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Fluids, Volume 9, Issue 9 (September 2024) – 27 articles

Cover Story (view full-size image): This study aims to quantify the blood loss and elucidate the blood flux and pressure redistribution between the kidneys and the aorta during a renal resection. We developed a new lumped-parameter mathematical model, using a non-Newtonian fluid and accounting for the total impedance variation, which offers near real-time estimations of the flow–pressure redistribution for several radical or partial nephrectomies. The findings indicate that the downstream aorta receives a 1.27 times higher percentage of the redistributed flow from the diseased kidney compared to that received by the healthy kidney. The implications are significant, as they can inform the development of surgical protocols to minimize blood loss and can assist surgeons in evaluating the adequacy of the remaining kidney vasculature. View this paper
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14 pages, 1926 KiB  
Article
The Response of Turbulent Channel Flow to Standing Wave-Like Wall Motion
by Matthew Brockhaus, Adrian Sescu and Jonathan Morrison
Fluids 2024, 9(9), 220; https://doi.org/10.3390/fluids9090220 - 23 Sep 2024
Abstract
The effect of wall deformations on wall turbulence is a topic of fundamental importance for the advancement of flow control strategies aimed at reducing the frictional drag. Dynamic wall deformations in the form of wall-normal of in-plane oscillations represents an area that is [...] Read more.
The effect of wall deformations on wall turbulence is a topic of fundamental importance for the advancement of flow control strategies aimed at reducing the frictional drag. Dynamic wall deformations in the form of wall-normal of in-plane oscillations represents an area that is yet to be fully explored, and that can open a new realm of drag control strategies, as well as provide fresh insights into the structure of wall turbulence. In this study, we present several results from direct numerical simulations aimed at understanding the response of wall turbulence to standing wave-like wall motion, arranged in a checkerboard pattern. More specifically, here we target a low Reynolds number turbulent channel flow, featuring standing wave-like dynamic wall deformations on both bottom and top walls, with parameterizations in terms of the frequency of oscillations, roughness height, and spacing. We quantify the effect that this type of wall motion can have on the skin friction drag, various turbulent statistics, and turbulent flow structures. In addition, taking advantage of the periodicity and the symmetry of the flow, an improved phase-averaging procedure is introduced, which enhances the smoothness of the data. The results show that this type of dynamic wall deflection can have a significant effect on the turbulent flow in proximity to the wall, and that the variation of the spatial wavenumbers of the wall deflections can make a big difference. A slight total drag reduction, in the order of 2%, was observed for some combinations of wavenumbers and frequencies, especially for the highest streamwise wavenumber case. Full article
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16 pages, 4467 KiB  
Article
Coulomb Driven Electro-Convection within Two Stacked Layers of Miscible Dielectric Liquids
by Philippe Traore, Alberto T. Pérez, Subhadeep Mondal, Anandaroop Bhattacharya, Pedro A. Vázquez and Zelu Yan
Fluids 2024, 9(9), 219; https://doi.org/10.3390/fluids9090219 - 19 Sep 2024
Abstract
This article investigates the behavior of two parallel layers of different miscible dielectric liquids enclosed and sandwiched between two electrodes. By applying an electric potential to one electrode while grounding the other, electro-convection occurs when the electric Rayleigh number exceeds a critical value, [...] Read more.
This article investigates the behavior of two parallel layers of different miscible dielectric liquids enclosed and sandwiched between two electrodes. By applying an electric potential to one electrode while grounding the other, electro-convection occurs when the electric Rayleigh number exceeds a critical value, setting the fluid into motion and resulting in rapid mixing between the two liquids. A numerical model is developed to account for the varying ionic mobility and permittivity of the two liquids, considering their evolution based on the relative concentration field. The simulations confirm that electro-convection significantly enhances the mixing between the two liquids, as expected. Additionally, intriguing ripples are observed near the initial interface during the early stages of electro-convection instability growth. To explain and describe the flow dynamics in terms of stability analysis, a semi-analytical model is presented. This study provides insights into the mixing behavior and flow dynamics of miscible dielectric liquids under the influence of electro-convection. The findings contribute to a better understanding of the underlying mechanisms and can be valuable for applications such as microfluidics, energy conversion, and mixing processes. Further research is encouraged to explore additional parameters and optimize the control of electro-convection for practical applications. Full article
(This article belongs to the Special Issue Advances in Electrohydrodynamic Flow)
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20 pages, 11233 KiB  
Article
Comparative Performance Assessment between Incompressible and Compressible Solvers to Simulate a Cavitating Wake
by Jian Chen, Linlin Geng, Esteve Jou and Xavier Escaler
Fluids 2024, 9(9), 218; https://doi.org/10.3390/fluids9090218 - 18 Sep 2024
Abstract
To study the effects of fluid compressibility on the dynamics of a cavitating vortex street flow in a regime where the vortex shedding frequency increases as a result of the cavitation increase, the cavitating wake behind a wedge was simulated employing both incompressible [...] Read more.
To study the effects of fluid compressibility on the dynamics of a cavitating vortex street flow in a regime where the vortex shedding frequency increases as a result of the cavitation increase, the cavitating wake behind a wedge was simulated employing both incompressible and compressible solvers. To do this, a compressible cavitation model was implemented, modifying the Zwart-Gerber-Belamri (ZGB) incompressible solver and including a pressure limit and absorbing boundary conditions to prevent a non-physical pressure field. To validate the performance of the compressible model, preliminary simulations were carried out on a 1D Sod cavitating tube and the cavitating vortex shedding behind a circular body at laminar flow conditions. The results of the cavitating wake behind the wedge with the incompressible and the compressible solvers showed similar predictions in terms of pressure, vortex shedding frequency, and instantaneous and average vapor volume fraction profiles. In spite of this, differences were obtained in the energy content of the fluid force fluctuations on the body at higher frequencies, which appear to be better resolved and amplified when the compressibility model is considered. Overall, both solvers provided comparable results in terms of cavitation phenomena that are well aligned with experimental observations. Full article
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23 pages, 7362 KiB  
Article
Thermal Efficiency Analysis of a 1 kW ORC System with a Solar Collection Stage and R-245fa Working Fluid: A Case Study
by Raúl Alejandro Martínez-Sánchez, José M. Álvarez-Alvarado, Gerardo I. Pérez-Soto, Idalberto Macías-Socarrás, Karla A. Camarillo-Gómez and Juvenal Rodríguez-Reséndiz
Fluids 2024, 9(9), 217; https://doi.org/10.3390/fluids9090217 - 15 Sep 2024
Abstract
A thermal efficiency analysis of an organic Rankine cycle (ORC) system enables its performance to be evaluated; for this purpose, critical system components, including the turbine and the boiler, must be scrutinized. ORC plants can operate under various regimes, such as simple, regeneration, [...] Read more.
A thermal efficiency analysis of an organic Rankine cycle (ORC) system enables its performance to be evaluated; for this purpose, critical system components, including the turbine and the boiler, must be scrutinized. ORC plants can operate under various regimes, such as simple, regeneration, and reheat work modes. Organic fluids such as R-245fa integrate low-temperature sources such as solar radiation. However, a literature review revealed limited research on the impact of a solar collection system on the overall thermal efficiency of an ORC system during the regeneration stage. In this study, we examined the thermal efficiency behavior of an ORC plant with a 1 kW generator operating in simple and regeneration modes with a solar collection stage. The results show that the thermal efficiency in simple mode was 35.27%, while in regeneration mode with solar collection it reached 51.30%. Improving the thermal efficiency of a thermodynamic cycle system can reduce CO2 emissions. The operating temperature ranges facilitate the development of a methodology for industries to implement ORC systems in their manufacturing processes, thereby utilizing waste heat from industrial operations. Full article
(This article belongs to the Special Issue Evaporation, Condensation and Heat Transfer)
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22 pages, 9034 KiB  
Review
Deep Reinforcement Learning for Fluid Mechanics: Control, Optimization, and Automation
by Innyoung Kim, Youngmin Jeon, Jonghyun Chae and Donghyun You
Fluids 2024, 9(9), 216; https://doi.org/10.3390/fluids9090216 - 14 Sep 2024
Abstract
A comprehensive review of recent advancements in applying deep reinforcement learning (DRL) to fluid dynamics problems is presented. Applications in flow control and shape optimization, the primary fields where DRL is currently utilized, are thoroughly examined. Moreover, the review introduces emerging research trends [...] Read more.
A comprehensive review of recent advancements in applying deep reinforcement learning (DRL) to fluid dynamics problems is presented. Applications in flow control and shape optimization, the primary fields where DRL is currently utilized, are thoroughly examined. Moreover, the review introduces emerging research trends in automation within computational fluid dynamics, a promising field for enhancing the efficiency and reliability of numerical analysis. Emphasis is placed on strategies developed to overcome challenges in applying DRL to complex, real-world engineering problems, such as data efficiency, turbulence, and partial observability. Specifically, the implementations of transfer learning, multi-agent reinforcement learning, and the partially observable Markov decision process are discussed, illustrating how these techniques can provide solutions to such issues. Finally, future research directions that could further advance the integration of DRL in fluid dynamics research are highlighted. Full article
(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2024)
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20 pages, 4248 KiB  
Article
Reducing Flow Resistance via Introduction and Enlargement of Microcracks in Convection Enhanced Delivery (CED) in Porous Tumors
by Md Jawed Naseem, Ronghui Ma and Liang Zhu
Fluids 2024, 9(9), 215; https://doi.org/10.3390/fluids9090215 - 13 Sep 2024
Abstract
A theoretical simulation is performed to evaluate how microcracks affect the flow resistance in tumors during the convection-enhanced delivery (CED) of nanofluids. Both Darcy’s law and the theory of poroelasticity are used to understand fluid transport with or without microcrack introduction and/or enlargement. [...] Read more.
A theoretical simulation is performed to evaluate how microcracks affect the flow resistance in tumors during the convection-enhanced delivery (CED) of nanofluids. Both Darcy’s law and the theory of poroelasticity are used to understand fluid transport with or without microcrack introduction and/or enlargement. The results demonstrate significantly altered pressure and velocity fields in a spherical tumor with a radius of 10 mm due to the presence of a microcrack with a radius of 0.05 mm and length of 3 mm. The non-uniform fluid pressure field enlarges the original cylindrical microcrack to a frustum, with the crack volume more than doubled. Due to the larger permeability and porosity in the microcrack, flow in the tumor is much easier. One finds that the flow resistance with the enlarged microcrack is reduced by 14% from the control without a microcrack. Parametric studies are conducted to show that larger crack radii, longer crack lengths and higher infusing pressures result in further resistance reductions. The largest resistance reduction occurs when the infusing pressure is 4 × 105 Pa and the microcrack is 9 mm long, up to 18% from the control. We conclude that introducing a microcrack is an effective way to facilitate nanofluid delivery in porous tumors using CED. Full article
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18 pages, 2268 KiB  
Article
Near Real-Time Estimation of Blood Loss and Flow–Pressure Redistribution during Unilateral Nephrectomy
by James Cowley, Justicia Kyeremeh, Grant D. Stewart, Xichun Luo, Wenmiao Shu and Asimina Kazakidi
Fluids 2024, 9(9), 214; https://doi.org/10.3390/fluids9090214 - 13 Sep 2024
Abstract
Radical or partial nephrectomy, commonly used for the treatment of kidney tumors, is a surgical procedure with a risk of high blood loss. The primary aim of this study is to quantify blood loss and elucidate the redistribution of blood flux and pressure [...] Read more.
Radical or partial nephrectomy, commonly used for the treatment of kidney tumors, is a surgical procedure with a risk of high blood loss. The primary aim of this study is to quantify blood loss and elucidate the redistribution of blood flux and pressure between the two kidneys and the abdominal aorta during renal resection. We have developed a robust research methodology that introduces a new lumped-parameter mathematical model, specifically focusing on the vasculature of both kidneys using a non-Newtonian Carreau fluid. This model, a first-order approximation, accounts for the variation in the total impedance of the vasculature when various vessels are severed in the diseased kidney (assumed to be the left in this work). The model offers near real-time estimations of the flow–pressure redistribution within the vascular network of the two kidneys and the downstream aorta for several radical or partial nephrectomy scenarios. Notably, our findings indicate that the downstream aorta receives an approximately 1.27 times higher percentage of the redistributed flow from the diseased kidney compared to that received by the healthy kidney, in nearly all examined cases. The implications of this study are significant, as they can inform the development of surgical protocols to minimize blood loss and can assist surgeons in evaluating the adequacy of the remaining kidney vasculature. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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20 pages, 6955 KiB  
Article
Flow Performance Analysis of Non-Return Multi-Door Reflux Valve: Experimental Case Study
by Xolani Prince Hadebe, Bernard Xavier Tchomeni Kouejou, Alfayo Anyika Alugongo and Desejo Filipeson Sozinando
Fluids 2024, 9(9), 213; https://doi.org/10.3390/fluids9090213 - 12 Sep 2024
Cited by 1
Abstract
Non-return multi-door reflux valves are essential in fluid control systems to prevent reverse flow and maintain system integrity. This study experimentally analyzes the flow performance of multi-door check valves under different operating conditions, focusing on pressure testing and evaluating their effectiveness in preventing [...] Read more.
Non-return multi-door reflux valves are essential in fluid control systems to prevent reverse flow and maintain system integrity. This study experimentally analyzes the flow performance of multi-door check valves under different operating conditions, focusing on pressure testing and evaluating their effectiveness in preventing backflow. A wide-ranging experimental setup was designed and implemented to simulate real-world scenarios, facilitating accurate measurement of flow rates, pressure differences, and valve response times. The collected experimental data were analyzed to evaluate the valve’s performance in terms of flow capacity, pressure drop, and hydraulic efficiency. Additionally, the effects of factors such as valve size, valve configuration, and fluid properties (water) on performance were considered. It was found that the non-return multi-door reflux valve has been proven effective and reliable in preserving system integrity and maintaining unidirectional flow at the same time during pressure testing. It exhibits no backflow, remains stable and constant across varied flow conditions, and demonstrates a low pressure drop and high flow capacity, making it suitable for critical pressure testing applications. The response curve revealed that valve opening takes longer to reach higher flow rates than closing, indicating pressure instability during transition periods. This non-linear relationship indicates possible irregularities in pressure drop response to flow rate changes, highlighting potential areas for further investigation. Full article
(This article belongs to the Special Issue Computational Fluid Dynamics in Fluid Machinery)
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25 pages, 5876 KiB  
Article
Effects of Expelled Air during Filling Operations with Blocking Columns in Water Pipelines of Undulating Profiles
by Vicente S. Fuertes-Miquel, Oscar E. Coronado-Hernández and Alfonso Arrieta-Pastrana
Fluids 2024, 9(9), 212; https://doi.org/10.3390/fluids9090212 - 11 Sep 2024
Abstract
Entrapped air pockets can cause failure in water distribution systems if air valves have not been appropriately designed for expelling air during filling manoeuvres performed by water utilities. One-dimensional mathematical models recently developed for studying this phenomenon do not consider the effect of [...] Read more.
Entrapped air pockets can cause failure in water distribution systems if air valves have not been appropriately designed for expelling air during filling manoeuvres performed by water utilities. One-dimensional mathematical models recently developed for studying this phenomenon do not consider the effect of blocking columns inside water pipelines. This research presents the development of a mathematical model for analysing the filling process in a pipeline with an undulating profile with various air valves, including blocking columns during starting-up water installations. The results show how different air pocket pressure peaks can be produced over transient events, which need to be analysed to ensure a successful procedure that guarantees pipeline safety during the pressure surge occurrence. In this study, an experimental set-up is analysed to observe the behaviour of two blocking columns during filling by comparing the air pocket pressure pulses. Full article
(This article belongs to the Special Issue Modelling Flows in Pipes and Channels)
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24 pages, 5293 KiB  
Article
Computational Fluid Dynamics Study on Bottom-Hole Multiphase Flow Fields Formed by Polycrystalline Diamond Compact Drill Bits in Foam Drilling
by Lihong Wei and Jaime Honra
Fluids 2024, 9(9), 211; https://doi.org/10.3390/fluids9090211 - 10 Sep 2024
Abstract
High-temperature geothermal wells frequently employ foam drilling fluids and Polycrystalline Diamond Compact (PDC) drill bits. Understanding the bottom-hole flow field of PDC drill bits in foam drilling is essential for accurately analyzing their hydraulic structure design. Based on computational fluid dynamics (CFD) and [...] Read more.
High-temperature geothermal wells frequently employ foam drilling fluids and Polycrystalline Diamond Compact (PDC) drill bits. Understanding the bottom-hole flow field of PDC drill bits in foam drilling is essential for accurately analyzing their hydraulic structure design. Based on computational fluid dynamics (CFD) and multiphase flow theory, this paper establishes a numerical simulation technique for gas-liquid-solid multiphase flow in foam drilling with PDC drill bits, combined with a qualitative and quantitative hydraulic structure evaluation method. This method is applied to simulate the bottom-hole flow field of a six-blade PDC drill bit. The results show that the flow velocity of the air phase in foam drilling fluid is generally higher than that of the water phase. Some blades’ cutting teeth exhibit poor cleaning and cooling effects, with individual cutting teeth showing signs of erosion damage and cuttings cross-flow between channels. To address these issues, optimizing the nozzle spray angle and channel design is necessary to improve hydraulic energy distribution, enhance drilling efficiency, and extend drill bit life. This study provides new ideas and methods for developing geothermal drilling technology in the numerical simulation of a gas-liquid-solid three-phase flow field. Additionally, the combined qualitative and quantitative evaluation method offers new insights and approaches for research and practice in drilling engineering. Full article
(This article belongs to the Special Issue Multiphase Flow and Granular Mechanics)
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15 pages, 3822 KiB  
Article
Soot and Flame Structures in Turbulent Partially Premixed Jet Flames of Pre-Evaporated Diesel Surrogates with Admixture of OMEn
by Steffen Walther, Tao Li, Dirk Geyer, Andreas Dreizler and Benjamin Böhm
Fluids 2024, 9(9), 210; https://doi.org/10.3390/fluids9090210 - 10 Sep 2024
Abstract
In this study, the soot formation and oxidation processes in different turbulent, pre-evaporated and partially premixed diesel surrogate flames are experimentally investigated. For this purpose, a piloted jet flame surrounded by an air co-flow is used. Starting from a defined diesel surrogate mixture, [...] Read more.
In this study, the soot formation and oxidation processes in different turbulent, pre-evaporated and partially premixed diesel surrogate flames are experimentally investigated. For this purpose, a piloted jet flame surrounded by an air co-flow is used. Starting from a defined diesel surrogate mixture, different fuel blends with increasing blending ratios of poly(oxymethylene) dimethyl ether (OME) are studied. The Reynolds number, equivalence ratio, and vaporization temperature are kept constant to ensure the comparability of the different fuel mixtures. The effects of OME addition on flame structures, soot precursors, and soot are investigated, showing soot reduction when OME is added to the diesel surrogate. Using chemiluminescence images of C2 radicals (line of sight) and subsequent Abel-inversion, flame lengths and global flame structure are analyzed. The flame structure is visualized by means of planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH). The spatial distribution of soot precursors, such as polycyclic aromatic hydrocarbons (PAHs), is simultaneously measured by PLIF using the same excitation wavelength. In particular, aromatic compounds with several benzene rings (e.g., naphthalene or pyrene), which are known to be actively involved in soot formation and growth, have been visualized. Spatially distributed soot particles are detected by using laser-induced incandescence (LII), which allows us to study the onset of soot clouds and its structures qualitatively. Evident soot formation is observed in the pure diesel surrogate flame, whereas a significant soot reduction with changing PAH and soot structures can be identified with increasing OME addition. Full article
(This article belongs to the Special Issue Turbulence and Combustion)
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16 pages, 10890 KiB  
Article
Numerical Simulation Studies on the Design of the Prosthetic Heart Valves Belly Curves
by Jingyuan Zhou, Yinkui Wu, Lu Chen, Tao Li, Yan Xiong and Yu Chen
Fluids 2024, 9(9), 209; https://doi.org/10.3390/fluids9090209 - 10 Sep 2024
Abstract
Prosthetic heart valves (PHVs) are employed to replace the diseased native valve as a treatment of severe aortic valve disease. This study aimed to evaluate the effect of curvature of the belly curve on valve performance, so as to support a better comprehension [...] Read more.
Prosthetic heart valves (PHVs) are employed to replace the diseased native valve as a treatment of severe aortic valve disease. This study aimed to evaluate the effect of curvature of the belly curve on valve performance, so as to support a better comprehension of the relationship between valve design and its performance. Five PHV models with different curvatures of the belly curve were established. Iterative implicit fluid–structure interaction simulations were carried out, analyzing in detail the effect of belly curvature on the geometric orifice area (GOA), coaptation area (CA), regurgitant fraction (RF), leaflet kinematics and stress distribution on the leaflets. Overall, GOA and CA were negatively and positively related to the curvature of the belly curve, respectively. Nevertheless, an excessive increase in curvature can lead to incomplete sealing of free edges of the valve during its closure, which resulted in a decrease in CA and an increase in regurgitation. The moderate curvature of the belly curve contributed to reducing RF and fluttering frequency. Valves with small curvature experienced a significantly higher frequency of fluttering. Furthermore, all stress concentrations intensified with the increase in the curvature of the belly curve. The valve with moderate curvature of the belly curve strikes the best compromise between valve performance parameters, leaflet kinematics and mechanical stress. Considering the different effects of the curvature of belly curve on valve performance parameters, the PHV design with variable curvature of belly curve may be a direction towards valve performance optimization. Full article
(This article belongs to the Special Issue Advances in Hemodynamics and Related Biological Flows)
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22 pages, 825 KiB  
Article
Influence of a Background Shear Flow on Cyclone–Anticyclone Asymmetry in Ageostrophic Balanced Flows
by William Joseph McKiver
Fluids 2024, 9(9), 208; https://doi.org/10.3390/fluids9090208 - 3 Sep 2024
Viewed by 205
Abstract
In this paper, we study how cyclonic and anticyclonic vortices adapt their shape and orientation to a background shear flow in an effort to understand geophysical vortices. Here we use a balanced model that incorporates the effects of rotation and density stratification to [...] Read more.
In this paper, we study how cyclonic and anticyclonic vortices adapt their shape and orientation to a background shear flow in an effort to understand geophysical vortices. Here we use a balanced model that incorporates the effects of rotation and density stratification to model the case of an isolated vortex of uniform potential vorticity subjected to a background shear flow that mimics the effect of surrounding vortices. We find equilibrium states and analyze their linear stability to determine the vortex characteristics at the margin of stability. Differences are found between the cyclonic and anticyclonic equilibria depending on the background flow parameters. When there is only horizontal strain, the vertical aspect ratio of the vortex does not change, whereas increasing the imposed background strain rate causes a change in the horizontal cross section, with cyclones being more deformed than anticyclones for a given value of strain. Vertical shear not only causes changes in the vertical axis but also causes the vortex to tilt away from it upright position. Overall, anticyclonic equilibria tend to have a more circular horizontal cross section, a longer vertical axis, and a larger tilt angle with respect to cyclonic equilibria. The strongest asymmetry between the horizontal cross section of cyclonic and anticyclonic vortices occurs for low values of vertical shear, while the strongest asymmetry in the vertical axes and tilt angle occurs for large vertical shear. Finally, by expanding the vortex shape and orientation in terms of the strain rate, we derive simple formulas that provide insights into how the vortex equilibria depend on the background flow. Full article
(This article belongs to the Collection Advances in Geophysical Fluid Dynamics)
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4 pages, 161 KiB  
Editorial
Computational Fluid Dynamics Modeling and Experiments of Two-Phase Flows
by Van-Tu Nguyen and Hemant J. Sagar
Fluids 2024, 9(9), 207; https://doi.org/10.3390/fluids9090207 - 3 Sep 2024
Viewed by 180
Abstract
Two-phase flows are prevalent in natural phenomena, as well as a wide range of marine engineering and industrial applications [...] Full article
(This article belongs to the Special Issue Numerical Modeling and Experimental Studies of Two-Phase Flows)
8 pages, 2651 KiB  
Article
Analysis of the Influence of the Size of Color-Calibrated Schlieren Filters on the General Sensitivity of Quantitative Schlieren Systems
by Emilia Georgiana Prisăcariu, Tudor Prisecaru and Mădălin Constantin Dombrovschi
Fluids 2024, 9(9), 206; https://doi.org/10.3390/fluids9090206 - 2 Sep 2024
Viewed by 233
Abstract
The quantitative color schlieren technique is renowned for its capacity to convert deflection angles into color ratios. This technique has been instrumental in providing data on 2D flows. The current study delves into assessing how the geometry and optical characteristics of color filters [...] Read more.
The quantitative color schlieren technique is renowned for its capacity to convert deflection angles into color ratios. This technique has been instrumental in providing data on 2D flows. The current study delves into assessing how the geometry and optical characteristics of color filters impact the sensitivity of the schlieren system. At present, there are many papers making the assumption that implementing a larger-sized color filter leads to better system sensitivity. However, having more calibration filter positions can lead to measurement errors due to the difficult calibration process. The present investigation focuses on the type of color filters created with a gradual evolution of colors. A turbulent, round water vapor jet serves as the test case. By comparing the results obtained with two different filter sizes, this analysis gives insight into the compromises made between system sensitivity and ease of calibration, helping one to better understand the trade-offs between the above-mentioned parameters. Moreover, the quantitative and qualitative results of the test case are presented to offer more comprehensive insights into quantitative color-calibrated schlieren. Full article
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15 pages, 3403 KiB  
Article
Machine Learning for Dynamic Pressure Coefficient Prediction in Vertical Water Jets
by Amin Salemnia, Seyedehmaryam Hosseini Boldaji, Vida Atashi and Manoochehr Fathi-Moghadam
Fluids 2024, 9(9), 205; https://doi.org/10.3390/fluids9090205 - 1 Sep 2024
Viewed by 317
Abstract
Vertical water jets present significant challenges for hydraulic structures due to their potential to cause erosion and structural damage. This study aimed to predict the dimensionless pressure coefficient (Cp) of vertical water jets by examining the relationships between experimental parameters, such [...] Read more.
Vertical water jets present significant challenges for hydraulic structures due to their potential to cause erosion and structural damage. This study aimed to predict the dimensionless pressure coefficient (Cp) of vertical water jets by examining the relationships between experimental parameters, such as Froude number, slope, and the ratio of waterfall height over the product of the Froude number and diameter, referred to as α, using machine learning models. Two hundred forty controlled experiments were conducted, with pressure data collected. To address the problem’s non-linearity, six machine learning models were tested: linear regression, K-nearest neighbors, decision tree, support vector regression, random forest, and XGBoost. The XGBoost model outperformed others, achieving an R-squared of 0.953 and a Root Mean Squared Error (RMSE) of 0.191. Residual analysis validated its better performance, demonstrating that it delivered the most accurate predictions with minimal bias. Feature importance analysis revealed the Froude number was the most significant predictor, followed by slope and diameter. This study emphasizes the importance of the Froude number in predicting jet behavior and shows the efficacy of advanced machine learning models in capturing complex fluid dynamics, providing valuable insights for optimizing engineering applications such as water jet cutting and cooling systems. Full article
(This article belongs to the Special Issue Machine Learning and Artificial Intelligence in Fluid Mechanics)
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15 pages, 4009 KiB  
Article
A Numerical Approach to Analyzing Shallow Flows over Rough Surfaces
by M. Nasimul Chowdhury, Abdul A. Khan and Oscar Castro-Orgaz
Fluids 2024, 9(9), 204; https://doi.org/10.3390/fluids9090204 - 1 Sep 2024
Viewed by 238
Abstract
The hydraulic characteristics (such as velocity profiles, near-bed velocity profile, bed shear stress, and resistance coefficients) of shallow flows over rough surfaces were investigated using numerical simulations. A novel method is presented to simulate shallow flows over rough surfaces in a two-dimensional (2D) [...] Read more.
The hydraulic characteristics (such as velocity profiles, near-bed velocity profile, bed shear stress, and resistance coefficients) of shallow flows over rough surfaces were investigated using numerical simulations. A novel method is presented to simulate shallow flows over rough surfaces in a two-dimensional (2D) numerical domain, where the physical numerical domain represents bed topography. Results reveal that the model can accurately predict spatially averaged velocity profiles, turbulence characteristics, shear stresses, and uniform flow depths. The analysis identified two distinct flow regions based on mean and turbulent flow profiles. Results show that the turbulent shear stress profiles provide a more accurate estimation of the bed shear stresses. Resistance coefficients (friction factor or Manning’s roughness coefficient) vary with Froude number and submergence ratio (depth divided by roughness height). Full article
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12 pages, 1539 KiB  
Article
Gel Diffusiophoresis of a Spherical Colloidal Particle
by Hiroyuki Ohshima
Fluids 2024, 9(9), 203; https://doi.org/10.3390/fluids9090203 - 1 Sep 2024
Viewed by 268
Abstract
A theoretical framework is established for the gel diffusiophoresis of a spherical colloidal particle moving through an uncharged dilute porous polymer gel medium when an electrolyte concentration gradient field is applied. The network of cross-linked polymer segments is treated as a porous skeleton [...] Read more.
A theoretical framework is established for the gel diffusiophoresis of a spherical colloidal particle moving through an uncharged dilute porous polymer gel medium when an electrolyte concentration gradient field is applied. The network of cross-linked polymer segments is treated as a porous skeleton containing an electrolyte solution using the Brinkman–Debye–Bueche model. We derive a general expression for the gel-diffusiophoretic mobility of a charged spherical colloidal particle. Based on this general mobility expression, we farther derive a closed-form approximate expression for the gel-diffusiophoretic mobility of a weakly charged spherical particle correct to the second order of the particle’s zeta potential. The obtained mobility expression depends on the Debye–Hückel parameter and the Brinkmann parameter. Full article
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17 pages, 911 KiB  
Article
Turbulent Micropolar Open-Channel Flow
by George Sofiadis, Antonios Liakopoulos, Apostolos Palasis and Filippos Sofos
Fluids 2024, 9(9), 202; https://doi.org/10.3390/fluids9090202 - 31 Aug 2024
Viewed by 182
Abstract
The present paper focuses on the investigation of the turbulent characteristics of an open-channel flow by employing the micropolar model. The underlying model has already been proven to correctly describe the secondary phase of turbulent wall-bounded flows. The open-channel case comprises an ideal [...] Read more.
The present paper focuses on the investigation of the turbulent characteristics of an open-channel flow by employing the micropolar model. The underlying model has already been proven to correctly describe the secondary phase of turbulent wall-bounded flows. The open-channel case comprises an ideal candidate to further test the micropolar model as many environmental flows carry a secondary phase, the behavior of which is of great interest for applications such as sedimentation transport and debris flow. Direct Numerical Simulations (DNSs) have been carried out on an open channel for Reb = 11,200 based on mean crossectional velocity, channel height, and the fluid kinematic viscosity. The simulated results are compared against previous experimental as well as Langrangian DNS data of similar flows, with excellent agreement. The micropolar model is capable of describing the same problem but in an Eulerian frame, thus significantly simplifying the computational cost and complexity. Full article
(This article belongs to the Special Issue Modelling Flows in Pipes and Channels)
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15 pages, 2857 KiB  
Article
An Analysis of Water Leakages in Series Pipelines Using the Rigid Column Model
by Vicente S. Fuertes-Miquel, Oscar E. Coronado-Hernández and Alfonso Arrieta-Pastrana
Fluids 2024, 9(9), 201; https://doi.org/10.3390/fluids9090201 - 29 Aug 2024
Viewed by 352
Abstract
Water leakages are assessed with an extended period simulation (traditional analysis) in water distribution networks. However, when rapid manoeuvres regulate valves, the conventional analysis proves unsuitable for representing the water volume of leaks. Under these circumstances, the Rigid Water Column Model can be [...] Read more.
Water leakages are assessed with an extended period simulation (traditional analysis) in water distribution networks. However, when rapid manoeuvres regulate valves, the conventional analysis proves unsuitable for representing the water volume of leaks. Under these circumstances, the Rigid Water Column Model can be utilised to accurately compute the quantity of water volume leaks. This method has been applied to single and parallel pipelines to predict water leakages, considering the opening and closure manoeuvres of regulating valves. This research develops governing equations using the Rigid Column Model to predict water volume leaks in a general scheme of series pipelines, accounting for the opening and closure manoeuvres in regulating valves. The practical application involves two branches of a series pipeline with internal pipe diameters of 0.45 and 0.25 m and pipe lengths of 1200 and 1400 m, respectively. The results of this practical application demonstrate the accuracy of the Rigid Column Model and its relevance in real-world scenarios. For instance, the model shows that traditional analysis can overestimate water volume leaks by approximately 72% when a closure manoeuvre regulates valves for 200 s and by about 28% for 30 s, highlighting the need for a more accurate method. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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18 pages, 1949 KiB  
Article
EHD Instabilities in Two Layers of Insulating and Conducting Immiscible Liquids Subjected to Unipolar Charge Injection
by Dantchi Koulova and Pierre Atten
Fluids 2024, 9(9), 200; https://doi.org/10.3390/fluids9090200 - 28 Aug 2024
Viewed by 352
Abstract
In this paper, the instability of two layers of insulating and conducting immiscible liquids separated by a deformable interface and subjected to unipolar injection is examined. Taking into account the slight deformation of the interface between the two liquids, a system of equations [...] Read more.
In this paper, the instability of two layers of insulating and conducting immiscible liquids separated by a deformable interface and subjected to unipolar injection is examined. Taking into account the slight deformation of the interface between the two liquids, a system of equations and boundary conditions is derived at marginal state. Non zero numerical solutions for both layers exist only for eigenvalues of the instability parameter T, which depends on the following parameters: injection level C, Bond number Bo, a new non-dimensional parameter P proportional to interfacial tension and the ratio of the layers’ thickness and of liquids viscosity. The variations in the instability criterion Tc, corresponding to the smallest eigenvalue, are examined in detail as a function of the main characteristic parameters C, P and the Bond number. We find that for some values of P, two instability mechanisms convective and interfacial ones can take place. When the strength of interfacial tension or the liquid thickness ratio is very low, the critical number tends to a value corresponding to interfacial instability. The influence of injection-induced convection in the insulating layer and the effect of interfacial deformation on interfacial instability are also discussed. Full article
(This article belongs to the Special Issue Advances in Electrohydrodynamic Flow)
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0 pages, 7102 KiB  
Article
Pressure Transient Solutions for Unbounded and Bounded Reservoirs Produced and/or Injected via Vertical Well Systems with Constant Bottomhole Pressure
by Ruud Weijermars and Clement Afagwu
Fluids 2024, 9(9), 199; https://doi.org/10.3390/fluids9090199 - 28 Aug 2024
Viewed by 262
Abstract
Various analytical solutions for computing production and injection-induced pressure changes in aquifers and oil reservoirs have been derived over the past century. All prior solutions assumed a constant well rate as the boundary condition. However, in many practical situations, the fluid withdrawal from [...] Read more.
Various analytical solutions for computing production and injection-induced pressure changes in aquifers and oil reservoirs have been derived over the past century. All prior solutions assumed a constant well rate as the boundary condition. However, in many practical situations, the fluid withdrawal from and/or injection into such subsurface reservoirs occurs with the aid of pump devices that maintain a constant bottomhole pressure in the well. Until now, how the well rate will decline over time, based on the pressure difference in the well relative to the initial reservoir pressure, could not be rapidly computed analytically (using the diffusivity as the key governing system parameter), because no concise expression had been derived with the boundary condition of a constant bottomhole pressure. The present study shows how the pressure diffusion equation can be readily solved for wells acting as sinks and sources with a constant bottomhole pressure condition. We consider both fractured and unfractured completions, as well as injection and production modes. The new solutions do not require an elaborate time-stepped pressure-matching procedure as in nodal analysis, the only other physics-based analytical method currently available to compute the well rate decline when a constant bottomhole pressure production system is used, which unlike our new method proposed here is limited to single well systems. Full article
(This article belongs to the Topic Advances in Environmental Hydraulics)
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18 pages, 4302 KiB  
Article
Prediction of Geometrical Characteristics of an Inclined Negatively Buoyant Jet Using Group Method of Data Handling (GMDH) Neural Network
by Hassan Alfaifi and Hossein Bonakdari
Fluids 2024, 9(9), 198; https://doi.org/10.3390/fluids9090198 - 28 Aug 2024
Viewed by 224
Abstract
A new approach to predicting the geometrical characteristics of the mixing behavior of an inclined dense jet for angles ranging from 15° to 85° is proposed in this study. This approach is called the group method of data handling (GMDH) and is based [...] Read more.
A new approach to predicting the geometrical characteristics of the mixing behavior of an inclined dense jet for angles ranging from 15° to 85° is proposed in this study. This approach is called the group method of data handling (GMDH) and is based on the artificial neural network (ANN) technique. The proposed model was trained and tested using existing experimental data reported in the literature. The model was then evaluated using statistical indices, as well as being compared with analytical models from previous studies. The results of the coefficient of determination (R2) indicate the high accuracy of the proposed model, with values of 0.9719 and 0.9513 for training and testing for the dimensionless distance from the nozzle to the return point xr/D and 0.9454 and 0.9565 for training and testing for the dimensionless terminal rise height yt/D. Moreover, four previous analytical models were used to evaluate the GMDH model. The results showed the superiority of the proposed model in predicting the geometrical characteristics of the inclined dense jet for all tested angles. Finally, the standard error of the estimate (SEE) was applied to demonstrate which model performed the best in terms of approaching the actual data. The results illustrate that all fitting lines of the GMDH model performed very well for all geometrical parameter predictions and it was the best model, with an approximately 10% error, which was the lowest error value among the models. Therefore, this study confirms that the GMDH model can be used to predict the geometrical properties of the inclined negatively buoyant jet with high performance and accuracy. Full article
(This article belongs to the Special Issue Experimental Fluid Mechanics on Bluff Body Wakes and Jets)
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27 pages, 17970 KiB  
Article
Determination of Local Heat Transfer Coefficients and Friction Factors at Variable Temperature and Velocity Boundary Conditions for Complex Flows
by Christopher Hartmann and Jens von Wolfersdorf
Fluids 2024, 9(9), 197; https://doi.org/10.3390/fluids9090197 - 27 Aug 2024
Viewed by 316
Abstract
Transient conjugate heat transfer measurements under varying temperature and velocity inlet boundary conditions at incompressible flow conditions were performed for flat plate and ribbed channel geometries. Therefrom, local adiabatic wall temperatures and heat transfer coefficients were determined. The data were analyzed using typical [...] Read more.
Transient conjugate heat transfer measurements under varying temperature and velocity inlet boundary conditions at incompressible flow conditions were performed for flat plate and ribbed channel geometries. Therefrom, local adiabatic wall temperatures and heat transfer coefficients were determined. The data were analyzed using typical heat transfer correlations, e.g., Nu=CRemPrn, determining the local distributions of C and m. It is shown that they are closely linked. A relationship lnC=A−mB is observed, with A and B as modeling parameters. They could be related to parameters in log-law or power-law representations for turbulent boundary layer flows. The parameter m is shown to have a close link to local pressure gradients and, therewith, near wall streamlines as well as friction factor distributions. A normalization of the C parameter allows one to derive a Reynolds analogy factor and, therefrom, local wall shear stresses. Full article
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21 pages, 7824 KiB  
Article
POD Analysis of the Wake of Two Tandem Square Cylinders
by Jingcheng Hao, Siva Ramalingam, Md. Mahbub Alam, Shunlin Tang and Yu Zhou
Fluids 2024, 9(9), 196; https://doi.org/10.3390/fluids9090196 - 26 Aug 2024
Viewed by 450
Abstract
This study aims to investigate the wake of two tandem square cylinders based on the Proper Orthogonal Decomposition (POD) analyses of the PIV and hotwire data. The cylinder centre-to-centre spacing ratio L/w examined is from 1.2 to 4.2, covering the four [...] Read more.
This study aims to investigate the wake of two tandem square cylinders based on the Proper Orthogonal Decomposition (POD) analyses of the PIV and hotwire data. The cylinder centre-to-centre spacing ratio L/w examined is from 1.2 to 4.2, covering the four flow regimes, i.e., extended body, reattachment, transition and co-shedding. The Reynolds number examined was 1.3 × 104. A novel Proper Orthogonal Decomposition (POD) technique (hereafter referred to as PODHW) is developed to analyse data from single point hotwire measurements, offering a new perspective compared to the conventional POD analysis (PODPIV) based on Particle Image Velocimetry (PIV) data. A key finding is the identification of two distinct states, reattachment and co-shedding, within the transition flow regime at L/w = 2.8, which PODPIV fails to capture due to the limited duration of the PIV data obtained. This study confirms, for the first time, the existence of these states as proposed by Zhou et al. (2024), highlighting the advantage of using PODHW for capturing intermittent flow phenomena. Furthermore, the analysis reveals how the predominant coherent structures contribute to the total fluctuating velocity energy in each individual regime. Other aspects of the flow are also discussed, including the Strouhal numbers, the contribution to the total fluctuating energy of the flow from the first four POD modes, and a comparison between different regimes. Full article
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18 pages, 6740 KiB  
Article
Simulation of Corner Solidification in a Cavity Using the Lattice Boltzmann Method
by Runa Samanta and Himadri Chattopadhyay
Fluids 2024, 9(9), 195; https://doi.org/10.3390/fluids9090195 - 25 Aug 2024
Viewed by 355
Abstract
This study investigates corner solidification in a closed cavity in which the left and bottom walls are kept at a temperature lower than its initial temperature. The liquid material in the cavity initially lies at its phase transition temperature and, due to cold [...] Read more.
This study investigates corner solidification in a closed cavity in which the left and bottom walls are kept at a temperature lower than its initial temperature. The liquid material in the cavity initially lies at its phase transition temperature and, due to cold boundary conditions at the left–bottom walls, solidification starts. The simulation of corner solidification was performed using a kinetic-based lattice Boltzmann method (LBM), and the tracking of the solid–liquid interface was captured through the evaluation of time. The present investigation addresses the effect of natural convection over conduction across a wide range of higher Rayleigh numbers, from 106 to 108. The total-enthalpy-based lattice Boltzmann method (ELBM) was used to observe the thermal profiles in the entire cavity with a two-phase interface. The isotherms reveal the relative dominance of natural convection over conduction, and the pattern of interface reveals the effective growth of the solidified layer in the cavity. To quantify the uniformity of cooling, a coefficient of variation (COV) for the thermal field was calculated in the effective solidified zone at a wide range of Ra. The results show that the value of COV increases with Ra and reduces with time. The thermal instability in the flow field is also quantified through FFT analyses. Full article
(This article belongs to the Special Issue Lattice Boltzmann Methods: Fundamentals and Applications)
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14 pages, 1911 KiB  
Article
Nonlinear Wrinkling Dynamics of a Multi-Component Vesicle (2D)
by Meng Zhao and Kai Liu
Fluids 2024, 9(9), 194; https://doi.org/10.3390/fluids9090194 - 23 Aug 2024
Viewed by 205
Abstract
This paper investigates wrinkling dynamics of two-dimensional multicomponent vesicles subjected to time-dependent extensional flow. By employing a non-stiff, pseudo-spectral boundary integral approach, we inspect the wrinkling patterns that arise due to negative surface tension and differential bending within a two-phase system. We focus [...] Read more.
This paper investigates wrinkling dynamics of two-dimensional multicomponent vesicles subjected to time-dependent extensional flow. By employing a non-stiff, pseudo-spectral boundary integral approach, we inspect the wrinkling patterns that arise due to negative surface tension and differential bending within a two-phase system. We focus on the formation and evolution of the wrinkling behaviors under diverse phase concentrations, extensional rates, and vesicle sphericity. Our findings demonstrate that for slightly perturbed circular vesicles, the numerical simulations align well with perturbation theory. For elongated vesicles, the wrinkling patterns vary significantly between phases, primarily influenced by their respective bending moduli. In weak flows, buckling behaviors are observed for elongated vesicles, where the membrane bends inward in regions with lower bending modulus. Full article
(This article belongs to the Special Issue Non-Newtonian Flow: Interfacial and Bulk Phenomena)
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