
Microplastics (MPs) have emerged as a major pollutant in aquatic ecosystems, primarily originating from industrial activities and plastic waste degradation. Understanding their transport dynamics is crucial for assessing environmental risks and developing mitigation strategies. This study employs Computational Fluid Dynamics (CFD) simulations to model the trajectory of MPs in section B of the Salado Estuary in the city of Guayaquil, Ecuador, using ANSYS FLUENT 2024 R2. The transient behavior of Polyethylene Terephthalate (PET) particles was analyzed using the Volume of Fluid (VOF) multiphase model, k-omega SST turbulence model, and Discrete Phase Model (DPM) under a continuous flow regime. Spherical PET particles (5 mm diameter, 1340 kg/m3 density) were used to establish a simplified baseline scenario. Two water velocities, 0.5 m/s and 1.25 m/s, were selected based on typical flow rates reported in similar estuarine systems. Density contour analysis facilitated the modeling of the air-water interface, while particle trajectory analysis revealed that at 0.5 m/s, particles traveled 18–22.5 m before sedimentation, whereas at 1.25 m/s, they traveled 50–60 m before reaching the bottom. These findings demonstrate that higher flow velocities enhance MP transport distances before deposition, emphasizing the role of hydrodynamics in microplastic dispersion. While limited to one particle type and idealized conditions, this study underscores the potential of CFD as a predictive tool for assessing MP behavior in aquatic environments, contributing to improved pollution control and remediation efforts. Full article
(This article belongs to the Section Flow of Multi-Phase Fluids and Granular Materials)
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37 pages, 1062 KiB  
Review
The Universal Presence of the Reynolds Number
by Aldo Tamburrino and Yarko Niño
Fluids 2025, 10(5), 117; https://doi.org/10.3390/fluids10050117 - 1 May 2025
Viewed by 334
Abstract
The Reynolds number is a fundamental parameter in fluid dynamics, initially introduced by O. Reynolds in 1883 to characterize the transition between laminar and turbulent flow in fluids and necessary in the scaling of viscous resistance. Over time, its application has expanded significantly, [...] Read more.
The Reynolds number is a fundamental parameter in fluid dynamics, initially introduced by O. Reynolds in 1883 to characterize the transition between laminar and turbulent flow in fluids and necessary in the scaling of viscous resistance. Over time, its application has expanded significantly, becoming essential for studying a vast range of fluid phenomena—from microscopic scales such as cellular motion to macroscopic scales like turbulent flows and even intergalactic dynamics. The article highlights the universal relevance of the Reynolds number across various fields, including its adaptation to non-Newtonian fluids and granular flows. It emphasizes how the Reynolds number has evolved from a simple dimensionless group to a critical tool for understanding complex physical processes across different scales and environments. Full article
(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2024)
24 pages, 10198 KiB  
Article
Analysis of Two Protection Strategies for Reducing Aerosol Expulsion from Wind Instruments
by Miriam Baron, Bogac Tur, Marie Köberlein, Laila Ava Hermann, Sophia Gantner, Matthias Echternach and Stefan Kniesburges
Fluids 2025, 10(5), 116; https://doi.org/10.3390/fluids10050116 - 30 Apr 2025
Viewed by 146
Abstract
(1) Background: the aim of this study is to assess the effectiveness of two protection systems for aerosol cloud reduction while playing different wind instruments. (2) Methods: The protection systems used were a cotton molton construction combined with a bell filter attached at [...] Read more.
(1) Background: the aim of this study is to assess the effectiveness of two protection systems for aerosol cloud reduction while playing different wind instruments. (2) Methods: The protection systems used were a cotton molton construction combined with a bell filter attached at the bell of the instruments, as well as a household paper towel. For visualization of the emitted aerosol particles, e-cigarettes were used. With three full HD cameras, cloud dispersion was captured in the forward, sideways, and upwards directions. The effectiveness of aerosol spread reduction was statistically evaluated. (3) Results: Without protection, aerosol clouds dispersed, on average, up to 1.23 m in the forward direction, 0.46 m sideways, and 0.86 m upwards. The cotton molton mask reduced forward spread by 42%, while the paper towel achieved a 15% reduction, although both systems increased lateral and vertical dispersion. Specifically, the cotton molton mask yielded a 9% increase to the side and 7% in the upward direction, while the paper towel resulted in a 66% increase to the sides and a 10% increase in the upward direction. The cotton molton mask’s effectiveness was attributed to its additional coverage of the tone holes, which contribute to aerosol emission in woodwind instruments. A statistical analysis via the Friedman test confirmed significant reductions in forward dispersion with the cotton molton system. (4) Conclusions: Protective systems can partially reduce aerosol emissions. However, these alone are not sufficient, and further measures to reduce the spread of particles are necessary. Full article
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22 pages, 4445 KiB  
Article
Research on Dual-Mode Self-Calibration Tensioning System
by Xuling Liu, Yusong Zhang, Chaofeng Peng, Le Bo, Kaiyi Zhang, Guoyong Ye, Jinggan Shao, Jinghui Peng and Songjing Li
Fluids 2025, 10(5), 115; https://doi.org/10.3390/fluids10050115 - 30 Apr 2025
Viewed by 184
Abstract
In this paper, a double-mode self-calibration tension system is proposed, which adopts the conversion of hydraulic meter tension and the monitoring of standard force sensors. According to the material characteristics of the jack and the viscosity and temperature characteristics of the hydraulic oil, [...] Read more.
In this paper, a double-mode self-calibration tension system is proposed, which adopts the conversion of hydraulic meter tension and the monitoring of standard force sensors. According to the material characteristics of the jack and the viscosity and temperature characteristics of the hydraulic oil, the differential model of heat conduction in the hydraulic cylinder and the mathematical model of oil film friction heat generation are established, and the internal thermodynamic characteristics of the jack are theoretically analyzed, which provides theoretical support for the temperature compensation of the hydraulic oil pressure gauge of the jack. A simulation analysis was conducted on the thermodynamic characteristics of the hydraulic jack, and the distribution patterns of the temperature field, thermal stress field, and thermal strain field inside the hydraulic cylinder during normal operation were determined by measuring the temperature changes in five different parts of the jack at different times (t = 200 s, 2600 s, 5000 s, 7400 s, and 10,000 s). For the issue of heat generation due to oil film friction in the hydraulic jack, a simulation calculation model is developed by integrating Computational Fluid Dynamics (CFD) techniques with dynamic grid and slip grid methods. By simulating and analyzing frictional heating under conditions where the inlet pressures are 0.1 MPa, 0.3 MPa, 0.5 MPa, 0.7 MPa, and 0.9 MPa, respectively, we can obtain the temperature distribution on the jack, determine the frictional resistance, and subsequently conduct a theoretical analysis of the simulation results. Using the high-precision standard force sensor after data processing and the hydraulic oil gauge after temperature compensation, the online self-calibration of the tensioning system is carried out, and the regression equation of the tensioning system under different oil temperatures is obtained. The double-mode self-calibration tensioning system with temperature compensation is used to verify the compensation accuracy of the proposed double-mode self-calibration tensioning system. Full article
(This article belongs to the Topic Applied Heat Transfer)
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22 pages, 3296 KiB  
Article
Performance of an L-Shaped Duct OWC-WEC Integrated into Vertical and Sloped Breakwaters by Using a Free-Surface RANS-Based Numerical Model
by Eric Didier and Paulo R. F. Teixeira
Fluids 2025, 10(5), 114; https://doi.org/10.3390/fluids10050114 - 30 Apr 2025
Viewed by 260
Abstract
Waves generated by the wind in oceans and seas have a significant available quantity of clean and renewable energy. However, harvesting their energy is still a challenge. The integration of an oscillating water column (OWC) wave energy converter into a breakwater leads to [...] Read more.
Waves generated by the wind in oceans and seas have a significant available quantity of clean and renewable energy. However, harvesting their energy is still a challenge. The integration of an oscillating water column (OWC) wave energy converter into a breakwater leads to more viability, since it allows working as both harbor and coastal protection and harvesting wave energy. The main objective of this study is to investigate different configurations of L-shaped duct OWC devices inserted into vertical and sloped (2:3) impermeable breakwaters for different lengths of the lip by using a numerical model based on the Reynolds-Averaged Navier-Stokes equations. The ANSYS FLUENT® software (2016) is used in 2D numerical simulations by adopting the volume of fluid method to consider the two-phase free surface flow (water and air). It was observed that both the length of the lip and the length of the L-shaped duct OWC significantly influence the resonance and the efficiency of the OWC device. In addition, the performance of the OWC device varies significantly with its geometric configuration, which needs to be adapted for the local sea state. Full article
(This article belongs to the Special Issue Computational Fluid Dynamics Applied to Transport Phenomena)
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26 pages, 9399 KiB  
Article
Investigation of Multiphase Flow in Continuous-Casting Water Model with Measurements and Computational Modeling
by Hamed Olia, Dylan Palmer, Ehsan Jebellat and Brian G. Thomas
Fluids 2025, 10(5), 113; https://doi.org/10.3390/fluids10050113 - 28 Apr 2025
Viewed by 283
Abstract
This work introduces a 0.6-scale water model of the continuous slab-casting process and a MATLAB-based model to study the effects of non-primed and multiphase flow on pressure and flow rate. The water model uses stopper-rod flow control and features pressure and velocity measurements [...] Read more.
This work introduces a 0.6-scale water model of the continuous slab-casting process and a MATLAB-based model to study the effects of non-primed and multiphase flow on pressure and flow rate. The water model uses stopper-rod flow control and features pressure and velocity measurements at multiple locations. The new computational model, PFSR V4 (Pressure-drop Flow-rate model of Stopper Rod metal delivery systems, Version 4), improves upon a prior one-dimensional Bernoulli-based framework by incorporating a bubble accumulation zone. This zone represents a region of bubbly flow with an intermediate gas fraction between constant-pressure gas pockets below the stopper tip and the downstream bubbly flow regime. Parametric studies with the water model show that flow remains fully primed at low gas flow rates but transitions to non-primed flow as the gas flow rate exceeds 10–16 SLPM. Three different flow regions are observed inside the water model nozzle: air pocket, bubble accumulation, and bubbly flow, which are also captured by the new computational model. Above a critical gas flow rate, the flow becomes unstable and difficult to control, though higher hot gas flow rates are expected for similar transitions in a real steel caster due to gas expansion at high temperatures. Pressure changes are minimal in the air pocket region and increase significantly in the upper bubble accumulation zone, where liquid velocity is much higher than in the classic bubbly-flow region, found lower in the nozzle. The new model was successfully calibrated to match the observed flow regimes and shows good agreement with the water-model measurements. Full article
(This article belongs to the Special Issue Multiphase Flow for Industry Applications)
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19 pages, 5624 KiB  
Article
Research on the Improvement of BEM Method for Ultra-Large Wind Turbine Blades Based on CFD and Artificial Intelligence Technologies
by Shiyu Yang, Mingming Zhang, Yu Feng, Haikun Jia, Na Zhao and Qingwei Chen
Fluids 2025, 10(5), 112; https://doi.org/10.3390/fluids10050112 - 27 Apr 2025
Viewed by 248
Abstract
With the development of the wind power industry, wind turbine blades are increasingly adopting ultra-large-scale designs. However, as the size of blades continues to increase, existing aerodynamic calculation methods struggle to achieve both relatively high computational accuracy and efficiency simultaneously. To tackle this [...] Read more.
With the development of the wind power industry, wind turbine blades are increasingly adopting ultra-large-scale designs. However, as the size of blades continues to increase, existing aerodynamic calculation methods struggle to achieve both relatively high computational accuracy and efficiency simultaneously. To tackle this challenge, this research focuses on the low accuracy issues of the traditional Blade Element Momentum theory (BEM) in predicting the aerodynamic performance of wind turbine blades. Consequently, a correction framework is proposed, to integrate the Computational Fluid Dynamics (CFD) method with the Multilayer Perceptron (MLP) neural network. In this approach, the CFD method is used to predict the airflow characteristics around the blades, and the MLP neural network is employed to model the intricate functional relationships between multiple influencing factors and key aerodynamic parameters. This process results in high-precision predictive functions for key aerodynamic parameters, which are then used to correct the traditional BEM. When this correction framework is applied to the rotor of the IEA 15 MW wind turbine, the effectiveness of MLP in predicting key aerodynamic parameters is demonstrated. The research findings suggest that this framework can enhance the accuracy of BEM aerodynamic load predictions to a level comparable to that of RANS. Full article
(This article belongs to the Special Issue Drag Reduction Through Traditional or Machine Learning-Based Flow Control)
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15 pages, 326 KiB  
Article
Weakly Nonlinear Instability of a Convective Flow in a Plane Vertical Channel
by Natalja Budkina, Valentina Koliskina, Andrei Kolyshkin and Inta Volodko
Fluids 2025, 10(5), 111; https://doi.org/10.3390/fluids10050111 - 26 Apr 2025
Viewed by 163
Abstract
The weakly nonlinear stability analysis of a convective flow in a planar vertical fluid layer is performed in this paper. The base flow in the vertical direction is generated by internal heat sources distributed within the fluid. The system of Navier–Stokes equations under [...] Read more.
The weakly nonlinear stability analysis of a convective flow in a planar vertical fluid layer is performed in this paper. The base flow in the vertical direction is generated by internal heat sources distributed within the fluid. The system of Navier–Stokes equations under the Boussinesq approximation and small-Prandtl-number approximation is transformed to one equation containing a stream function. Linear stability calculations with and without a small-Prandtl-number approximation lead to the range of the Prantdl numbers for which the approximation is valid. The method of multiple scales in the neighborhood of the critical point is used to construct amplitude evolution equation for the most unstable mode. It is shown that the amplitude equation is the complex Ginzburg–Landau equation. The coefficients of the equation are expressed in terms of integrals containing the linear stability characteristics and the solutions of three boundary value problems for ordinary differential equations. The results of numerical calculations are presented. The type of bifurcation (supercritical bifurcation) predicted by weakly nonlinear calculations is in agreement with experimental data. Full article
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20 pages, 3854 KiB  
Article
EHD Instability Modes of Power-Law Fluid Jet Issuing in Gaseous Streaming via Permeable Media
by Mohamed F. El-Sayed, Mohamed F. E. Amer and Doaa M. Mostafa
Fluids 2025, 10(5), 110; https://doi.org/10.3390/fluids10050110 - 25 Apr 2025
Viewed by 270
Abstract
The instability of a non-Newtonian dielectric fluid jet of power-law (P-L) type injected when streaming dielectric gas through porous media is examined using electrohydrodynamic (EHD) linear analysis. The interfacial boundary conditions (BCs) are used to derive the dispersion relation for both shear-thinning (s-thin) [...] Read more.
The instability of a non-Newtonian dielectric fluid jet of power-law (P-L) type injected when streaming dielectric gas through porous media is examined using electrohydrodynamic (EHD) linear analysis. The interfacial boundary conditions (BCs) are used to derive the dispersion relation for both shear-thinning (s-thin) and shear-thickening (s-thick) fluids. A detailed discussion is outlined on the impact of dimensionless flow parameters. The findings show that jet breakup can be categorized into two instability modes: Rayleigh (RM) and Taylor (TM), respectively. For both fluids, the system in TM is found to be more unstable than that found in RM, and, for s-thick fluids, it is more unstable. For all P-L index values, the system is more unstable if a porous material exists than when it does not. It is demonstrated that the generalized Reynolds number (Ren), Reynolds number (Re), P-L index, dielectric constants, gas-to-liquid density, and viscosity ratios have destabilizing influences; moreover, the Weber number (We), electric field (EF), porosity, and permeability of the porous medium have a stabilizing impact. Depending on whether its value is less or more than one, the velocity ratio plays two different roles in stability, and the breakup length and size of P-L fluids are connected to the maximal growth level and the instability range in both modes. Full article
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