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Volume 10
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Fluids, Volume 10, Issue 3 (March 2025) – 22 articles

Cover Story (view full-size image): In this study, linear stability analysis is conducted on the flow-induced vibration of an elastically mounted rotating cylinder. We consider different combinations of degrees of freedom in oscillation and Reynolds numbers. The results of the analysis are used to interpret specific phenomena that were observed in previous nonlinear simulations. The stability properties of leading modes provide fresh insight into the influences of forced rotation on flow-induced vibration. The findings of the current study have important implications in the design of offshore structures and energy-harvesting devices. View this paper
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17 pages, 4269 KiB  
Article
Optimising Air Change Rates: A CFD Study on Mitigating Pathogen Transmission in Aircraft Cabins
by Jaydon Benn and Lin Tian
Fluids 2025, 10(3), 74; https://doi.org/10.3390/fluids10030074 - 20 Mar 2025
Viewed by 332
Abstract
Amid the COVID-19 pandemic, understanding airborne pathogen transmission within confined spaces became critically important. The release of infectious aerosols through activities such as breathing, speaking, and coughing poses significant health risks, especially in confined spaces like airplane cabins. This study addresses gaps in [...] Read more.
Amid the COVID-19 pandemic, understanding airborne pathogen transmission within confined spaces became critically important. The release of infectious aerosols through activities such as breathing, speaking, and coughing poses significant health risks, especially in confined spaces like airplane cabins. This study addresses gaps in the research by evaluating the impact of air changes per hour (ACH) on pathogen transmission in an aircraft cabin using computational fluid dynamics (CFD) simulations. A detailed computer-aided design (CAD) model representing half of a four-row section of a Boeing 737 cabin was developed, utilising symmetry boundary conditions to optimise the computational resources while maintaining accuracy. Using ANSYS Fluent 2024, four scenarios were simulated at ACH rates of 15, 20, 25, and 30, with 4 µm pathogens injected into the cabin from a single infector. Airflow patterns and pathogen residence times were analysed for each case. The results indicate that ACH 15 presents the highest risk of pathogen transmission, while increasing the ACH to 20 significantly reduces this risk, with diminishing returns observed beyond ACH 20. Thus, this study underscores the importance of balancing ventilation efficiency, energy consumption, and passenger comfort. The findings provide valuable insights into optimising the ventilation systems to mitigate the airborne transmission in aircraft cabins. Future research should explore higher ACH rates, validate their impact, and conduct a comprehensive optimisation study to further improve the infection control measures. Full article
(This article belongs to the Special Issue CFD Applications in Environmental Engineering)
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21 pages, 7744 KiB  
Article
CFD Analysis of Heat Transfer Enhancement for Twisted Tape Inserted in Spirally Corrugated Tubes and Proposal of a New Vane-Inserted Geometry
by Mouhsine M. Benmbarek and Samir F. Moujaes
Fluids 2025, 10(3), 73; https://doi.org/10.3390/fluids10030073 - 20 Mar 2025
Viewed by 1697
Abstract
This research investigates the enhancement of heat transfer in a heat exchanger that is made of a corrugated tube which has a twisted plate inserted in it; the corrugation and twisted plate are expected to increase the amount of heat transfer since the [...] Read more.
This research investigates the enhancement of heat transfer in a heat exchanger that is made of a corrugated tube which has a twisted plate inserted in it; the corrugation and twisted plate are expected to increase the amount of heat transfer since the plate is acting as a connection between the center of the flow and the edges of the tube. The turbulence will cause an increase in pressure drop along the channel length, so the investigation will try to find the best compromise between the gain in heat transfer and loss of hydraulic energy by using well-established metrics. A positive heat transfer gain is achieved if the metric indicates a value equal to or greater than 1. This CFD research will be compared with the experimental results found in previous studies cited in the text. After validating the CFD results, it is proposed to investigate a new insert geometry to further improve the efficiency of the heat exchanger. The computational fluid dynamics (CFD) simulation was conducted to investigate and validate the CFD model, which evaluates the heat transfer performance in a spirally corrugated tube that has a twisted tape inserted. The heat transfer was then compared to a simple corrugated tube without the twisted tape and to a smooth tube with no corrugations and no twisted tape. Full article
(This article belongs to the Special Issue Pipe Flow: Research and Applications)
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17 pages, 5109 KiB  
Article
Numerical Mixing Index: Definition and Application on Concrete Mixer
by Cristian Ferrari, Nicolò Beccati and Luca Magri
Fluids 2025, 10(3), 72; https://doi.org/10.3390/fluids10030072 - 20 Mar 2025
Viewed by 427
Abstract
In this work, a statistical method is applied to a multiphase CFD simulation of concrete mixing performed in a truck mixer. The numerical model is based on an Eulerian–Eulerian approach in a transient regime. The aggregate materials are simulated as dispersed solid particles [...] Read more.
In this work, a statistical method is applied to a multiphase CFD simulation of concrete mixing performed in a truck mixer. The numerical model is based on an Eulerian–Eulerian approach in a transient regime. The aggregate materials are simulated as dispersed solid particles of various diameters, while the cement paste is simulated as a non-Newtonian continuous fluid. The first ten drum revolutions are analyzed from the condition of the completely segregated materials. The cell mixing index, defined by a statistical method in terms of mean, variance, and density probability function, is applied to the analysis of the simulation results. The statistical variables are implemented using the fluid dynamics code in the post-processing result analyses. The method predicts the distribution efficiency of the materials within a truck mixer as a function of its internal geometry, rotation speed, and mixture composition. As the number of revolutions increases, the distribution qualitatively improves, as shown by the motion fields, velocities, and vortices of the various materials, quantified through the calculation of the mixing index. The illustrated method can be used to predictively calculate the distribution effectiveness of new truck mixer designs before prototyping them and can be applied to other types of mixers. Furthermore, this study can be applied to liquid–solid mixing processes analyzed via the Eulerian multiphase numerical approach. Full article
(This article belongs to the Special Issue Industrial CFD and Fluid Modelling in Engineering, 2nd Edition)
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14 pages, 3927 KiB  
Article
Exergy and Irreversibility Analysis in Non-Equilibrium Thermal Porous Rectangular Channel
by Billel Yessad, Abdessamed Medelfef, Abderraouf Arabi and Ferhat Souidi
Fluids 2025, 10(3), 71; https://doi.org/10.3390/fluids10030071 - 18 Mar 2025
Viewed by 262
Abstract
This paper deals with laminar forced convection in a rectangular channel through a non-equilibrium thermal gas saturated porous medium. The thermodynamic aspects of this flow, including the entropy generation rate, irreversibility, and exergy, are carefully investigated. The governing conservation equations of momentum, mass, [...] Read more.
This paper deals with laminar forced convection in a rectangular channel through a non-equilibrium thermal gas saturated porous medium. The thermodynamic aspects of this flow, including the entropy generation rate, irreversibility, and exergy, are carefully investigated. The governing conservation equations of momentum, mass, and energy are solved numerically using the finite volume method. The effects of Reynolds number Re (ranging from 100 to 2000), Darcy number Da from 10−6 to 10−1, and Biot number Bi (from 10−3 to 103) on the entropy generation, exergy, and irreversibility, for which the Gouy-Stodola relation is employed, are then presented. The results reveal that at low Re and high Bi, thermal equilibrium between the two phases is achieved, leading to a reduction in entropy generation and, consequently, less exergy destruction. However, in the limit of high Re and low Da, irreversibility is significant due to large velocity gradients, leading to greater exergy destruction. Furthermore, it was observed that the thermal non-equilibrium intensity (LNTE) significantly influences entropy generation, leading to critical exergy destruction. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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10 pages, 4498 KiB  
Article
Observation of Laser-Induced Bubbles in Glycerol–Water Mixtures
by Laurel ONeill and Tim Kane
Fluids 2025, 10(3), 70; https://doi.org/10.3390/fluids10030070 - 18 Mar 2025
Viewed by 301
Abstract
This work presents a method of directly imaging the growth and collapse of laser induced-bubbles in glycerol and water mixtures. The direct optical imaging is augmented with interferometric measurements of the vibration spectrum of the bubble-vessel system. Experimentation confirms the expectation that fluid [...] Read more.
This work presents a method of directly imaging the growth and collapse of laser induced-bubbles in glycerol and water mixtures. The direct optical imaging is augmented with interferometric measurements of the vibration spectrum of the bubble-vessel system. Experimentation confirms the expectation that fluid viscosity affects the bubble formation and lifetime. During the experiment, deviation from the Rayleigh–Plesset equation is observed. Given this deviation of the observed bubble dynamics from the expected results, it is possible that the limited size of the sample volume and the walls of the container impact the bubble dynamics. The optical observations are supported by the observations of the system’s vibration spectrum. Full article
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12 pages, 866 KiB  
Article
An Image-Based Technique for Measuring Velocity and Shape of Air Bubbles in Two-Phase Vertical Bubbly Flows
by Giulio Tribbiani, Lorenzo Capponi, Tommaso Tocci, Martina Mengoni, Marco Marrazzo and Gianluca Rossi
Fluids 2025, 10(3), 69; https://doi.org/10.3390/fluids10030069 - 17 Mar 2025
Viewed by 279
Abstract
Bubbly flow is a flow regime common in many industrial applications involving heat and mass transfer, such as reactors, cooling systems, and separation units. Accurate knowledge of bubble velocity, shape, and volume is crucial as these parameters directly influence the efficiency of phase [...] Read more.
Bubbly flow is a flow regime common in many industrial applications involving heat and mass transfer, such as reactors, cooling systems, and separation units. Accurate knowledge of bubble velocity, shape, and volume is crucial as these parameters directly influence the efficiency of phase interaction and the mixing process performance. Over the past few decades, numerous techniques have been developed to measure the velocity, shape, and volume of bubbles. Most efforts have focused on non-intrusive methods to minimize disturbance to the flow. However, a technique capable of simultaneously measuring these bubble characteristics across a dense spatial domain remains elusive. In this research, an image-based technique that enables simultaneous measurement of bubble velocity, shape, and volume in bubbly flows over a densely sampled linear domain is presented. A high-speed camera captures the variation in light intensity as bubbles pass in front of a collimated laser sheet, providing real-time, high-resolution data. The accuracy of the proposed methodology is evaluated and the uncertainties associated with the velocity and volume measurements are quantified. Given the promising results and the simplicity of the hardware and setup, this study represents an important step toward developing a technique for online monitoring of industrial processes involving bubbly flows. Full article
(This article belongs to the Special Issue Pipe Flow: Research and Applications, 2nd Edition)
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16 pages, 19757 KiB  
Article
Experimental Particle Image Velocimetry Apparatus with Known Displacement of Synthetic Particles
by Anderson Gomes Girardi, Sigeo Kitatani Júnior, João Paulo da Silva Fonseca and Felipe Pamplona Mariano
Fluids 2025, 10(3), 68; https://doi.org/10.3390/fluids10030068 - 16 Mar 2025
Viewed by 408
Abstract
The study of velocimetry is important for characterizing and comprehending the effects of fluid flow, and the particle image velocimetry (PIV) technique is one of the primary approaches for understanding the velocity vector field in a test section. Commercial PIV systems are expensive, [...] Read more.
The study of velocimetry is important for characterizing and comprehending the effects of fluid flow, and the particle image velocimetry (PIV) technique is one of the primary approaches for understanding the velocity vector field in a test section. Commercial PIV systems are expensive, with one of the main cost factors being high-speed camera equipment capable of capturing images at high frames per second (fps), rendering them impractical for many applications. This study proposes an evaluation of utilizing smartphones as accessible image acquisition systems for PIV technique application. An experimental setup inspired by the known angular displacement of synthetic particles is proposed. A stepper motor rotates a plate containing an image of synthetic particles on its surface. The motion of the plate is captured by the smartphone camera, and the images are processed using PIVlab-MatLab® software. The use of two smartphones is assessed, with acquisition rates of either 240 fps or 960 fps and varying angular velocities. The results were satisfactory for velocities up to 0.7 m/s at an acquisition rate of 240 fps and up to 1.8 m/s at 960 fps, validating the use of smartphones as a cost-effective alternative for applying the PIV technique, both for educational purposes and for research carried out in low-income organizations. Full article
(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques, 2nd Edition)
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19 pages, 3145 KiB  
Article
Solar Thermal Collector Roughened with S-Shaped Ribs: Parametric Optimization Using AHP-MABAC Technique
by Khushmeet Kumar, Sushil Kumar, Deoraj Prajapati, Sushant Samir, Sashank Thapa and Raj Kumar
Fluids 2025, 10(3), 67; https://doi.org/10.3390/fluids10030067 - 10 Mar 2025
Viewed by 605
Abstract
The current examination used a multi-criteria decision-making (MCDM) approach to optimize the roughness parameters of S-shaped ribs (SSRs) in a solar thermal collector (STC) duct using air as the working fluid. Different SSRs were tested to identify the combination of parameters resulting in [...] Read more.
The current examination used a multi-criteria decision-making (MCDM) approach to optimize the roughness parameters of S-shaped ribs (SSRs) in a solar thermal collector (STC) duct using air as the working fluid. Different SSRs were tested to identify the combination of parameters resulting in the best performance. Geometrical parameters such as relative roughness pitch (PR/eRH) varied from 4 to 12, relative roughness height (eRH/Dhd) from 0.022 to 0.054, arc angle (αArc) from 30° to 75°, and relative roughness width (WDuct/wRS) from 1 to 4. The Nusselt number (NuRP) and friction factor (fRP), findings which impact the STC performance, rely on SSRs. The performance measurements show that no combination of SSR parameters lead to the best enhancement heat transfer rate at low enhancement in the friction. So, a hybrid multi-criteria decision-making strategy using the Analytical Hierarchy Process (AHP) for criterion significance and Multi Attributive Border Approximation Area Comparison (MABAC) for alternative ranking was used to determine which combination of geometrical parameters will result in the optimum performance of a roughened STC. This work employs a hybrid MCDM technique to optimise the effectiveness of an STC roughened with SSRs. To optimize the SSR design parameters, this study used the hybrid AHP-MABAC technique for analytical assessment of a roughened STC. The optimization results showed that the STC roughened with SSRs achieved the optimum performance at PR/eRH = 8, eRH/Dhd = 0.043, αArc = 60° and WDuct/wRS = 3. Full article
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17 pages, 833 KiB  
Article
ARES: A Meanline Code for Outboard Dynamic-Inlet Waterjet Axial-Flow Pumps Design
by Filippo Avanzi, Francesco De Vanna, Andrea Magrini and Ernesto Benini
Fluids 2025, 10(3), 66; https://doi.org/10.3390/fluids10030066 - 10 Mar 2025
Viewed by 603
Abstract
We introduce the solver ARES: Axial-flow pump Radial Equilibrium through Streamlines. The code implements a meanline method, enforcing the conservation of flow momentum and continuity across a set of discrete streamlines in the axial-flow pump’s meridional channel. Real flow effects are modeled with [...] Read more.
We introduce the solver ARES: Axial-flow pump Radial Equilibrium through Streamlines. The code implements a meanline method, enforcing the conservation of flow momentum and continuity across a set of discrete streamlines in the axial-flow pump’s meridional channel. Real flow effects are modeled with empirical correlations, including off-design deviation and losses due to profile shape, secondary flows, tip leakage, and the end-wall boundary layer (EWBL). Inspired by aeronautical fan and compressor methods, this implementation is specifically tailored for the analysis of the Outboard Dynamic-inlet Waterjet (ODW), the latest aero-engine-derived innovation in marine engineering. To ensure the reliable application of ARES for the systematic designs of ODW pumps, the present investigation focuses on prediction accuracy. Global and local statistics are compared between numerical estimates and available measurements of three test cases: two single rotors and a rotor–stator waterjet configuration. At mass flow rates near the design point, hydraulic efficiency is predicted within 1% discrepancy to tests. Differently, as the flow coefficient increases, the loss prediction accuracy degrades, incrementing the error for off-design estimates. Spanwise velocity and pressure distributions exhibit good alignment with experiments near midspan, especially at the rotor exit, while end-wall boundary layer complex dynamics are hardly recovered by the present implementation. Full article
(This article belongs to the Special Issue Industrial CFD and Fluid Modelling in Engineering, 2nd Edition)
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17 pages, 7918 KiB  
Article
A Method for Measuring the Rheology of a Non-Newtonian Fluid Based on the Analysis of the Recirculation Angle of Secondary Flows in a Curved U-Shaped Channel
by Alexander S. Lobasov, Andrey V. Minakov and Sergey A. Filimonov
Fluids 2025, 10(3), 65; https://doi.org/10.3390/fluids10030065 - 8 Mar 2025
Viewed by 2270
Abstract
The field of hydrodynamics, specifically microfluidics, is currently undergoing rapid development, with significant progress being made in the creation of new devices and technologies that outperform their macroscopic counterparts. Concurrently, determining the parameters of a non-Newtonian fluid is becoming an important task in [...] Read more.
The field of hydrodynamics, specifically microfluidics, is currently undergoing rapid development, with significant progress being made in the creation of new devices and technologies that outperform their macroscopic counterparts. Concurrently, determining the parameters of a non-Newtonian fluid is becoming an important task in many areas of industry and production, particularly in the oil industry. Both the drilling fluids (needed to create wells) and the polymer-based displacers and surfactants (needed to extract oil) have non-Newtonian properties. This paper presents a method for determining the indices of consistency and flow behaviour of the non-Newtonian fluid (power-law model) based on the analysis of secondary Dean vortices generated in a curved channel. This phenomenon is conveniently described using the recirculation angle. The structure of the flow of non-Newtonian fluids in a U-shaped micromixer has been studied. The dependence of the recirculation angle on the fluid flow rate was obtained for different fluid parameters. A universal correlation was proposed to describe the dependence of the inverse Dean number on the recirculation angle of the flow. The consistency and flow behaviour indices of the power-law model of non-Newtonian fluids found using the above correlation can be measured in the experiments. Full article
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22 pages, 8709 KiB  
Article
Performance of a Pharmaceutical Single-Use Stirred Tank Operating at Different Filling Volumes: Mixing Time, Fluid Dynamics and Power Consumption
by Federico Alberini, Andrea Albano, Pushpinder Singh, Giuseppina Montante, Francesco Maluta, Nicodemo Di Pasquale and Alessandro Paglianti
Fluids 2025, 10(3), 64; https://doi.org/10.3390/fluids10030064 - 8 Mar 2025
Viewed by 712
Abstract
Single-use bioreactors (SUBs) are revolutionizing biotechnology and biopharmaceutical manufacturing by offering cost-efficient, flexible, and scalable alternatives to traditional reusable systems. These bioreactors, made from disposable and pre-sterilized materials, streamline cell cultivation for biological production while minimizing the need for complex cleaning and sterilization. [...] Read more.
Single-use bioreactors (SUBs) are revolutionizing biotechnology and biopharmaceutical manufacturing by offering cost-efficient, flexible, and scalable alternatives to traditional reusable systems. These bioreactors, made from disposable and pre-sterilized materials, streamline cell cultivation for biological production while minimizing the need for complex cleaning and sterilization. A critical aspect of SUB performance lies in optimizing hydrodynamic parameters flow field, power consumption, mixing time, and energy efficiency, which directly influence process outcomes. This study investigates the hydrodynamic performance of an SUB system through stereo Particle Image Velocimetry (PIV) to analyze flow fields, Planar Laser-Induced Fluorescence (PLIF) for mixing time, and Electro Resistance Tomography (ERT) for further insights into mixing dynamics. The results, evaluated at varying impeller speeds and fill heights, provide a comprehensive understanding of flow behavior, mixing efficiency, and power requirements. This work highlights the importance of hydrodynamic characterization in optimizing SUB design and operation, contributing to more sustainable and efficient biopharmaceutical production. Full article
(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques, 2nd Edition)
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15 pages, 74958 KiB  
Article
Hybridization of a Micro-Scale Savonius Rotor Using a Helical Darrieus Rotor
by Martin Moreno, Iván Trejo-Zúñiga, Jesús Terrazas, Arturo Díaz-Ponce and Andrés Pérez-Terrazo
Fluids 2025, 10(3), 63; https://doi.org/10.3390/fluids10030063 - 6 Mar 2025
Viewed by 685
Abstract
This study presents a micro-scale hybrid wind turbine that integrates a Savonius rotor with a Helical Darrieus rotor, aiming to enhance energy conversion efficiency and adaptability for decentralized renewable energy generation. The hybrid design leverages the high torque generation of the Savonius rotor [...] Read more.
This study presents a micro-scale hybrid wind turbine that integrates a Savonius rotor with a Helical Darrieus rotor, aiming to enhance energy conversion efficiency and adaptability for decentralized renewable energy generation. The hybrid design leverages the high torque generation of the Savonius rotor and the aerodynamic efficiency of the Helical Darrieus rotor. Computational analyses using CFD simulations and experimental validation with a 3D-printed prototype in a closed wind tunnel were conducted at speeds ranging from 3 to 8 m/s. The results demonstrate that the hybrid turbine achieves a power coefficient of 0.26 at an optimal tip-speed ratio of 2.7, marking a 180% improvement over standalone Savonius rotors. The hybridization process mitigates the low-speed inefficiencies of the Savonius rotor. It compensates for the high-speed limitations of the Darrieus rotor, resulting in a turbine capable of operating efficiently over a wider range of wind speeds. This balanced integration maximizes energy harvesting and improves adaptability to varying wind conditions, achieving balanced and synergistic performance. Full article
(This article belongs to the Special Issue CFD Applications in Environmental Engineering)
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24 pages, 11607 KiB  
Article
Thermal Performance Analysis of LOX/LCH4 Engine Feed Systems Using CFD Modeling
by Iram Hernandez, Salvador Orozco, Md Amzad Hossain and Ahsan Choudhuri
Fluids 2025, 10(3), 62; https://doi.org/10.3390/fluids10030062 - 5 Mar 2025
Viewed by 1083
Abstract
This study examines the thermal management of the Centennial Restartable Oxygen Methane Engine (CROME) feed system under two propellant tank pressure conditions: 33 psi (227.5 kPa) and 100 psi (689.5 kPa), at a constant liquid methane flow rate of 0.9 lbm/s (0.4 kg/s). [...] Read more.
This study examines the thermal management of the Centennial Restartable Oxygen Methane Engine (CROME) feed system under two propellant tank pressure conditions: 33 psi (227.5 kPa) and 100 psi (689.5 kPa), at a constant liquid methane flow rate of 0.9 lbm/s (0.4 kg/s). Using the Eulerian Single-Phase (ESP) model, the initial test validated experimental data, showing close agreement in total pressure (experimental: 31 psi; CFD: 33 psi) and temperature measurements (experimental: −287.3 °F and −300 °F; CFD: −299 °F and −294 °F) with deviations of 6.4% and ≤4.1%, respectively. For the second test, a simplified Volume of Fluid (VOF) model was used, adjusted for varying liquid-to-gas volume fractions. The best agreement with experimental data was found with 100% GN2, showing a 3.1 psi pressure rise and a 3.3% error. These findings show the importance of improving thermal management and precision control in cryogenic LOX-LCH4 feedline systems for optimal engine performance. Future research will focus on exploring pressures up to the propellant tank’s maximum rated limit of 400 psi. Full article
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20 pages, 4126 KiB  
Article
Evolution of Wind-Generated Shallow-Water Waves in the Framework of a Modified Kadomtsev–Petviashvili Equation
by Montri Maleewong and Roger Grimshaw
Fluids 2025, 10(3), 61; https://doi.org/10.3390/fluids10030061 - 27 Feb 2025
Cited by 1 | Viewed by 341
Abstract
In a recent paper, denoted by MG24 in this text, we used a modified Korteweg–de Vries (KdV) equation to describe the evolution of wind-driven water wave packets in shallow water. The modifications were several forcing/friction terms describing wave growth due to critical-level instability [...] Read more.
In a recent paper, denoted by MG24 in this text, we used a modified Korteweg–de Vries (KdV) equation to describe the evolution of wind-driven water wave packets in shallow water. The modifications were several forcing/friction terms describing wave growth due to critical-level instability in the air, wave decay due to laminar friction in the water at the air–water interface, wave growth due to turbulent wave stress in the air near the interface, and wave decay due to a turbulent bottom boundary layer. The outcome was a KdV–Burgers type of equation that can be a stable or unstable model depending on the forcing/friction parameters. In most cases that we examined, many solitary waves are generated, suggesting the formation of a soliton gas. In this paper, we extend that model in the horizontal direction transverse to the wind forcing to produce a similarly modified Kadomtsev–Petviashvili equation (KPII for water waves in the absence of surface tension). A modulation theory is described for the cnoidal and solitary wave solutions of the unforced KP equation, focusing on the forcing/friction terms and the transverse dependence. Then, using similar initial conditions to those used in MG24, that is a sinusoidal wave with a slowly varying envelope, but supplemented here with a transverse sinusoidal term, we find through numerical simulations that the radiation field upstream is enhanced, but that a soliton gas still emerges downstream as in MG24. Full article
(This article belongs to the Special Issue Numerical Modeling and Experimental Studies of Two-Phase Flows, 2nd Edition)
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27 pages, 25794 KiB  
Article
Numerical Investigation of the Influence of Temperature on Fluidization Behavior: Importance of Particle Collision Parameters and Inter-Particle Forces
by Milan Mihajlović, Juan G. Ramírez, Ildefonso Campos Velarde, Martin Van Sint Annaland and Ivo Roghair
Fluids 2025, 10(3), 60; https://doi.org/10.3390/fluids10030060 - 27 Feb 2025
Viewed by 487
Abstract
Fluidized bed reactors (FBRs) are integral to various industries due to their exceptional capability in facilitating efficient gas–solid interactions, resulting in superior mixing and heat and mass transfer. This research delves into the impact of temperature on fluidization dynamics, particularly focusing on the [...] Read more.
Fluidized bed reactors (FBRs) are integral to various industries due to their exceptional capability in facilitating efficient gas–solid interactions, resulting in superior mixing and heat and mass transfer. This research delves into the impact of temperature on fluidization dynamics, particularly focusing on the collisional properties of particles within the bed. The investigation builds upon foundational research, notably Geldart’s classification of fluidization regimes and recent advancements in high-temperature experimental techniques, such as High-Temperature Endoscopic-Laser particle image velocimetry/digital image analysis. To explore these temperature effects, a coupled Discrete Element Method and Computational Fluid Dynamics (cfd–dem) model was employed. This approach enables a detailed examination of gas–particle and particle–particle interactions under varying temperature conditions. The simulations in this study explore the friction coefficient, as well as changes in both tangential and normal restitution coefficients, which affect the fluidization behavior. These changes were systematically analyzed to determine their influence on minimum fluidization velocity and bubble formation. The numerical results are compared with experimental data from high-temperature fluidization studies, highlighting the necessity of incorporating inter-particle forces to fully capture the observed phenomena. The findings underscore the critical role of particle collisional properties in high-temperature fluidization and suggest the potential increasing role of inter-particle forces. Overall, this paper provides new insights into the complex dynamics of fluidized beds at elevated temperatures, emphasizing the need for further experimental–numerical research to enhance the reliability and understanding of these systems in industrial applications. Full article
(This article belongs to the Special Issue Experimental and Computational Advances in Bubbling Fluidized Beds Hydrodynamics)
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15 pages, 16118 KiB  
Article
Hysteresis-Induced Onset and Progressive Decay of Periodic and Metastable Sheet Cavitation in a Chamfered Circular Orifice
by Min Son, Michael Börner, Wolfgang Armbruster and Justin S. Hardi
Fluids 2025, 10(3), 59; https://doi.org/10.3390/fluids10030059 - 26 Feb 2025
Viewed by 527
Abstract
This study investigates the onset and decay mechanisms of sheet cavitation within a chamfered orifice under turbulent conditions, using high-speed backlight imaging for detailed frame-by-frame analysis. A distinctive metastable sheet cavitation regime was identified, distinguished by its unique hysteresis behavior during onset conditions, [...] Read more.
This study investigates the onset and decay mechanisms of sheet cavitation within a chamfered orifice under turbulent conditions, using high-speed backlight imaging for detailed frame-by-frame analysis. A distinctive metastable sheet cavitation regime was identified, distinguished by its unique hysteresis behavior during onset conditions, with the ability to control periodicity through variations in cavitation numbers. This new sheet cavitation regime appears at high cavitation numbers, contrary to typical expectations of cavitation inception, highlighting a new potential risk within the range of safe operation for hydraulic systems equipped with control valves. Furthermore, linear growth and rapid collapse of the bubble sheet were observed, which differs from the conventional periodic behavior of sheet cavitation on hydrofoils. The new mechanism to intentionally initiate and control this sheet cavitation regime by manipulating the pressure drop across the orifice could potentially be adopted for industrial applications, particularly in the generation of controlled and dispersed bubbles. Future research should include quantifying bubble dynamics within this regime and assessing the effects of fluid properties and orifice geometries on cavitation characteristics. In summary, this study introduces a new perspective on metastable sheet cavitation, emphasizing its potential applications and importance in the design and operation of fluid systems. Full article
(This article belongs to the Special Issue Cavitation and Bubble Dynamics)
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13 pages, 5889 KiB  
Article
Non-Newtonian Interfacial Modeling of Protein Drops Sheared in Microgravity
by Joe A. Adam, Frank P. Riley, Juan M. Lopez, Patrick T. Underhill and Amir H. Hirsa
Fluids 2025, 10(3), 58; https://doi.org/10.3390/fluids10030058 - 26 Feb 2025
Viewed by 545
Abstract
Complex fluid interfaces are commonplace in natural and engineered systems and a major topic in the fields of rheology and soft matter physics, providing boundary conditions for a system’s hydrodynamics. The relationship between structure and function dictates how constituents within complex fluids govern [...] Read more.
Complex fluid interfaces are commonplace in natural and engineered systems and a major topic in the fields of rheology and soft matter physics, providing boundary conditions for a system’s hydrodynamics. The relationship between structure and function dictates how constituents within complex fluids govern flow behavior via constituents changing conformation in response to the local microenvironment to minimize free energy. Both hydrodynamics, such as shear flow, and the presence of air–liquid interfaces are principal aspects of a complex fluid’s environment. The study of fluid interfaces coupled to bulk flows can be uniquely advanced through experimentation in microgravity, where surface tension containment can be achieved at relatively large length scales. This computational investigation assesses flow in the ring-sheared drop (RSD), a containerless biochemical reactor operating aboard the International Space Station for the study of complex fluids and soft matter physics. Specifically, the hydrodynamic effects of a generalized Boussinesq–Scriven interface with a shear-thinning surface shear viscosity are examined in flow regimes where the air–liquid interface remains coupled to the Newtonian bulk fluid. The results verify this interfacial model’s ability to affect system-wide hydrodynamics under specific parameter regimes, enabling future model validation with high-precision rheological measurements. Full article
(This article belongs to the Special Issue Non-Newtonian Flow: Interfacial and Bulk Phenomena)
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16 pages, 2639 KiB  
Article
Analysis of the Evolution of Shock Waves When a Muzzle Jet Impacts a Constrained Moving Body
by Zijie Li and Hao Wang
Fluids 2025, 10(3), 57; https://doi.org/10.3390/fluids10030057 - 26 Feb 2025
Viewed by 314
Abstract
The evolution form of the flow field generated via the impact of a muzzle jet on a constrained moving body changes from the state of fully free-spatial development to that of constrained development, and it involves the problem of interference, owing to the [...] Read more.
The evolution form of the flow field generated via the impact of a muzzle jet on a constrained moving body changes from the state of fully free-spatial development to that of constrained development, and it involves the problem of interference, owing to the spatiotemporal coupling of various kinds of shock waves and vortices. Against this backdrop, the authors use the dynamic mesh method to establish two models for simulating the flow field and exploring the mechanism of development and the characteristics of propagation of disturbances induced via the shock waves as the muzzle jet impacts a constrained moving body. The results show that the muzzle jet exhibited a circumferentially asymmetric shape under the influence of the constrained track. The shock wave leaned towards the upper part of the muzzle, and its speed of propagation above the muzzle was higher than that below the muzzle. Meanwhile, the vortex that should have been present below the muzzle disappeared, and it was replaced with a separation line. Changes in the pressure of the flow field and important parameters of the moving body also became more complex due to the influence of the constrained track. Full article
(This article belongs to the Topic Fluid Mechanics, 2nd Edition)
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17 pages, 13494 KiB  
Article
Linear Stability Analysis on Flow-Induced Vibration of an Elastically Mounted Rotating Cylinder
by Jianfeng Lu, Zhiyu Zhang and Xing Zhang
Fluids 2025, 10(3), 56; https://doi.org/10.3390/fluids10030056 - 21 Feb 2025
Viewed by 481
Abstract
In this paper, we present a linear stability analysis on flow-induced vibration of an elastically mounted cylinder subjected to forced rotation. Four series of cases, with different combinations of degrees of freedoms in oscillation and Reynolds number are investigated. For each series of [...] Read more.
In this paper, we present a linear stability analysis on flow-induced vibration of an elastically mounted cylinder subjected to forced rotation. Four series of cases, with different combinations of degrees of freedoms in oscillation and Reynolds number are investigated. For each series of cases, a wide range of reduced velocity at various rotation rates are considered. The variations of growth and frequency with reduced velocity for the leading modes are presented. Some phenomena observed in previous numerical studies are interpreted by using the results of linear stability analysis. The supressing of vortex shedding at moderate rotation rate is explained by the absence of unstable fluid mode. The amplitude enhancement in high range of rotaton rate is explained by the emergence of unstable elastic mode. The stability properties of the leading modes provide some new insight into the influences of forced rotation on flow-induced vibration. The results of the current study have important implications in the design of offshore structures and energy-havesting devices. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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24 pages, 6077 KiB  
Article
On the Modular Design Application for the Gas Turbine Sector: A Performance Optimization Approach in the Context of Industry 4.0
by Lucas Chavanel-Precloux, Roland Maranzana and Lucas A. Hof
Fluids 2025, 10(3), 55; https://doi.org/10.3390/fluids10030055 - 21 Feb 2025
Viewed by 415
Abstract
Production changes enabled by Industry 4.0 (I4.0) allow industries to respond to customer needs in a much more precise and agile manner. It also permits companies to focus on the development of sustainable and more efficient solutions. The energy sector is still lacking [...] Read more.
Production changes enabled by Industry 4.0 (I4.0) allow industries to respond to customer needs in a much more precise and agile manner. It also permits companies to focus on the development of sustainable and more efficient solutions. The energy sector is still lacking progress in this context, however, and the implementation of I4.0 and modularity could help to solve such issues. The present research study contributes to addressing the research gap in I4.0 implementation in the Gas Turbine (GT) sector by developing a design application for modular GT configuration. The main objective of the developed modular design application (MDA) is to facilitate the relationship between customer and engineer by providing an accessible application (program), including pre-designed heat cycles (HCs), that proposes optimized modular solutions, according to customer requirements, using simulation. Indeed, this study presents the functioning of the novel application, the different deployed components and their variables, such as the compressor efficiency, heat exchangers, or turbine stages, and the decision variable, e.g., the costs of generated energy. Simulations and comparisons using reported HCs in the literature have been performed to validate the accuracy of the simulation processes. Finally, a study case is presented, placing the MDA in an industrial context to illustrate its benefits and to provide solutions for GT modularity. It is concluded that the developed MDA correctly simulates HCs and enables a first step towards modular HC design. Indeed, the proposed MDA architecture allows for continuous improvement and expansion, e.g., by the addition of HC-related components or the integration of different entry variables, such as the company’s financial scope, world location, desired power, and available components. Full article
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17 pages, 2442 KiB  
Article
On the Aerodynamic Performance of a Blended-Wing-Body, Low-Mach Number Unmanned Aerial Vehicle
by Nikolaos Lampropoulos, Alexandros Vouros, Ioannis Templalexis and Theodoros Lekas
Fluids 2025, 10(3), 54; https://doi.org/10.3390/fluids10030054 - 20 Feb 2025
Viewed by 845
Abstract
A study on aerodynamic design studies of a blended wing–body (BWB) unmanned aerial vehicle (UAV) operating at low Mach numbers is presented. First, a parametric investigation based on analytical equations is carried out to identify the range of the necessary wetted area for [...] Read more.
A study on aerodynamic design studies of a blended wing–body (BWB) unmanned aerial vehicle (UAV) operating at low Mach numbers is presented. First, a parametric investigation based on analytical equations is carried out to identify the range of the necessary wetted area for the UAV to maximize endurance at a Mach number close to 0.1. A base-of-reference configuration is designed, and its aerodynamic performance is evaluated by utilizing a panel method in Xflr5. An optimization algorithm is then incorporated to trim the UAV and produce the ‘clean’ configuration. Computational fluid dynamics (CFD) simulations are performed within the OpenFoam environment to produce first the updated drag polars, and then, to analyze the integration of the nacelle and the pair of electric ducted fans (EDFs) used for the propulsion system. In particular, when examining the integration of the nacelle with a spinning electric ducted fan (EDF) standing as the propulsion system of the vehicle, a rotating, sliding mesh computational approach is adopted. Results indicate that the clean configuration is characterized by strong longitudinal stability so that the UAV has the potential to fly trimmed at very low speeds. Mounting EDFs on the back of the fuselage is conducive to higher loading with minimal drag penalty. An increased lift-to-drag ratio is achieved. Reduced wake mixing due to the EDF’s jet flow is observed. The spanwise flow that is conducive to pitch brake and loss of stability is also weak, as the suction produced by the EDF diverts the flow inboard. Full article
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20 pages, 3395 KiB  
Article
A Validation of the Junction Riemann Problem in Networks of Channels Under Transitory Flow Conditions
by Juan Mairal, Javier Murillo and Pilar Garcia-Navarro
Fluids 2025, 10(3), 53; https://doi.org/10.3390/fluids10030053 - 20 Feb 2025
Viewed by 381
Abstract
Transitory states and supercritical regimes in channel junctions have been challenging to model using 1D models, and it has been common to resort to 2D models to locally solve networks of channels. In this paper, we present a methodology based on the Junction [...] Read more.
Transitory states and supercritical regimes in channel junctions have been challenging to model using 1D models, and it has been common to resort to 2D models to locally solve networks of channels. In this paper, we present a methodology based on the Junction Riemann Problem that manages to include supercritical solutions by making use of suitable limiting coefficients. The method is compared against a pure 2D model in a series of test cases with challenging geometries that include transitory flow conditions. The results show that the method hereby presented is robust across all regimes and is able to capture the main features of wave propagation along a network of channels. Full article
(This article belongs to the Special Issue Modelling Flows in Pipes and Channels)
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