Fluids

10 pages, 1325 KiB

Open AccessArticle

Molecular Dynamics of Nanodroplet Coalescence in Quasi-Saturated Vapor

by Dmitry Beloborodov and Aleksey Vishnyakov

Fluids 2023, 8(2), 77; https://doi.org/10.3390/fluids8020077 - 20 Feb 2023

Cited by 2 | Viewed by 1480

The dynamics of coalescence of small Lennard–Jones droplets as a function of droplet size and temperature is explored with molecular simulations. Droplet sizes vary from several hundred to several thousand molecules, and three different temperatures are explored. As the droplets establish contact, a [...] Read more.

The dynamics of coalescence of small Lennard–Jones droplets as a function of droplet size and temperature is explored with molecular simulations. Droplet sizes vary from several hundred to several thousand molecules, and three different temperatures are explored. As the droplets establish contact, a liquid-like bridge between them forms and grows, ultimately leading to a complete coalescence. The dynamics of the bridge growth are consistent with the “collective molecular jumps” mechanism reported in the literature rather than with the continuous interpretation of the coalescence process in terms of capillary and viscous forces. The effective coalescence time shows a linear growth with the droplet sizes. The influence of the larger droplet size is weaker but non-negligible. Surprisingly, practically no dependence of the coalescence time on the temperature is observed. Comparison of the coalescence times with the droplet lifespan in a suspension shows that for reasonably dense suspensions and small droplet sizes, the coalescence time becomes significant and should be accounted for in the theoretical models of aggregation. Full article

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

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18 pages, 9811 KiB

Open AccessArticle

The Use of Uncertainty Quantification and Numerical Optimization to Support the Design and Operation Management of Air-Staging Gas Recirculation Strategies in Glass Furnaces

by Carlo Cravero, Davide De Domenico and Davide Marsano

Fluids 2023, 8(2), 76; https://doi.org/10.3390/fluids8020076 - 18 Feb 2023

Cited by 15 | Viewed by 2616

Abstract

The reduction in energy consumption and the increasingly demanding emissions regulations have become strategic challenges for every industrial sector. In this context, the glass industry would be one of the most affected sectors due to its high energy demand and emissions productions, especially [...] Read more.

The reduction in energy consumption and the increasingly demanding emissions regulations have become strategic challenges for every industrial sector. In this context, the glass industry would be one of the most affected sectors due to its high energy demand and emissions productions, especially in terms of NO_x. For this reason, various emission abatement systems have been developed in this field and one of the most used is the air staging system. It consists in injecting air into the upper part of the regenerative chamber on the exhaust gases side in order to create the conditions for combustion that reduces NO_x emissions. In this work, the combined use of CFD with data analysis techniques offers a tool for the design and management of a hybrid air staging system. Surrogate models of the bypass mass flow rate and uniformity index in the regenerative chamber have been obtained starting from DoE based on different simulations by varying the air mass flow rate of the two injectors located in a bypass duct that connects the two regenerative chambers. This model allows a UQ analysis to verify how the uncertainty of the air injectors can affect the bypass mass flow rate. Finally, an optimization procedure has identified the optimal condition for the best bypass mass flow rates and uniformity of the oxygen concentration in the chamber. High values of the mass flow rate of the pros injector and medium-low values for the cons injectors are identified as operating parameters for best conditions. Full article

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11 pages, 1661 KiB

Open AccessArticle

Effect of Internal Waves on Moving Small Vessels in the Sea

by Andrey Serebryany

Fluids 2023, 8(2), 75; https://doi.org/10.3390/fluids8020075 - 18 Feb 2023

Viewed by 2070

Abstract

Internal waves are responsible for many important processes in the ocean environment (ocean ventilation, energy transfer from large-scale processes to turbulence, etc.). Our goal is to draw attention to the relatively little studied effect of internal waves on ships at sea. We encountered [...] Read more.

Internal waves are responsible for many important processes in the ocean environment (ocean ventilation, energy transfer from large-scale processes to turbulence, etc.). Our goal is to draw attention to the relatively little studied effect of internal waves on ships at sea. We encountered this effect many times while working on the study of internal waves in the shelf zone. The work was carried out from a yacht equipped with the “Rio Grande 600 kHz” ADCP, which makes it possible to measure both the parameters of internal waves and other important parameters of the medium. Two typical examples of the impact of internal waves on a yacht are given. One, when the yacht was at anchor and a train of soliton-like internal waves passed under it. The second, when the yacht was moving and passing over a train of internal waves. Internal waves passing under the moored yacht shifted its position synchronously with the periods of passing waves. A uniformly moving yacht, passing over a package of internal waves synchronously with the period of waves, alternately increased and decreased the speed of its movement. The described effect is explained by the impact on the vessel of the orbital currents of internal waves. Under our conditions, at heights of internal waves reaching 10–15 m, the ship’s speed fluctuations reached 0.30 m/s, which was more than 10% of the ship’s speed. Full article

(This article belongs to the Special Issue Fluid Dynamics: Wave–Structure Interactions)

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13 pages, 1914 KiB

Open AccessArticle

Kinetic Features of Cd and Zn Cathodic Formations in the Membrane Electrolysis Process

by Vasyl Serdiuk, Ivan Pavlenko, Svitlana Bolshanina, Vsevolod Sklabinskyi, Sylwia Włodarczak, Andżelika Krupińska, Magdalena Matuszak, Zdzisław Bielecki and Marek Ochowiak

Fluids 2023, 8(2), 74; https://doi.org/10.3390/fluids8020074 - 17 Feb 2023

Cited by 1 | Viewed by 1369

Abstract

Chromate and dichromate solutions used for the activation and passivation of cadmium and zinc galvanic coatings of metal products are widely used due to their ability to form corrosion-protective films. Therefore, in this article, we examined the kinetic features of the cathodic deposition [...] Read more.

Chromate and dichromate solutions used for the activation and passivation of cadmium and zinc galvanic coatings of metal products are widely used due to their ability to form corrosion-protective films. Therefore, in this article, we examined the kinetic features of the cathodic deposition of Cd and Zn during membrane electrolysis. As a result of comprehensive experimental and theoretical studies, the features of Cd and Zn cathodic depositions were analyzed under different hydrodynamic conditions in a submembrane zone of an anolyte. Experimental physicochemical methods such as the experimental analysis of solutions, analytical modeling, and a statistical analysis were used during the research. A regression dependence for evaluating a reaction rate constant was assessed based on the least-square approximation of the proposed model. As a result, the peculiarities of the cathodic formations for Cd and Zn during the membrane electrolysis process were analyzed. The effect of mechanical mixing with different values of the Reynolds number on the deposition of Cd and Zn on a cathode was evaluated. A change in Cd²⁺ and Zn²⁺ ion concentrations was also considered during the research. Overall, the obtained results increased the Cd deposition rate by 2.2 times using an active hydrodynamic environment with the anolyte. Full article

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17 pages, 7238 KiB

Open AccessArticle

Investigating the Linear Dynamics of the Near-Field of a Turbulent High-Speed Jet Using Dual-Particle Image Velocimetry (PIV) and Dynamic Mode Decomposition (DMD)

by Vishal Chaugule, Alexis Duddridge, Tushar Sikroria, Callum Atkinson and Julio Soria

Fluids 2023, 8(2), 73; https://doi.org/10.3390/fluids8020073 - 17 Feb 2023

Viewed by 1915

Abstract

The quest for the physical mechanisms underlying turbulent high-speed jet flows is underpinned by the extraction of spatio-temporal coherent structures from their flow fields. Experimental measurements to enable data decomposition need to comprise time-resolved velocity fields with a high-spatial resolution—qualities which current particle [...] Read more.

The quest for the physical mechanisms underlying turbulent high-speed jet flows is underpinned by the extraction of spatio-temporal coherent structures from their flow fields. Experimental measurements to enable data decomposition need to comprise time-resolved velocity fields with a high-spatial resolution—qualities which current particle image velocimetry hardware are incapable of providing. This paper demonstrates a novel approach that addresses this challenge through the implementation of an experimental high-spatial resolution dual-particle image velocimetry methodology coupled with dynamic mode decomposition. This new approach is exemplified by its application in studying the dynamics of the near-field region of a turbulent high-speed jet, enabling the spatio-temporal structure to be investigated by the identification of the spatial structure of the dominant dynamic modes and their temporal dynamics. The spatial amplification of these modes is compared with that predicted by classical linear stability theory, showing close agreement, which demonstrates the powerful capability of this technique to identify the dominant frequencies and their associated spatial structures in high-speed turbulent flows. Full article

(This article belongs to the Special Issue Experimental Fluid Mechanics on Bluff Body Wakes and Jets)

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8 pages, 349 KiB

Open AccessArticle

Calculation of Thermodynamic Characteristics and Sound Velocity for Two-Dimensional Yukawa Fluids Based on a Two-Step Approximation for the Radial Distribution Function

by Ilnaz I. Fairushin and Anatolii V. Mokshin

Fluids 2023, 8(2), 72; https://doi.org/10.3390/fluids8020072 - 17 Feb 2023

Cited by 2 | Viewed by 1529

Abstract

We propose a simple two-step approximation for the radial distribution function of a one-component two-dimensional Yukawa fluid. This approximation is specified by the key parameters of the system: coupling parameter and screening parameter. On the basis of this approximation, analytical expressions are obtained [...] Read more.

We propose a simple two-step approximation for the radial distribution function of a one-component two-dimensional Yukawa fluid. This approximation is specified by the key parameters of the system: coupling parameter and screening parameter. On the basis of this approximation, analytical expressions are obtained for the same thermodynamic quantities as internal energy, internal pressure, excess entropy in the two-particle approximation, and also longitudinal sound velocity. The theoretical results show an agreement with the results obtained in the case of a true radial distribution function. Full article

(This article belongs to the Special Issue Radial Basis Functions and their Applications in Fluids)

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15 pages, 2924 KiB

Open AccessArticle

Hemodynamic Assessment of the Pathological Left Ventricle Function under Rest and Exercise Conditions

by Jana Korte, Thomas Rauwolf, Jan-Niklas Thiel, Andreas Mitrasch, Paulina Groschopp, Michael Neidlin, Alexander Schmeißer, Rüdiger Braun-Dullaeus and Philipp Berg

Fluids 2023, 8(2), 71; https://doi.org/10.3390/fluids8020071 - 16 Feb 2023

Cited by 4 | Viewed by 1551

Abstract

Purpose: The analysis of pathological human left ventricular hemodynamics using high-resolved image-based blood flow simulations shows a major potential for examining mitral valve insufficiency (MI) under exercise conditions. Since capturing and simulating the patient-specific movement of the left ventricle (LV) during rest and [...] Read more.

Purpose: The analysis of pathological human left ventricular hemodynamics using high-resolved image-based blood flow simulations shows a major potential for examining mitral valve insufficiency (MI) under exercise conditions. Since capturing and simulating the patient-specific movement of the left ventricle (LV) during rest and exercise is challenging, this study aims to propose a workflow to analyze the hemodynamics within the pathologically moving LV. Methods: Patient-specific ultrasound (US) data of ten patients with MI in different stages were captured with three-dimensional real-time echocardiography. US measurements were performed while patients were resting and while doing handgrip exercise (2–4 min work). Patient-specific hemodynamic simulations were carried out based on the captured ventricular wall movement. Velocity and kinetic energy were analyzed for rest and exercise and for the different MI stages. Results: The results reveal a dependency of the kinetic energy over time in the ventricular volume curves. Concerning the comparison between rest and exercise, the left ventricular function reveals lower systolic kinetic energy under exercise (kinetic energy normalized by EDV; mean ± standard deviation: rest = 0.16 ± 0.14; exercise = 0.06 ± 0.05; p-value = 0.04). Comparing patients with non-limiting (MI I) and mild/moderate (MI II/III) MI, lower velocities (mean ± standard deviation: non-limiting = 0.10 ± 0.03; mild/moderate = 0.06 ± 0.02; p-value = 0.01) and lower diastolic kinetic energy (kinetic energy normalized by EDV; mean ± standard deviation: non-limiting = 0.45 ± 0.30; mild/moderate = 0.20 ± 0.19; p-value = 0.03) were found for the latter. Conclusion: With the proposed workflow, the hemodynamics within LVs with MI can be analyzed under rest and exercise. The results reveal the importance of the patient-specific wall movement when analyzing intraventricular hemodynamics. These findings can be further used within patient-specific simulations, based on varying the imaging and segmentation methods. Full article

(This article belongs to the Special Issue Image-Based Computational and Experimental Biomedical Flows)

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13 pages, 6149 KiB

Open AccessArticle

High Power Output Augmented Vertical Axis Wind Turbine

by Hayder Salem, Adel Mohammedredha and Abdullah Alawadhi

Fluids 2023, 8(2), 70; https://doi.org/10.3390/fluids8020070 - 16 Feb 2023

Cited by 4 | Viewed by 2378

Abstract

Nowadays, wind energy is one of the most cost-effective and environmentally friendly energies in high demand due to shortages in fossil fuels and the necessity to reduce global carbon footprint. One of the main goals of wind turbine development is to increase the [...] Read more.

Nowadays, wind energy is one of the most cost-effective and environmentally friendly energies in high demand due to shortages in fossil fuels and the necessity to reduce global carbon footprint. One of the main goals of wind turbine development is to increase the power output of the turbine either by increasing the turbine blade swept area or increasing the velocity of the wind. In this article, a proprietary augmentation system was introduced to increase the power output of vertical axis wind turbines (VAWT) by increasing the free stream velocity to more than two folds. The system comprises two identical airfoiled casings within which the turbine/turbines are seated. The results showed that the velocity slightly increases when decreasing the gap between the casing. It was also found that changing the angle of attack of the housing has more impact on the augmented airspeed. CFD technique was used to assess the velocity and flow of air around the system. Full article

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

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17 pages, 4812 KiB

Open AccessArticle

Numerical Study of the Effects of Asymmetric Velocity Profiles in a Curvilinear Channel on Migration of Neutral Buoyant Particle

by Ryo Naito and Tomohiro Fukui

Fluids 2023, 8(2), 69; https://doi.org/10.3390/fluids8020069 - 16 Feb 2023

Cited by 1 | Viewed by 1586

Abstract

The microstructure and suspended particle behavior should be considered when studying the flow properties exhibited by particle suspension. In addition, particle migration, also known as Segré–Silberberg effects, alters the microstructure of the suspension and significantly affects the viscosity properties of the suspension. Therefore, [...] Read more.

The microstructure and suspended particle behavior should be considered when studying the flow properties exhibited by particle suspension. In addition, particle migration, also known as Segré–Silberberg effects, alters the microstructure of the suspension and significantly affects the viscosity properties of the suspension. Therefore, particle behavior with respect to the changes in mechanical factors should be considered to better understand suspension. In this study, we investigated the particle behavior in asymmetric velocity profiles with respect to the channel center numerically using the lattice Boltzmann method and a two-way coupling scheme. Our findings confirmed that the final equilibrium position of particles in asymmetric velocity profiles converged differently between the outer and inner wall sides with respect to the channel center. This indicates that the mechanical equilibrium position of particles can be changed by asymmetric velocity profiles. In addition, centrifugal force acting on the particles is also important in the study of equilibrium position. These results suggest that the microstructure and viscosity characteristics of a suspension in a pipe could be handled by changes in velocity profiles. Full article

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

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18 pages, 3773 KiB

Open AccessArticle

The LNG Flow Simulation in Stationary Conditions through a Pipeline with Various Types of Insulating Coating

by Ildar Shammazov and Ekaterina Karyakina

Fluids 2023, 8(2), 68; https://doi.org/10.3390/fluids8020068 - 14 Feb 2023

Cited by 7 | Viewed by 3436

Abstract

Liquefied natural gas (LNG) is one of the most promising fuels for energy supply because it has a favorable combination of environmental and economic properties in connection with new trends aimed at the development of ecological and sustainable consumption of natural resources, which [...] Read more.

Liquefied natural gas (LNG) is one of the most promising fuels for energy supply because it has a favorable combination of environmental and economic properties in connection with new trends aimed at the development of ecological and sustainable consumption of natural resources, which ensure a constant growth in LNG consumption. The article presents an analytical review of the main technical solutions for the construction of cryogenic pipelines and insulating coating structures. The ANSYS Fluent software was used for simulation of the LNG flow in a pipeline section 10 m long with an outer diameter of 108 mm for three types of insulating coating (polyurethane (PU) foam, aerogel, and vacuum-insulated pipe (VIP)). In addition, an assessment was made of the insulating effect on the LNG temperature distribution along the length of the pipeline. The largest increase in temperature from 113 K to 113.61 K occurs in PU foam-insulated pipes; the smallest was observed in VIP. Further, as an alternative to steel, the use of ultra-high molecular weight polyethylene (UHMWPE) for pipeline material was considered. The optimal result in terms of temperature distributions was obtained while simulating the flow of an LNG pipeline with PU foam by increasing the thickness of the insulating coating to 0.05 m. Full article

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11 pages, 942 KiB

Open AccessArticle

Unsteady Coupled Heat Transfer in the Air and Surrounding Rock Mass for Mine Excavations with Distributed Heat Sources

by Artem Zaitsev, Andrey Shalimov and Dmitriy Borodavkin

Fluids 2023, 8(2), 67; https://doi.org/10.3390/fluids8020067 - 14 Feb 2023

Cited by 2 | Viewed by 1284

Abstract

This paper presents an unsteady coupled heat transfer model in mine air and surrounding rock mass in the presence of distributed heat sources. The case of distributed heat sources is typical when analyzing the temperature distribution in mine excavations equipped with conveyor systems. [...] Read more.

This paper presents an unsteady coupled heat transfer model in mine air and surrounding rock mass in the presence of distributed heat sources. The case of distributed heat sources is typical when analyzing the temperature distribution in mine excavations equipped with conveyor systems. For this case, the asymptotic value of the air temperature at the end of the mine excavation is determined not only by the heat exchange between the air and surrounding rock mass but also by the thermal power of distributed heat sources and the total airflow. This conclusion is confirmed by the experimental data presented in the paper for a longwall in a potash mine. We formulate the mathematical model and calculate the distribution of air parameters along the length of an excavation, considering heat release from the conveyor and surrounding rock mass. The results show that a distributed heat release is necessary for correctly calculating the air temperature in working areas. The numerical simulations allow us to recommend a redistribution of air between the haulage and conveyor roadways in the presence of distributed heat sources. Full article

(This article belongs to the Special Issue Fluid Flows in Geotechnical Engineering)

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18 pages, 3044 KiB

Open AccessArticle

Heat Transfer Correlations for Smooth and Rough Airfoils

by Sepehr Samadani and François Morency

Fluids 2023, 8(2), 66; https://doi.org/10.3390/fluids8020066 - 13 Feb 2023

Cited by 5 | Viewed by 2732

Abstract

Low-fidelity methods such as the Blade Element Momentum Theory frequently provide rotor aerodynamic performances. However, these methods must be coupled to databases or correlations to compute heat transfer. The literature lacks correlations to compute the average heat transfer around airfoil. The present study [...] Read more.

Low-fidelity methods such as the Blade Element Momentum Theory frequently provide rotor aerodynamic performances. However, these methods must be coupled to databases or correlations to compute heat transfer. The literature lacks correlations to compute the average heat transfer around airfoil. The present study develops correlations for an average heat transfer over smooth and rough airfoil. The correlation coefficients were obtained from a CFD database using RANS equations and the Spalart–Allmaras turbulent model. This work studies the NACA 0009, NACA 0012, and NACA 0015 with and without the leading roughness representative of a small ice accretion. The numerical results are validated against lift and drag coefficients from the literature. The heat transfer at the stagnation point compares well with the experimental results. The database indicates a negligible dependency on airfoil thickness. The work presents two correlations from the database analysis: one for the smooth airfoils and one for the rough airfoils. For the zero lift coefficient, the average Nusselt number is maximum. This increases with

R e^{0.636}

for the smooth surface and with

R e^{0.85}

for the rough surface. As the lift increases, the average Nusselt is reduced by values proportional to the square of the lift coefficient for the smooth surface, while it is reduced by values proportional to

R e

and the square of the lift coefficient for the rough surface. Full article

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

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17 pages, 3268 KiB

Open AccessArticle

Stability and Resolution Analysis of the Wavelet Collocation Upwind Schemes for Hyperbolic Conservation Laws

by Bing Yang, Jizeng Wang, Xiaojing Liu and Youhe Zhou

Fluids 2023, 8(2), 65; https://doi.org/10.3390/fluids8020065 - 13 Feb 2023

Cited by 2 | Viewed by 1600

Abstract

The numerical solution of hyperbolic conservation laws requires algorithms with upwind characteristics. Conventional methods such as the numerical difference method can realize this characteristic by constructing special distributions of nodes. However, there are still no reports on how to construct algorithms with upwind [...] Read more.

The numerical solution of hyperbolic conservation laws requires algorithms with upwind characteristics. Conventional methods such as the numerical difference method can realize this characteristic by constructing special distributions of nodes. However, there are still no reports on how to construct algorithms with upwind characteristics through wavelet theory. To solve this problem, a system of high-order and stable wavelet collocation upwind schemes was successfully proposed by constructing interpolation wavelets with specific symmetry and smoothness. The effects of the characteristics of the scaling functions on the schemes were explored based on numerical tests and Fourier analysis. The numerical results revealed that the stability of the constructed scheme is affected by the smoothness order, N, and the asymmetry of the scaling function. The dissipation analysis suggested that schemes with N ∈ even have negative dissipation coefficients, leading to unstable behaviors. Only scaling functions with N ∈ odd and a bias magnitude of 1 can be used to construct stable upwind schemes due to the non-negative dissipation coefficients. Typical numerical examples verified the effectiveness of the proposed method, which is proved to have high accuracy and efficiency in solving high-speed flow problems with multi-scale smooth structures and discontinuities. Full article

(This article belongs to the Special Issue Wavelets and Fluids)

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16 pages, 6066 KiB

Open AccessArticle

Experimental and Numerical Investigation of the Aerodynamic Ventilation Drag of Heavy-Duty Vehicle Wheels

by Carlos Peiró Frasquet, Daniel Stoll, Timo Kuthada and Andreas Wagner

Fluids 2023, 8(2), 64; https://doi.org/10.3390/fluids8020064 - 10 Feb 2023

Cited by 1 | Viewed by 2410

Abstract

Due to current EU regulations, constant-speed testing on test tracks is used for aerodynamic certification of heavy-duty vehicles (HDV). However, the aerodynamic development of HDVs is performed using wind tunnels and computational fluid dynamics (CFD). Both techniques commonly neglect the rotational aerodynamic losses [...] Read more.

Due to current EU regulations, constant-speed testing on test tracks is used for aerodynamic certification of heavy-duty vehicles (HDV). However, the aerodynamic development of HDVs is performed using wind tunnels and computational fluid dynamics (CFD). Both techniques commonly neglect the rotational aerodynamic losses of the wheels—the so-called ventilation drag—that are present when driving on the road. This is due to the fact that there is no full-scale wind tunnel for this type of vehicle with a suitable belt system for the simulation of the wheel rotation. Furthermore, the ventilation drag of HDV wheels has been neglected in CFD due to their almost completely closed rim design. These constraints lead to an underprediction of the aerodynamic forces in comparison to the results under on-road conditions when performing constant-speed tests. In order to investigate the ventilation drag of HDV wheels, measurements were carried out on a 1:4.5 scale generic tractor-trailer model in the Model Scale Wind Tunnel of the University of Stuttgart. The measured aerodynamic forces as well as the measured flow field data provide the basis for the definition and validation of a procedure for analyzing the ventilation drag in CFD. Accordingly, the ventilation drag of a full scale HDV was investigated in CFD. The results show that the tire treading and rim geometry have a significant influence on ventilation drag that contributes to the total aerodynamic drag of the HDV. The present work shows that the ventilation drag has a relevant impact on the total aerodynamic drag of HDVs and should therefore not be neglected. The presented CFD approach thus allows to assess the aerodynamic drag under real on-road conditions in an early stage of the vehicle development. Full article

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15 pages, 1925 KiB

Open AccessArticle

Modeling and Experimental Study on Drying Characteristics of Corn Particles with Hot Air in Downward Moving Bed

by Hairui Wang, Shuangming Zhang, Haodong Fan, Man Zhang, Nan Hu and Hairui Yang

Fluids 2023, 8(2), 63; https://doi.org/10.3390/fluids8020063 - 10 Feb 2023

Cited by 1 | Viewed by 2275

Abstract

With regard to drying fresh grain prior to storage, the drying tower with a downward moving bed with hot air is often used, which always has high energy consumption during operation. To optimize the operation, according to the actual operating parameters of a [...] Read more.

With regard to drying fresh grain prior to storage, the drying tower with a downward moving bed with hot air is often used, which always has high energy consumption during operation. To optimize the operation, according to the actual operating parameters of a corn drying tower with hot air, a heat balance model was established, and the heat transfer between the hot air and corn flow in a downward moving bed was analyzed. Since the downward moving time is short, the heat absorbed by corn significantly depends on the heat transfer coefficient, mainly the convective heat transfer, between the hot air and corn surface. To determine the convective heat transfer coefficient, a hot air drying experimental system for corn grains was established, and the effects of hot air temperature and wind speed on the central temperature and moisture content of corn grains were analyzed. Utilizing the heat balance model, the convective heat transfer coefficients between corn particles and hot air were calculated. The total convective heat transfer coefficients are in the range of 39.4–53.8 W/m² · K. With an average value of 46.7 W/m² · K, drying energy efficiencies in different drying zones in the drying tower were calculated, and the accuracy of the model was verified by the operation data. Due to the high inlet temperature of hot air, the maximum energy efficiency of the first zone is 60.15%, whereas when the temperature of hot air in the second drying tower is 140 °C, the energy efficiency is only 41.97%. Therefore, under the premise of ensuring the drying rate, the temperature of hot air of the second zone should be appropriately reduced to improve the whole drying energy efficiency. Full article

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

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13 pages, 6748 KiB

Open AccessArticle

Optimization of the Circular Experimental Channel for PIV Measurements of Internal Aerodynamic Cases

by Jaroslav Pulec, Petra Dančová and Jan Novosád

Fluids 2023, 8(2), 62; https://doi.org/10.3390/fluids8020062 - 10 Feb 2023

Cited by 1 | Viewed by 1702

Abstract

This paper deals with the design of an experimental circular channel model specially adapted for particle image velocimetry (PIV) measurements of airflow. The goal is to find a simple, fast, and functional approach for creating experimental models. The issue of the required PIV [...] Read more.

This paper deals with the design of an experimental circular channel model specially adapted for particle image velocimetry (PIV) measurements of airflow. The goal is to find a simple, fast, and functional approach for creating experimental models. The issue of the required PIV signal quality is defined, and the limits of acquiring relevant PIV data in the interior of circular channels are described, primarily as reflections of direct and scattered laser light caused by light passage through the cylindrical wall. As part of the experiment, measurements of reflections were made on differently coated surfaces of a plexiglass plate. Samples of various combinations of black matt spray, Rhodamine 6G coating, and roughened surface were created. From the presented results, a combination of black matt spray with a layer of Rhodamine 6G spray paint was chosen. The selected modification was further used for the internal modification of the experimental channel surface. Furthermore, a geometry modification to prevent light spread through the tube material is described. Data obtained from measurements before and after channel modification are presented and explained. Full article

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24 pages, 43961 KiB

Open AccessArticle

Investigation of Flow-Induced Instabilities in a Francis Turbine Operating in Non-Cavitating and Cavitating Part-Load Conditions

by Mohammad Hossein Arabnejad, Håkan Nilsson and Rickard E. Bensow

Fluids 2023, 8(2), 61; https://doi.org/10.3390/fluids8020061 - 10 Feb 2023

Cited by 3 | Viewed by 2100

Abstract

The integration of intermittent renewable energy resources to the grid system requires that hydro turbines regularly operate at part-load conditions. Reliable operation of hydro turbines at these conditions is typically limited by the formation of a Rotating Vortex Rope (RVR) in the draft [...] Read more.

The integration of intermittent renewable energy resources to the grid system requires that hydro turbines regularly operate at part-load conditions. Reliable operation of hydro turbines at these conditions is typically limited by the formation of a Rotating Vortex Rope (RVR) in the draft tube. In this paper, we investigate the formation of this vortex using the scale-resolving methods SST-SAS, wall-modeled LES (WMLES), and zonal WMLES. The numerical results are first validated against the available experimental data, and then analyzed to explain the effect of using different scale-resolving methods in detail. It is revealed that although all methods can capture the main features of the RVRs, the WMLES method provides the best quantitative agreement between the simulation results and experiment. Furthermore, cavitating simulations are performed using WMLES method to study the effect of cavitation on the flow in the turbine. These effects of cavitation are shown to be highly dependent on the amount of vapor in the RVR. If the amount of vapor is small, cavitation induces broadband high-frequency fluctuations in the pressure and forces exerted on the turbine. As the amount of cavitation increases, these fluctuations tend to have a distinct dominant frequency which is different from the frequency of the RVR. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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22 pages, 7697 KiB

Open AccessArticle

Numerical Study of Flow Downstream a Step with a Cylinder Part 2: Effect of a Cylinder on the Flow over the Step

by Milad Abdollahpour, Paola Gualtieri, David F. Vetsch and Carlo Gualtieri

Fluids 2023, 8(2), 60; https://doi.org/10.3390/fluids8020060 - 10 Feb 2023

Cited by 3 | Viewed by 2095

Abstract

In this study, divided into two parts, the effect on a two-dimensional backward-facing step flow (BFSF) of a cylinder placed downstream of the step was numerically investigated. While in Part 1, the numerical simulations carried out without the cylinder were validated using the [...] Read more.

In this study, divided into two parts, the effect on a two-dimensional backward-facing step flow (BFSF) of a cylinder placed downstream of the step was numerically investigated. While in Part 1, the numerical simulations carried out without the cylinder were validated using the available literature data, in Part 2 the effect of the cylinder was investigated. In the laminar regime, different Reynolds numbers were considered. In the turbulent regime, the effects on the flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step were comparatively studied. When the cylinder was positioned below the step edge mid-plane, flow over the step was not altered by a cylinder. However, in other locations of a cylinder, the added cylinder modified the structure of flow, increasing the skin friction coefficient in the recirculation zone. Furthermore, the pressure coefficient of the bottom wall increased immediately downstream of the cylinder and farther downstream of the reattachment point and remained stable in the flow recovery process. Moreover, the presence of the step significantly influenced the dynamics of the vortex generation and shedding leading to an asymmetric wake distribution. Full article

(This article belongs to the Collection Advances in Turbulence)

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15 pages, 2931 KiB

Open AccessArticle

The Aerodynamic Effects of a 3D Streamlined Tail on the Windsor Body

by Jeff Howell, Max Varney, Martin Passmore and Daniel Butcher

Fluids 2023, 8(2), 59; https://doi.org/10.3390/fluids8020059 - 8 Feb 2023

Cited by 1 | Viewed by 1975

Abstract

The aerodynamic drag reduction of road vehicles is of continuing interest. The drag arising from the rear surfaces is usually the dominant component, but this can be alleviated by the tapering of the rear body. The effects on the aerodynamic characteristics of a [...] Read more.

The aerodynamic drag reduction of road vehicles is of continuing interest. The drag arising from the rear surfaces is usually the dominant component, but this can be alleviated by the tapering of the rear body. The effects on the aerodynamic characteristics of a simple body from adding an elongated tapered tail have been investigated in a wind tunnel experiment. The streamlined tail consists of a constant rear body side taper added to a constant upper body taper. The results have been compared with an earlier study of the same body with upper body tapering only. The effects of truncating the long tail are explored. Adding the planform tapering reduces the impact of the slant edge vortices, and drag and lift are substantially reduced. The lateral aerodynamic characteristics are largely unaffected. Full article

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16 pages, 322 KiB

Open AccessArticle

Ideal Reactors as an Illustration of Solving Transport Phenomena Problems in Engineering

by Santiago Laín and Mario A. Gandini

Fluids 2023, 8(2), 58; https://doi.org/10.3390/fluids8020058 - 8 Feb 2023

Cited by 1 | Viewed by 1955

Abstract

This contribution aims at emphasizing the importance of ideal reactors in the field of environmental engineering and in the education of the corresponding engineers. The exposition presents the mass flow governing equations of the ideal reactors (batch, completely mixed flow, and plug flow [...] Read more.

This contribution aims at emphasizing the importance of ideal reactors in the field of environmental engineering and in the education of the corresponding engineers. The exposition presents the mass flow governing equations of the ideal reactors (batch, completely mixed flow, and plug flow reactors) as particular cases derived from the integral version of the conservation of mass of a chemical/biological species. In the case of transient problems and simple kinetics, such expressions result in first-order ordinary differential equations amenable to be solved analytically when they are linear. In this article, it is shown that when they are non-linear, due to the presence of a second-order kinetics reaction, an analytical solution is also possible, a situation not dealt with in the textbooks. Finally, the previous findings are integrated into a teaching proposal addressed to help undergraduate students to solve more efficiently ideal reactor problems. Full article

18 pages, 1940 KiB

Open AccessArticle

A New Rheological Model for Phosphate Slurry Flows

by Zeineb Ghoudi, Souhail Maazioui, Fayssal Benkhaldoun and Noureddine Hajjaji

Fluids 2023, 8(2), 57; https://doi.org/10.3390/fluids8020057 - 8 Feb 2023

Cited by 1 | Viewed by 1924

Abstract

In this paper, a new rheological model for the flow of phosphate-water suspensions is proposed. The model’s ability to replicate the rheological characteristics of phosphate-water suspensions under different shear rate conditions is evaluated using rheometric tests, and it is found to be in [...] Read more.

In this paper, a new rheological model for the flow of phosphate-water suspensions is proposed. The model’s ability to replicate the rheological characteristics of phosphate-water suspensions under different shear rate conditions is evaluated using rheometric tests, and it is found to be in good agreement with experimental data. A comprehensive methodology for obtaining the model parameters is presented. The proposed model is then incorporated into the OpenFoam numerical code. The results demonstrate that the model is capable of reproducing the rheological behavior of phosphate suspensions at both low and high concentrations by comparing it with suitable models for modeling the rheological behavior of phosphate suspensions. The proposed model can be applied to simulate and monitor phosphate slurry flows in industrial applications. Full article

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

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12 pages, 1016 KiB

Open AccessArticle

Non-Singular Burton–Miller Boundary Element Method for Acoustics

by Qiang Sun and Evert Klaseboer

Fluids 2023, 8(2), 56; https://doi.org/10.3390/fluids8020056 - 5 Feb 2023

Cited by 2 | Viewed by 1550

Abstract

The problem of non-unique solutions at fictitious frequencies that can appear in the boundary element method for external acoustic phenomena described by the Helmholtz equation is studied. We propose a method to fully desingularise in an analytical way the otherwise hyper-singular Burton–Miller framework, [...] Read more.

The problem of non-unique solutions at fictitious frequencies that can appear in the boundary element method for external acoustic phenomena described by the Helmholtz equation is studied. We propose a method to fully desingularise in an analytical way the otherwise hyper-singular Burton–Miller framework, where the original boundary element method and its normal derivative are combined. The method considerably simplifies the use of higher-order elements, for example, quadratic curved surface elements. The concept is validated using the example of scattering on a rigid sphere and a rigid cube, and its robustness and effectiveness for external sound-wave problems are confirmed. Full article

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20 pages, 3182 KiB

Open AccessArticle

Numerical Study of Flow Downstream a Step with a Cylinder Part 1: Validation of the Numerical Simulations

by Milad Abdollahpour, Paola Gualtieri, David F. Vetsch and Carlo Gualtieri

Fluids 2023, 8(2), 55; https://doi.org/10.3390/fluids8020055 - 3 Feb 2023

Cited by 5 | Viewed by 2187

Abstract

The backward-facing step flow (BFSF) is a classical problem in fluid mechanics, hydraulic engineering, and environmental hydraulics. The nature of this flow, consisting of separation and reattachment, makes it a problem worthy of investigation. In this study, divided into two parts, the effect [...] Read more.

The backward-facing step flow (BFSF) is a classical problem in fluid mechanics, hydraulic engineering, and environmental hydraulics. The nature of this flow, consisting of separation and reattachment, makes it a problem worthy of investigation. In this study, divided into two parts, the effect of a cylinder placed downstream of the step on the 2D flow structure was investigated. In Part 1, the classical 2D BFSF was validated by using OpenFOAM. The BFSF characteristics (reattachment, recirculation zone, velocity profile, skin friction coefficient, and pressure coefficient) were validated for a step-height Reynolds number in the range from 75 to 9000, covering both laminar and turbulent flow. The numerical results at different Reynolds numbers of laminar flow and four RANS turbulence models (standard k-ε, RNG k-ε, standard k-ω, and SST k-ω) were found to be in good agreement with the literature data. In laminar flow, the average error between the numerical results and experimental data for velocity profiles and reattachment lengths and the skin friction coefficient were lower than 8.1, 18, and 20%, respectively. In turbulent flow, the standard k-ε was the most accurate model in predicting pressure coefficients, skin friction coefficient, and reattachment length with an average error lower than 20.5, 17.5, and 6%, respectively. In Part 2, the effect on the 2D flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step was investigated. Full article

(This article belongs to the Collection Advances in Turbulence)

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11 pages, 525 KiB

Open AccessArticle

Thermal Convection of an Ellis Fluid Saturating a Porous Layer with Constant Heat Flux Boundary Conditions

by Pedro Vayssière Brandão, Michele Celli, Antonio Barletta and Stefano Lazzari

Fluids 2023, 8(2), 54; https://doi.org/10.3390/fluids8020054 - 2 Feb 2023

Viewed by 1407

Abstract

The present work analyzes the thermal instability of mixed convection in a horizontal porous channel that is saturated by a shear-thinning fluid following Ellis’ rheology. The fluid layer is heated from below by a constant heat flux and cooled from above by the [...] Read more.

The present work analyzes the thermal instability of mixed convection in a horizontal porous channel that is saturated by a shear-thinning fluid following Ellis’ rheology. The fluid layer is heated from below by a constant heat flux and cooled from above by the same heat flux. The instability of such a system is investigated by means of a small-disturbances analysis and the resulting eigenvalue problem is solved numerically by means of a shooting method. It is demonstrated that the most unstable modes on the instability threshold are those with infinite wavelength and an analytical expression for such conditions is derived from an asymptotic analysis. Results show that the non-Newtonian character of the fluid has a destabilizing role. Full article

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16 pages, 3628 KiB

Open AccessArticle

A Meshless Algorithm for Modeling the Gas-Dynamic Interaction between High-Inertia Particles and a Shock Layer

by Andrey Sposobin and Dmitry Reviznikov

Fluids 2023, 8(2), 53; https://doi.org/10.3390/fluids8020053 - 2 Feb 2023

Viewed by 1207

Abstract

This paper is devoted to numerical modeling of a supersonic flow around a blunt body by a viscous gas with an admixture of relatively large high-inertia particles that, after reflection from the surface, may go beyond the shock layer and change the flow [...] Read more.

This paper is devoted to numerical modeling of a supersonic flow around a blunt body by a viscous gas with an admixture of relatively large high-inertia particles that, after reflection from the surface, may go beyond the shock layer and change the flow structure dramatically. To calculate the gas-dynamic interaction of moving particles with the shock layer, it is important to take into account the large difference in scales of the flow around the particles and around the body. To make the computations effective, we use a meshless method to solve non-stationary Navier–Stokes equations. The algorithm is based on the approximation of partial derivatives by the least squares method on a set of nodes distributed in the calculation area. Each moving particle is surrounded by a cloud of calculation nodes belonging to its domain and moving with it in space. The algorithm has been tested on the problem of the motion of a single particle and a pair of particles in a supersonic flow around a sphere. Full article

(This article belongs to the Special Issue Focus on Supercritical Fluids: Control and Extraction)

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32 pages, 5437 KiB

Open AccessArticle

Evolution of Water Wave Groups in the Forced Benney–Roskes System

by Montri Maleewong and Roger H. J. Grimshaw

Fluids 2023, 8(2), 52; https://doi.org/10.3390/fluids8020052 - 2 Feb 2023

Cited by 1 | Viewed by 1408

Abstract

For weakly nonlinear waves in one space dimension, the nonlinear Schrödinger Equation is widely accepted as a canonical model for the evolution of wave groups described by modulation instability and its soliton and breather solutions. When there is forcing such as that due [...] Read more.

For weakly nonlinear waves in one space dimension, the nonlinear Schrödinger Equation is widely accepted as a canonical model for the evolution of wave groups described by modulation instability and its soliton and breather solutions. When there is forcing such as that due to wind blowing over the water surface, this can be supplemented with a linear growth term representing linear instability leading to the forced nonlinear Schrödinger Equation. For water waves in two horizontal space dimensions, this is replaced by a forced Benney–Roskes system. This is a two-dimensional nonlinear Schrödinger Equation with a nonlocal nonlinear term. In deep water, this becomes a local nonlinear term, and it reduces to a two-dimensional nonlinear Schrödinger Equation. In this paper, we numerically explore the evolution of wave groups in the forced Benney–Roskes system using four cases of initial conditions. In the one-dimensional unforced nonlinear Schrödinger equa tion, the first case would lead to a Peregrine breather and the second case to a line soliton; the third case is a long-wave perturbation, and the fourth case is designed to stimulate modulation instability. In deep water and for finite depth, when there is modulation instability in the one-dimensional nonlinear Schdrödinger Equation, the two-dimensional simulations show a similar pattern. However, in shallow water where there is no one-dimensional modulation instability, the extra horizontal dimension is significant in producing wave growth through modulation instability. Full article

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

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10 pages, 1404 KiB

Open AccessArticle

Understanding Atmospheric Convection Using Large Eddy Simulation

by Gaurav Dogra, Anupam Dewan and Sandeep Sahany

Fluids 2023, 8(2), 51; https://doi.org/10.3390/fluids8020051 - 2 Feb 2023

Cited by 1 | Viewed by 1969

Abstract

Cloud formation is based on the fundamental principle of atmospheric convection, which involves the vertical transport of heat and moisture into an unstable environment. Convective transfer of moisture and heat in the form of turbulent fluxes over the Bay of Bengal (BoB) has [...] Read more.

Cloud formation is based on the fundamental principle of atmospheric convection, which involves the vertical transport of heat and moisture into an unstable environment. Convective transfer of moisture and heat in the form of turbulent fluxes over the Bay of Bengal (BoB) has not been explored much and is not resolved in global and regional climate models (GCMs and RCMs) due to the coarser grid resolutions used. Therefore, the present study is an attempt to understand the convection phenomenon over the BoB using a high-resolution cloud-resolving large eddy simulation. Due to the lack of observational data over the BoB, initial and boundary conditions were generated using reanalysis data. We found that the LES successfully captured the cloud formation and convection phenomenon. The turbulence in the convection was analyzed by using Reynolds averaging to obtain variances and covariances. The presence of turbulence over the region was observed. The cloud characteristics were verified by conditionally averaging the output fields. The present study paves a pathway to perform various simulations at different atmospheric conditions over the region in order to create a library of high-resolution simulations. Full article

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

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13 pages, 3705 KiB

Open AccessArticle

Aerodynamic Interaction of Migratory Birds in Gliding Flight

by Fabien Beaumont, Sébastien Murer, Fabien Bogard and Guillaume Polidori

Fluids 2023, 8(2), 50; https://doi.org/10.3390/fluids8020050 - 1 Feb 2023

Cited by 2 | Viewed by 2893

Abstract

(1) Background: Many studies suggest that migratory bird groups fly in a V-formation to improve their aerodynamic efficiency, the goal being to reduce their energy expenditure to fly longer distances. To further validate this hypothesis, we numerically simulated the aerodynamic interaction of two [...] Read more.

(1) Background: Many studies suggest that migratory bird groups fly in a V-formation to improve their aerodynamic efficiency, the goal being to reduce their energy expenditure to fly longer distances. To further validate this hypothesis, we numerically simulated the aerodynamic interaction of two gliding migratory birds and evaluated the aerodynamic forces as a function of the bird spacing. (2) Methods: Computational Fluid Dynamics (CFD) was used to model the flow pattern in and around the wake of Canada geese flying at an altitude of 1000 m and a speed of 13.9 m/s. (3) Results: The post-processing of the 3D results revealed a complex flow structure composed of two contra-rotating vortices developing at the wing tip. (4) Conclusions: In a plane perpendicular to the main flow direction, we showed that the bird’s wake could be broken down into two distinct zones: the downwash zone and the upwash zone, the latter being used by birds flying in formation to reduce their energy expenditure. The results of our study suggested an optimal wingtip spacing of -26cm to maximize the lift/drag ratio that characterizes aerodynamic efficiency. Full article

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15 pages, 7111 KiB

Open AccessArticle

Motion of a Light Free Sphere and Liquid in a Rotating Vertical Cylinder of Finite Length

by Victor Kozlov, Ekaterina Zvyagintseva, Ekaterina Kudymova and Vlada Romanetz

Fluids 2023, 8(2), 49; https://doi.org/10.3390/fluids8020049 - 1 Feb 2023

Cited by 2 | Viewed by 1677

Abstract

The paper is devoted to an experimental study of the fluid motion excited by a light spherical body floating along the axis of a rotating vertical cylinder. The experiments are performed with fast rotation. The high-speed video recording examines the behavior of the [...] Read more.

The paper is devoted to an experimental study of the fluid motion excited by a light spherical body floating along the axis of a rotating vertical cylinder. The experiments are performed with fast rotation. The high-speed video recording examines the behavior of the body depending on the rotation rate and liquid viscosity. PIV-method is used to investigate the velocity fields of liquid. In the cavity frame, the body excites the motion liquid in the form of a Taylor–Proudman column, the diameter of which is consistent with the body diameter. In the upper column, the liquid performs a retrograde differential rotation, and in the lower, a prograde one. Outside the columns, the differential rotation is practically absent. It is found that the intensity of the retrograde azimuthal motion in the frontal column increases as the body goes up, while the intensity of the prograde rotation in the rear column decreases. As a result, the free body simultaneously with motion along the axis performs differential rotation: in the lower part of the cavity it is prograde, while in the upper one it is retrograde. The intensity of the body differential rotation varies with the longitudinal coordinate linearly and decreases with the dimensionless rotation velocity. Full article

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23 pages, 1329 KiB

Open AccessArticle

Continuum Models for Bulk Viscosity and Relaxation in Polyatomic Gases

by Elena Kustova, Mariia Mekhonoshina, Anna Bechina, Semen Lagutin and Yulia Voroshilova

Fluids 2023, 8(2), 48; https://doi.org/10.3390/fluids8020048 - 31 Jan 2023

Cited by 8 | Viewed by 2063

Abstract

Bulk viscosity and acoustic wave propagation in polyatomic gases and their mixtures are studied in the frame of one-temperature and multi-temperature continuum models developed using the generalized Chapman–Enskog method. Governing equations and constitutive relations for both models are written, and the dispersion equations [...] Read more.

Bulk viscosity and acoustic wave propagation in polyatomic gases and their mixtures are studied in the frame of one-temperature and multi-temperature continuum models developed using the generalized Chapman–Enskog method. Governing equations and constitutive relations for both models are written, and the dispersion equations are derived. In the vibrationally nonequilibrium multi-component gas mixture, wave attenuation mechanisms include viscosity, thermal conductivity, bulk viscosity, diffusion, thermal diffusion, and vibrational relaxation; in the proposed approach these mechanisms are fully coupled contrarily to commonly used models based on the separation of classical Stokes–Kirchhoff attenuation and relaxation. Contributions of rotational and vibrational modes to the bulk viscosity coefficient are evaluated. In the one-temperature approach, artificial separation of rotational and vibrational modes causes great overestimation of bulk viscosity whereas using the effective internal energy relaxation time yields good agreement with experimental data and molecular-dynamic simulations. In the multi-temperature approach, the bulk viscosity is specified only by rotational modes. The developed two-temperature model provides excellent agreement of theoretical and experimental attenuation coefficients in polyatomic gases; both the location and the value of its maximum are predicted correctly. One-temperature dispersion relations do not reproduce the non-monotonic behavior of the attenuation coefficient; large bulk viscosity improves its accuracy only in the very limited frequency range. It is emphasized that implementing large bulk viscosity in the one-temperature Navier–Stokes–Fourier equations may lead to unphysical results. Full article

(This article belongs to the Special Issue Bulk Viscosity and Relaxation Processes: Revisited)

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