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Fluids, Volume 5, Issue 4 (December 2020) – 89 articles

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Article
Multi-Scale Localized Perturbation Method in OpenFOAM
Fluids 2020, 5(4), 250; https://doi.org/10.3390/fluids5040250 - 19 Dec 2020
Cited by 2 | Viewed by 670
Abstract
A modified set of governing differential equations for geophysical fluid flows is derived. All of the simulation fields are decomposed into a nominal large-scale background state and a small-scale perturbation from this background, and the new system is closed by the assumption that [...] Read more.
A modified set of governing differential equations for geophysical fluid flows is derived. All of the simulation fields are decomposed into a nominal large-scale background state and a small-scale perturbation from this background, and the new system is closed by the assumption that the perturbation is one-way coupled to the background. The decomposition method, termed the multi-scale localized perturbation method (MSLPM), is then applied to the governing equations of stratified fluid flows, implemented in OpenFOAM, and exercised in order to simulate the interaction of a vertically-varying background shear flow with an axisymmetric perturbation in a turbulent ocean environment. The results demonstrate that the MSLPM can be useful in visualizing the evolution of a perturbation within a complex background while retaining the complex physics that are associated with the original governing equations. The simulation setup may also be simplified under the MSLPM framework. Further applications of the MSLPM, especially to multi-scale simulations that encompass a large range of spatial and temporal scales, may be beneficial for researchers. Full article
(This article belongs to the Special Issue Selected Papers from the 15th OpenFOAM Workshop)
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Article
Stokes Equation in a Semi-Infinite Region: Generalization of the Lamb Solution and Applications to Marangoni Flows
Fluids 2020, 5(4), 249; https://doi.org/10.3390/fluids5040249 - 18 Dec 2020
Cited by 1 | Viewed by 506
Abstract
We consider the creeping flow of a Newtonian fluid in a hemispherical region. In a domain with spherical or nearly spherical geometry, the solution of the Stokes equation can be expressed as a series of spherical harmonics. However, the original Lamb solution is [...] Read more.
We consider the creeping flow of a Newtonian fluid in a hemispherical region. In a domain with spherical or nearly spherical geometry, the solution of the Stokes equation can be expressed as a series of spherical harmonics. However, the original Lamb solution is not complete when the flow is restricted to a semi-infinite space. The general solution in hemispherical geometry is then constructed explicitly. As an application, we discuss the solutions of Marangoni flows due to a local source at the liquid–air interface. Full article
(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers)
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Article
Anisotropic RANS Turbulence Modeling for Wakes in an Active Ocean Environment
Fluids 2020, 5(4), 248; https://doi.org/10.3390/fluids5040248 - 18 Dec 2020
Viewed by 678
Abstract
The problem of simulating wakes in a stratified oceanic environment with active background turbulence is considered. Anisotropic RANS turbulence models are tested against laboratory and eddy-resolving models of the problem. An important aspect of our work is to acknowledge that the environment is [...] Read more.
The problem of simulating wakes in a stratified oceanic environment with active background turbulence is considered. Anisotropic RANS turbulence models are tested against laboratory and eddy-resolving models of the problem. An important aspect of our work is to acknowledge that the environment is not quiescent; therefore, additional sources are included in the models to provide a non-zero background turbulence. The RANS models are found to reproduce some key features from the eddy-resolving and laboratory descriptions of the problem. Tests using the freestream sources show the intuitive result that background turbulence causes more rapid wake growth and decay. Full article
(This article belongs to the Special Issue Selected Papers from the 15th OpenFOAM Workshop)
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Article
Computational Aerodynamics Analysis of Non-Symmetric Multi-Element Wing in Ground Effect with Humpback Whale Flipper Tubercles
Fluids 2020, 5(4), 247; https://doi.org/10.3390/fluids5040247 - 17 Dec 2020
Viewed by 1271
Abstract
The humpback whale flipper tubercles have been shown to improve the aerodynamic coefficients of a wing, especially in stall conditions, where the flow is almost fully detached. In this work, these tubercles were implemented on a F1 front-wing geometry, very close to a [...] Read more.
The humpback whale flipper tubercles have been shown to improve the aerodynamic coefficients of a wing, especially in stall conditions, where the flow is almost fully detached. In this work, these tubercles were implemented on a F1 front-wing geometry, very close to a Tyrrell wing. Numerical simulations were carried out employing the kω SST turbulence model and the overall effects of the tubercles on the flow behavior were analyzed. The optimal amplitude and number of tubercles was determined in this study for this front wing where an improvement of 22.6% and 9.4% is achieved, respectively, on the lift and the L/D ratio. On the main element, the stall was delayed by 167.7%. On the flap, the flow is either fully detached, in the large circulation zone, or fully attached. Overall, in stall conditions, tubercles improve the downforce generation but at the cost of increased drag. Furthermore, as the tubercles are case-dependent, an optimal configuration for tubercles implementation also exists for any geometry. Full article
(This article belongs to the Special Issue Aerodynamics and Aeroacoustics of Vehicles)
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Article
Asymmetrical Thermal Boundary Condition Influence on the Flow Structure and Heat Transfer Performance of Paramagnetic Fluid-Forced Convection in the Strong Magnetic Field
Fluids 2020, 5(4), 246; https://doi.org/10.3390/fluids5040246 - 16 Dec 2020
Viewed by 895
Abstract
Continuous interest in space journeys opens the research fields, which might be useful in non-terrestrial conditions. Due to the lack of the gravitational force, there will be a need to force the flow for mixing or heat transfer. Strong magnetic field offers the [...] Read more.
Continuous interest in space journeys opens the research fields, which might be useful in non-terrestrial conditions. Due to the lack of the gravitational force, there will be a need to force the flow for mixing or heat transfer. Strong magnetic field offers the conditions, which can help to obtain the flow. In light of this origin, presented paper discusses the dually modified Graetz-Brinkman problem. The modifications were related to the presence of the magnetic field influencing the flow and asymmetrical thermal boundary condition. Dimensionless numerical analysis was performed, and two dimensionless numbers (magnetic Grashof number and magnetic Richardson number) were defined for paramagnetic fluid flow. The results revealed the heat transfer enhancement due to the strong magnetic field influence accompanied by possible but not essential flow structure modifications. On the other hand, the flow structure changes can be utilized to prevent the solid particles’ sedimentation. The explanation of the heat transfer enhancement including energy budget and vorticity distribution was presented. Full article
(This article belongs to the Special Issue Fluids in Magnetic/Electric Fields)
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Article
Effects of Shell Thickness on Cross-Helicity Generation in Convection-Driven Spherical Dynamos
Fluids 2020, 5(4), 245; https://doi.org/10.3390/fluids5040245 - 16 Dec 2020
Cited by 1 | Viewed by 705
Abstract
The relative importance of the helicity and cross-helicity electromotive dynamo effects for self-sustained magnetic field generation by chaotic thermal convection in rotating spherical shells is investigated as a function of shell thickness. Two distinct branches of dynamo solutions are found to coexist in [...] Read more.
The relative importance of the helicity and cross-helicity electromotive dynamo effects for self-sustained magnetic field generation by chaotic thermal convection in rotating spherical shells is investigated as a function of shell thickness. Two distinct branches of dynamo solutions are found to coexist in direct numerical simulations for shell aspect ratios between 0.25 and 0.6—a mean-field dipolar regime and a fluctuating dipolar regime. The properties characterising the coexisting dynamo attractors are compared and contrasted, including differences in temporal behaviour and spatial structures of both magnetic fields and rotating thermal convection. The helicity α-effect and the cross-helicity γ-effect are found to be comparable in intensity within the fluctuating dipolar dynamo regime, where their ratio does not vary significantly with the shell thickness. In contrast, within the mean-field dipolar dynamo regime the helicity α-effect dominates by approximately two orders of magnitude and becomes stronger with decreasing shell thickness. Full article
(This article belongs to the Special Issue Thermal Flows)
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Article
The Effect of Single Dielectric Barrier Discharge Actuators in Reducing Drag on an Ahmed Body
Fluids 2020, 5(4), 244; https://doi.org/10.3390/fluids5040244 - 15 Dec 2020
Viewed by 658
Abstract
The feasibility of a single dielectric barrier discharge (SDBD) actuator in controlling flow over an Ahmed body, representing a simplified car model, has been numerically and experimentally investigated at Reynolds numbers of 7.68×105 and 2.25×105. The [...] Read more.
The feasibility of a single dielectric barrier discharge (SDBD) actuator in controlling flow over an Ahmed body, representing a simplified car model, has been numerically and experimentally investigated at Reynolds numbers of 7.68×105 and 2.25×105. The Ahmed body had slant angles of 25 and 35. The results showed that SDBD actuators could significantly enhance the aerodynamic performance of the Ahmed body. Several arrangements of the actuators on the slant surface and the rear face of the model were examined to identify the most effective arrangement for drag reduction. This arrangement resulted in an approximately 6.1% drag reduction. This improvement in aerodynamic performance is attributed to the alteration of three-dimensional wake structures due to the presence of SDBD, which coincides with surface pressure variations on the slant and rear faces of the Ahmed body. Full article
(This article belongs to the Special Issue Aerodynamics and Aeroacoustics of Vehicles)
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Article
Impact of Leading Edge Roughness in Cavitation Simulations around a Twisted Foil
Fluids 2020, 5(4), 243; https://doi.org/10.3390/fluids5040243 - 14 Dec 2020
Cited by 2 | Viewed by 476
Abstract
The simulation of fully turbulent, three-dimensional, cavitating flow over Delft twisted foil is conducted by an implicit large eddy simulation (LES) approach in both smooth and tripped conditions, the latter by including leading-edge roughness. The analysis investigates the importance of representing the roughness [...] Read more.
The simulation of fully turbulent, three-dimensional, cavitating flow over Delft twisted foil is conducted by an implicit large eddy simulation (LES) approach in both smooth and tripped conditions, the latter by including leading-edge roughness. The analysis investigates the importance of representing the roughness elements on the flow structures in the cavitation prediction. The results include detailed comparisons of cavitation pattern, vorticity distribution, and force predictions with the experimental measurements. It is noted that the presence of roughness generates very small cavitating vortical structures which interact with the main sheet cavity developing over the foil to later form a cloud cavity. Very similar to the experimental observation, these interactions create a streaky sheet cavity interface which cannot be captured in the smooth condition, influencing both the richness of structures in the detached cloudy cavitation as well as the extent and transport of vapour. It is further found to have a direct impact on the pressure distribution, especially in the mid-chord region where the shed cloud cavity collapses. Full article
(This article belongs to the Special Issue Cavitating Flows)
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Article
Numerical Analysis of Tube Heat Exchanger with Perforated Star-Shaped Fins
Fluids 2020, 5(4), 242; https://doi.org/10.3390/fluids5040242 - 13 Dec 2020
Cited by 1 | Viewed by 583
Abstract
This article discusses the possibility of further reducing the mass of the heat exchanger with stainless steel star-shaped fins while achieving good heat transfer performance. For this purpose, we perforated the fins with holes Ø2, Ø3, and Ø4 mm. Applying computational fluid dynamics [...] Read more.
This article discusses the possibility of further reducing the mass of the heat exchanger with stainless steel star-shaped fins while achieving good heat transfer performance. For this purpose, we perforated the fins with holes Ø2, Ø3, and Ø4 mm. Applying computational fluid dynamics (CFD) numerical analysis, we determined the influence of each perforation on the characteristics of the flow field in the liquid–gas type of heat exchanger and the heat transfer for the range of Re numbers from 2300 to 16,000. With a reduction in the mass of the fins to 17.65% (by Ø4 mm), perforated fins had greater heat transfer from 5.5% to 11.3% than fins without perforation. A comparison of perforated star-shaped fins with annular fins was also performed. Perforated fins had 51.8% less mass than annular fins, with an increase in heat transfer up to 26.5% in terms of Nusselt number. Full article
(This article belongs to the Special Issue Heat Transfer and Fluid Dynamics in Energy Systems)
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Article
Thermophysical Properties of Nanofluids Composed of Ethylene Glycol and Long Multi-Walled Carbon Nanotubes
Fluids 2020, 5(4), 241; https://doi.org/10.3390/fluids5040241 - 12 Dec 2020
Cited by 3 | Viewed by 823
Abstract
In this work, thermal conductivity, viscosity, isobaric heat capacity, and density of stable carbon-based nanofluids are presented. The nanofluids under study are composed of 1,2-ethanediol (ethylene glycol, EG) and long multi-walled carbon nanotubes (MWCNTs), so-called ‘in-house 16h’ (synthesized in our laboratory via catalytic [...] Read more.
In this work, thermal conductivity, viscosity, isobaric heat capacity, and density of stable carbon-based nanofluids are presented. The nanofluids under study are composed of 1,2-ethanediol (ethylene glycol, EG) and long multi-walled carbon nanotubes (MWCNTs), so-called ‘in-house 16h’ (synthesized in our laboratory via catalytic chemical vapor deposition during 16 h with a diameter of 60–80 nm and length of 770 μm). Poly(N-vinylpyrrolidone) (PVP) was used to increase the stability of nanofluids. The nanofluids were prepared via an ultrasonication-assisted, three-step method while their key thermophysical characteristics were obtained using the hot-wire technique and rotary viscometer. As a result, the addition of MWCNTs significantly improved the thermal conductivity of nanofluids by 31.5% for the highest 1.0 wt% (0.498 vol%) long MWCNT content, leaving the Newtonian character of the nanofluids practically intact. Full article
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Article
Multi-Physics Modeling of Electrochemical Deposition
Fluids 2020, 5(4), 240; https://doi.org/10.3390/fluids5040240 - 11 Dec 2020
Cited by 2 | Viewed by 663
Abstract
Electrochemical deposition (ECD) is a common method used in the field of microelectronics to grow metallic coatings on an electrode. The deposition process occurs in an electrolyte bath where dissolved ions of the depositing material are suspended in an acid while an electric [...] Read more.
Electrochemical deposition (ECD) is a common method used in the field of microelectronics to grow metallic coatings on an electrode. The deposition process occurs in an electrolyte bath where dissolved ions of the depositing material are suspended in an acid while an electric current is applied to the electrodes. The proposed computational model uses the finite volume method and the finite area method to predict copper growth on the plating surface without the use of a level set method or deforming mesh because the amount of copper layer growth is not expected to impact the fluid motion. The finite area method enables the solver to track the growth of the copper layer and uses the current density as a forcing function for an electric potential field on the plating surface. The current density at the electrolyte-plating surface interface is converged within each PISO (Pressure Implicit with Splitting Operator) loop iteration and incorporates the variance of the electrical resistance that occurs via the growth of the copper layer. This paper demonstrates the application of the finite area method for an ECD problem and additionally incorporates coupling between fluid mechanics, ionic diffusion, and electrochemistry. Full article
(This article belongs to the Special Issue Selected Papers from the 15th OpenFOAM Workshop)
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Article
Effect of Functional Surfaces with Gradient Mixed Wettability on Flow Boiling in a High Aspect Ratio Microchannel
Fluids 2020, 5(4), 239; https://doi.org/10.3390/fluids5040239 - 10 Dec 2020
Cited by 3 | Viewed by 995
Abstract
Flow boiling is one of the most effective phase-change heat transfer mechanisms and is strongly dependent on surface properties. The surface wettability is a crucial parameter, which has a considerable effect on the heat transfer performance, particularly in flow boiling. The contact angle [...] Read more.
Flow boiling is one of the most effective phase-change heat transfer mechanisms and is strongly dependent on surface properties. The surface wettability is a crucial parameter, which has a considerable effect on the heat transfer performance, particularly in flow boiling. The contact angle determines the number of nucleation sites as well as bubble dynamics and flow patterns. This study introduces three new generation mixed wettability surfaces and compares them with a wholly hydrophobic surface reference sample, in flow boiling in a high aspect ratio microchannel. The mixed wettability substrates have five regions as fully Al2O3, (hydrophobic zone) region, three different patterned configurations with various A* values, and fully SiO2 (hydrophilic zone) region, where A* is defined as A Al2O3/A total (hydrophobicity ratio). Boiling heat transfer results were obtained for each surface at various wall heat fluxes and three different mass fluxes. According to the obtained results, significant enhancements in heat transfer (by up to 56.7%) could be obtained with biphilic surfaces compared to the reference sample (hydrophobic surface). Performed flow visualization proves that the tested biphilic surfaces enhance heat transfer by reducing the bubbly flow regime and extending the slug regime. Full article
(This article belongs to the Special Issue Recent Advances in Single and Multiphase Flows in Microchannels)
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Article
Free Flow and Discharge Characteristics of Trapezoidal-Shaped Weirs
Fluids 2020, 5(4), 238; https://doi.org/10.3390/fluids5040238 - 10 Dec 2020
Cited by 1 | Viewed by 744
Abstract
A number of studies have considered the effects of weir design variations on the free- and submerged-flow characteristics of trapezoidal broad-crested weirs. It appears that the hydraulics of short-crested weir flows have received little attention; thus, the current knowledge is incomplete. By systematically [...] Read more.
A number of studies have considered the effects of weir design variations on the free- and submerged-flow characteristics of trapezoidal broad-crested weirs. It appears that the hydraulics of short-crested weir flows have received little attention; thus, the current knowledge is incomplete. By systematically analyzing a large set of experimental data, the present study aims to fill in this knowledge gap and to provide a complete description of the discharge characteristics of trapezoidal-shaped weirs, including the salient features of two-dimensional weir flows. The analysis of the axial free-surface profiles for short-crested weir flows attested that the location of the nearest station for the correct measurement of the overflow depth under free-flow conditions is at η0 from the heel of the weir, where η0 is the upstream free-surface elevation. Additionally, an empirical equation for the free-flow discharge coefficient is proposed as being valid for a trapezoidal-shaped weir with varying upstream- and downstream-face slopes. The results of this investigation reveal that the streamline curvature and the slopes of the upstream and downstream weir faces significantly affect the streamwise flow patterns and, hence, the free-flow discharge. Full article
(This article belongs to the Special Issue Modelling the Behaviour of Water Systems to Increase Sustainability)
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Article
Aerodynamic and Structural Design of a 2022 Formula One Front Wing Assembly
Fluids 2020, 5(4), 237; https://doi.org/10.3390/fluids5040237 - 09 Dec 2020
Cited by 1 | Viewed by 2296
Abstract
The aerodynamic loads generated in a wing are critical in its structural design. When multi-element wings with wingtip devices are selected, it is essential to identify and to quantify their structural behaviour to avoid undesirable deformations which degrade the aerodynamic performance. This research [...] Read more.
The aerodynamic loads generated in a wing are critical in its structural design. When multi-element wings with wingtip devices are selected, it is essential to identify and to quantify their structural behaviour to avoid undesirable deformations which degrade the aerodynamic performance. This research investigates these questions using numerical methods (Computational Fluid Dynamics and Finite Elements Analysis), employing exhaustive validation methods to ensure the accuracy of the results and to assess their uncertainty. Firstly, a thorough investigation of four baseline configurations is carried out, employing Reynolds Averaged Navier–Stokes equations and the k-ω SST (Shear Stress Transport) turbulence model to analyse and quantify the most important aerodynamic and structural parameters. Several structural configurations are analysed, including different materials (metal alloys and two designed fibre-reinforced composites). A 2022 front wing is designed based on a bidimensional three-element wing adapted to the 2022 FIA Formula One regulations and its structural components are selected based on a sensitivity analysis of the previous results. The outcome is a high-rigidity-weight wing which satisfies the technical regulations and lies under the maximum deformation established before the analysis. Additionally, the superposition principle is proven to be an excellent method to carry out high-performance structural designs. Full article
(This article belongs to the Special Issue Aerodynamics and Aeroacoustics of Vehicles)
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Article
Experimental Data for the Validation of Numerical Methods: DrivAer Model
Fluids 2020, 5(4), 236; https://doi.org/10.3390/fluids5040236 - 08 Dec 2020
Cited by 1 | Viewed by 1773
Abstract
As the automotive industry strives to increase the amount of digital engineering in the product development process, cut costs and improve time to market, the need for high quality validation data has become a pressing requirement. While there is a substantial body of [...] Read more.
As the automotive industry strives to increase the amount of digital engineering in the product development process, cut costs and improve time to market, the need for high quality validation data has become a pressing requirement. While there is a substantial body of experimental work published in the literature, it is rarely accompanied by access to the data and a sufficient description of the test conditions for a high quality validation study. This paper addresses this by reporting on a comprehensive series of measurements for a 25% scale model of the DrivAer automotive test case. The paper reports on the measurement of the forces and moments, pressures and off body PIV measurements for three rear end body configurations, and summarises and compares the results. A detailed description of the test conditions and wind tunnel set up are included along with access to the full data set. Full article
(This article belongs to the Special Issue Aerodynamics and Aeroacoustics of Vehicles)
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Article
Default versus Configured-Geostatistical Modeling of Suspended Particulate Matter in Potter Cove, West Antarctic Peninsula
Fluids 2020, 5(4), 235; https://doi.org/10.3390/fluids5040235 - 08 Dec 2020
Cited by 2 | Viewed by 829
Abstract
The glacier retreat observed during the last decades at Potter Cove (PC) causes an increasing amount of suspended particulate matter (SPM) in the water column, which has a high impact on sessile filter feeder’ species at PC located at the West Antarctic Peninsula. [...] Read more.
The glacier retreat observed during the last decades at Potter Cove (PC) causes an increasing amount of suspended particulate matter (SPM) in the water column, which has a high impact on sessile filter feeder’ species at PC located at the West Antarctic Peninsula. SPM presents a highly-fluctuating dynamic pattern on a daily, monthly, seasonal, and interannual basis. Geostatistical interpolation techniques are widely used by default to generate reliable spatial information and thereby to improve the ecological understanding of environmental variables, which is often fundamental for guiding decision-makers and scientists. In this study, we compared the results of default and configured settings of three geostatistical algorithms (Simple Kriging, Ordinary Kriging, and Empirical Bayesian) and developed a performance index. In order to interpolate SPM data from the summer season 2010/2011 at PC, the best performance was obtained with Empirical Bayesian Kriging (standard mean = −0.001 and root mean square standardized = 0.995). It showed an excellent performance (performance index = 0.004), improving both evaluation parameters when radio and neighborhood were configured. About 69% of the models showed improved standard means when configured compared to the default settings following a here proposed guideline. Full article
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Article
A Qualitative Numerical Study on Catalytic Hydrogenation of Nitrobenzene in Gas-Liquid Taylor Flow with Detailed Reaction Mechanism
Fluids 2020, 5(4), 234; https://doi.org/10.3390/fluids5040234 - 08 Dec 2020
Cited by 1 | Viewed by 646
Abstract
While the number of computational studies considering two-phase flows in microfluidic systems with or without mass transfer is increasing, numerical studies incorporating chemical reactions are still rare. This study aims to simulate the catalytic hydrogenation of nitrobenzene in gas-liquid Taylor flow by combining [...] Read more.
While the number of computational studies considering two-phase flows in microfluidic systems with or without mass transfer is increasing, numerical studies incorporating chemical reactions are still rare. This study aims to simulate the catalytic hydrogenation of nitrobenzene in gas-liquid Taylor flow by combining interface-resolving numerical simulations of two-phase flow and mass transfer by a volume-of-fluid method with detailed modeling of the heterogeneous chemical reaction by software package DETCHEMTM. Practically relevant physical properties are utilized for hydrodynamic and mass transfer simulations in combination with a preliminary reaction mechanism based on density functional theory. Simulations of mass transfer are conducted using a predetermined velocity field and Taylor bubble shape. At the beginning of the simulation when liquid nitrobenzene is not saturated by hydrogen, axial profiles of surface species concentrations and reaction rates show local variations. As hydrogen dissolves in nitrobenzene, the concentration profiles of surface species at the wall become uniform, eventually reaching an equilibrium state. Neglecting the local variation in a short initial period will allow further simplification of modeling surface reactions within a Taylor flow. Full article
(This article belongs to the Special Issue Recent Advances in Single and Multiphase Flows in Microchannels)
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Article
Effects of Inflow Condition on RANS and LES Predictions of the Flow around a High-Rise Building
Fluids 2020, 5(4), 233; https://doi.org/10.3390/fluids5040233 - 07 Dec 2020
Cited by 2 | Viewed by 969
Abstract
An increasing number of engineering applications require accurate predictions of the flow around buildings to guarantee performance and safety. This paper investigates the effects of variations in the turbulent inflow, as predicted in different numerical simulations, on the flow pattern prediction around buildings, [...] Read more.
An increasing number of engineering applications require accurate predictions of the flow around buildings to guarantee performance and safety. This paper investigates the effects of variations in the turbulent inflow, as predicted in different numerical simulations, on the flow pattern prediction around buildings, compared to wind tunnel tests. Turbulence characteristics were assessed at several locations around a model square high-rise building, namely, above the roof region, at the pedestrian level, and in the wake. Both Reynolds-averaged Navier–Stokes (RANS, where turbulence is fully modelled) equations and large-eddy simulation (LES, where turbulence is partially resolved) were used to model an experimental setup providing validation for the roof region. The performances of both techniques were compared in ability to predict the flow features. It was found that RANS provides reliable results in regions of the flow heavily influenced by the building model, and it is unreliable where the flow is influenced by ambient conditions. In contrast, LES is generally reliable, provided that a suitable turbulent inflow is included in the simulation. RANS also benefits when a turbulent inflow is provided in simulations. In general, LES should be the methodology of choice if engineering applications are involved with the highly separated and turbulent flow features around the building, and RANS provides reliable information when regions of high wind speed and low turbulence are investigated. Full article
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Article
Vertical Round Buoyant Jets and Fountains in a Linearly, Density-Stratified Fluid
Fluids 2020, 5(4), 232; https://doi.org/10.3390/fluids5040232 - 04 Dec 2020
Viewed by 514
Abstract
Turbulent round buoyant jets and fountains issuing vertically into a linearly density-stratified calm ambient have been investigated in a series of laboratory experiments. The terminal (steady-state) height of rise and the mean elevation of subsequent horizontal spreading have been measured in positively buoyant [...] Read more.
Turbulent round buoyant jets and fountains issuing vertically into a linearly density-stratified calm ambient have been investigated in a series of laboratory experiments. The terminal (steady-state) height of rise and the mean elevation of subsequent horizontal spreading have been measured in positively buoyant jets (at source level), including pure momentum jets and plumes, as well in momentum-driven negatively buoyant jets (fountains). The results from experiments confirmed the asymptotic analysis that was based on dimensional arguments. The normalized terminal height and spreading elevation with respect to the elevation of injection of momentum-driven (positively) buoyant jets and fountains attained the same asymptotic values. The numerical results from the solution of entrainment equations, using an improved entrainment coefficient function, confirmed the results related to buoyancy dominant flows (plumes), while their predictions in momentum-driven flows were quite low if compared to measurements. Full article
(This article belongs to the Special Issue Advances in Turbulent Buoyant Jets)
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Article
Experimental and Mathematical Tools to Predict Droplet Size and Velocity Distribution for a Two-Fluid Nozzle
Fluids 2020, 5(4), 231; https://doi.org/10.3390/fluids5040231 - 03 Dec 2020
Cited by 1 | Viewed by 695
Abstract
Despite progress in laser-based and computational tools, an accessible model that relies on fundamentals and offers a reasonably accurate estimation of droplet size and velocity is lacking, primarily due to entangled complex breakup mechanisms. Therefore, this study aims at using the integral form [...] Read more.
Despite progress in laser-based and computational tools, an accessible model that relies on fundamentals and offers a reasonably accurate estimation of droplet size and velocity is lacking, primarily due to entangled complex breakup mechanisms. Therefore, this study aims at using the integral form of the conservation equations to create a system of equations by solving which, the far-field secondary atomization can be analyzed through predicting droplet size and velocity distributions of the involved phases. To validate the model predictions, experiments are conducted at ambient conditions using water, methanol, and acetone as model fluids with varying formulation properties, such as density, viscosity, and surface tension. Droplet size distribution and velocity are measured with laser diffraction and a high-speed camera, respectively. Finally, an attempt is made to utilize non-scaled parameters to characterize the atomization process, useful for extrapolating the sensitivity analysis to other scales. The merit of this model lies in its simplicity for use in process control and optimization. Full article
(This article belongs to the Special Issue Trends in Spray Atomization)
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Review
Carbon Nanoparticles as Sources for a Cost-Effective Water Purification Method: A Comprehensive Review
Fluids 2020, 5(4), 230; https://doi.org/10.3390/fluids5040230 - 01 Dec 2020
Cited by 1 | Viewed by 1059
Abstract
As the global population grows, the demand for cost-effective and eco-friendly water purification methods is increasing, which presently is at its peak due to the increase of impurities in water and the increasing awareness of waterborne disease. Carbon-based materials, which includes activated carbon, [...] Read more.
As the global population grows, the demand for cost-effective and eco-friendly water purification methods is increasing, which presently is at its peak due to the increase of impurities in water and the increasing awareness of waterborne disease. Carbon-based materials, which includes activated carbon, carbon nanotubes (CNTs), graphene, graphene oxide (GO), reduced graphene oxide (rGO), fullerene, and carbon dots, are observed as potential candidates for water treatment. In the present review, developments related to water purification methods using carbon nanomaterials over the last decade are critically summarized, with an emphasis on their thermophysical properties. The fabrication techniques for activated carbon, CNTs, graphene, and graphene oxide are presented, with an emphasis on the properties of carbon materials that allow their usage for water purification. Then, an extensive review of 71 patents dedicated to water purification using carbon materials such as activated carbon and cotton fibers is performed. Subsequently, the more important research studies on water purification using carbon nanomaterials are discussed, showing that CNTs, GO, and rGO are widely used in water treatment processes. The present review critically discusses the recent developments and provides important information on water purification using carbon materials. Full article
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Article
Wave Modeling for the Establishment Potential Area of Offshore Aquaculture in Indonesia
Fluids 2020, 5(4), 229; https://doi.org/10.3390/fluids5040229 - 01 Dec 2020
Viewed by 723
Abstract
Aquaculture is expected to further improve in the future and can provide 57 percent of fish for human consumption by 2025. In Indonesia, the aquaculture sector produced 5.77% of the world total production in 2014 and increases annually by, on average, 0.62%. Prigi [...] Read more.
Aquaculture is expected to further improve in the future and can provide 57 percent of fish for human consumption by 2025. In Indonesia, the aquaculture sector produced 5.77% of the world total production in 2014 and increases annually by, on average, 0.62%. Prigi Bay, located in the south of east Java, is one potential area to develop sustainable aquaculture in Indonesia. This study presents numerical wave modeling to investigate the potential area for offshore aquaculture in Prigi Bay. The method used Delft3D Flow and CG WAVE model to simulate wave and current. The superimposed analysis is used to select potential areas between the results of the model and the criteria of environmental parameters. The result shows that the location which meets the aquaculture criteria is located at coordinates 8.311° S–8.322° S and 111.734° E–111.747° E. This site has a depth of around 18–26 m with current velocity between 0.10 and 0.14 m/s and significant a wave height between 0.2–0.4 m. This location is the most suitable location for aquaculture in the Prigi Bay. Full article
(This article belongs to the Special Issue Mathematical and Numerical Modeling of Water Waves)
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Article
Validations of the Microchannel Flow Model for Characterizing Vascularized Tissues
Fluids 2020, 5(4), 228; https://doi.org/10.3390/fluids5040228 - 30 Nov 2020
Cited by 1 | Viewed by 1086
Abstract
The microchannel flow model postulates that stress-strain behavior in soft tissues is influenced by the time constants of fluid-filled vessels related to Poiseuille’s law. A consequence of this framework is that changes in fluid viscosity and changes in vessel diameter (through vasoconstriction) have [...] Read more.
The microchannel flow model postulates that stress-strain behavior in soft tissues is influenced by the time constants of fluid-filled vessels related to Poiseuille’s law. A consequence of this framework is that changes in fluid viscosity and changes in vessel diameter (through vasoconstriction) have a measurable effect on tissue stiffness. These influences are examined through the theory of the microchannel flow model. Then, the effects of viscosity and vasoconstriction are demonstrated in gelatin phantoms and in perfused tissues, respectively. We find good agreement between theory and experiments using both a simple model made from gelatin and from living, perfused, placental tissue in vitro. Full article
(This article belongs to the Special Issue Recent Advances in Single and Multiphase Flows in Microchannels)
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Article
Lightning Solvers for Potential Flows
Fluids 2020, 5(4), 227; https://doi.org/10.3390/fluids5040227 - 30 Nov 2020
Cited by 2 | Viewed by 692
Abstract
We present a method for computing potential flows in planar domains. Our approach is based on a new class of techniques, known as “lightning solvers”, which exploit rational function approximation theory in order to achieve excellent convergence rates. The method is particularly suitable [...] Read more.
We present a method for computing potential flows in planar domains. Our approach is based on a new class of techniques, known as “lightning solvers”, which exploit rational function approximation theory in order to achieve excellent convergence rates. The method is particularly suitable for flows in domains with corners where traditional numerical methods fail. We outline the mathematical basis for the method and establish the connection with potential flow theory. In particular, we apply the new solver to a range of classical problems including steady potential flows, vortex dynamics, and free-streamline flows. The solution method is extremely rapid and usually takes just a fraction of a second to converge to a high degree of accuracy. Numerical evaluations of the solutions are performed in a matter of microseconds and can be compressed further with novel algorithms. Full article
(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers)
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Article
Kuramoto-Like Synchronization Mediated through Faraday Surface Waves
Fluids 2020, 5(4), 226; https://doi.org/10.3390/fluids5040226 - 29 Nov 2020
Cited by 1 | Viewed by 662
Abstract
A new class of problems in free surface hydrodynamics appeared after the groundbreaking discovery by Yves Couder and Emmanuel Fort. A bouncing droplet in association with Faraday surface waves gives rise to new nonlinear dynamics, in analogy with the pilot-wave proposed by de [...] Read more.
A new class of problems in free surface hydrodynamics appeared after the groundbreaking discovery by Yves Couder and Emmanuel Fort. A bouncing droplet in association with Faraday surface waves gives rise to new nonlinear dynamics, in analogy with the pilot-wave proposed by de Broglie. The droplet and the underlying vibrating bath are of silicon oil. A weakly viscous potential theory model should be used. Numerical simulations are presented with one and two bouncing droplets oscillating while confined to their cavities. These oscillators are implicitly coupled by the underlying surface wave field. In certain regimes, the oscillators can spontaneously synchronize, even when placed at a distance. Cavity parameters are varied in order to highlight the sensitive wave-mediated coupling. The present nonlinear wave-mediated oscillator synchronization is more general than that displayed by the celebrated Kuramoto model and therefore of general interest. Full article
(This article belongs to the Special Issue Recent Advances in Free Surface Hydrodynamics)
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Article
PyDA: A Hands-On Introduction to Dynamical Data Assimilation with Python
Fluids 2020, 5(4), 225; https://doi.org/10.3390/fluids5040225 - 29 Nov 2020
Cited by 4 | Viewed by 2133
Abstract
Dynamic data assimilation offers a suite of algorithms that merge measurement data with numerical simulations to predict accurate state trajectories. Meteorological centers rely heavily on data assimilation to achieve trustworthy weather forecast. With the advance in measurement systems, as well as the reduction [...] Read more.
Dynamic data assimilation offers a suite of algorithms that merge measurement data with numerical simulations to predict accurate state trajectories. Meteorological centers rely heavily on data assimilation to achieve trustworthy weather forecast. With the advance in measurement systems, as well as the reduction in sensor prices, data assimilation (DA) techniques are applicable to various fields, other than meteorology. However, beginners usually face hardships digesting the core ideas from the available sophisticated resources requiring a steep learning curve. In this tutorial, we lay out the mathematical principles behind DA with easy-to-follow Python module implementations so that this group of newcomers can quickly feel the essence of DA algorithms. We explore a series of common variational, and sequential techniques, and highlight major differences and potential extensions. We demonstrate the presented approaches using an array of fluid flow applications with varying levels of complexity. Full article
(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics, Volume II)
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Article
Numerical Study of the Effects of Twin-Fluid Atomization on the Suspension Plasma Spraying Process
Fluids 2020, 5(4), 224; https://doi.org/10.3390/fluids5040224 - 28 Nov 2020
Viewed by 737
Abstract
Suspension plasma spraying (SPS) is an effective technique to enhance the quality of the thermal barrier, wear-resistant, corrosion-resistant, and superhydrophobic coatings. To create the suspension in the SPS technique, nano and sub-micron solid particles are added to a base liquid (typically water or [...] Read more.
Suspension plasma spraying (SPS) is an effective technique to enhance the quality of the thermal barrier, wear-resistant, corrosion-resistant, and superhydrophobic coatings. To create the suspension in the SPS technique, nano and sub-micron solid particles are added to a base liquid (typically water or ethanol). Subsequently, by using either a mechanical injection system with a plain orifice or a twin-fluid atomizer (e.g., air-blast or effervescent), the suspension is injected into the high-velocity high-temperature plasma flow. In the present work, we simulate the interactions between the air-blast suspension spray and the plasma crossflow by using a three-dimensional two-way coupled Eulerian–Lagrangian model. Here, the suspension consists of ethanol (85 wt.%) and nickel (15 wt.%). Furthermore, at the standoff distance of 40 mm, a flat substrate is placed. To model the turbulence and the droplet breakup, Reynolds Stress Model (RSM) and Kelvin-Helmholtz Rayleigh-Taylor breakup model are used, respectively. Tracking of the fine particles is continued after suspension’s fragmentation and evaporation, until their deposition on the substrate. In addition, the effects of several parameters such as suspension mass flow rate, spray angle, and injector location on the in-flight behavior of droplets/particles as well as the particle velocity and temperature upon impact are investigated. It is shown that the injector location and the spray angle have a significant influence on the droplet/particle in-flight behavior. If the injector is far from the plasma or the spray angle is too wide, the particle temperature and velocity upon impact decrease considerably. Full article
(This article belongs to the Special Issue Trends in Spray Atomization)
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Article
Gas-Liquid Flow and Interphase Mass Transfer in LL Microreactors
Fluids 2020, 5(4), 223; https://doi.org/10.3390/fluids5040223 - 28 Nov 2020
Cited by 2 | Viewed by 964
Abstract
This work investigates the impact of fluid (CO2(g), water) flow rates, channel geometry, and the presence of a surfactant (ethanol) on the resulting gas–liquid flow regime (bubble, slug, annular), pressure drop, and interphase mass transfer coefficient (kla) [...] Read more.
This work investigates the impact of fluid (CO2(g), water) flow rates, channel geometry, and the presence of a surfactant (ethanol) on the resulting gas–liquid flow regime (bubble, slug, annular), pressure drop, and interphase mass transfer coefficient (kla) in the FlowPlateTM LL (liquid-liquid) microreactor, which was originally designed for immiscible liquid systems. The flow regime map generated by the complex mixer geometry is compared to that obtained in straight channels of a similar characteristic length, while the pressure drop is fitted to the separated flows model of Lockhart–Martinelli, and the kla in the bubble flow regime is fitted to a power dissipation model based on isotropic turbulent bubble breakup. The LL-Rhombus configuration yielded higher kla values for an equivalent pressure drop when compared to the LL-Triangle geometry. The Lockhart–Martinelli model provided good pressure drop predictions for the entire range of experimental data (AARE < 8.1%), but the fitting parameters are dependent on the mixing unit geometry and fluid phase properties. The correlation of kla with the energy dissipation rate provided a good fit for the experimental data in the bubble flow regime (AARE < 13.9%). The presented experimental data and correlations further characterize LL microreactors, which are part of a toolbox for fine chemical synthesis involving immiscible fluids for applications involving reactive gas–liquid flows. Full article
(This article belongs to the Special Issue Recent Advances in Single and Multiphase Flows in Microchannels)
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Article
Fast-Projection Methods for the Incompressible Navier–Stokes Equations
Fluids 2020, 5(4), 222; https://doi.org/10.3390/fluids5040222 - 27 Nov 2020
Cited by 1 | Viewed by 643
Abstract
An analysis of existing and newly derived fast-projection methods for the numerical integration of incompressible Navier–Stokes equations is proposed. Fast-projection methods are based on the explicit time integration of the semi-discretized Navier–Stokes equations with a Runge–Kutta (RK) method, in which only one Pressure [...] Read more.
An analysis of existing and newly derived fast-projection methods for the numerical integration of incompressible Navier–Stokes equations is proposed. Fast-projection methods are based on the explicit time integration of the semi-discretized Navier–Stokes equations with a Runge–Kutta (RK) method, in which only one Pressure Poisson Equation is solved at each time step. The methods are based on a class of interpolation formulas for the pseudo-pressure computed inside the stages of the RK procedure to enforce the divergence-free constraint on the velocity field. The procedure is independent of the particular multi-stage method, and numerical tests are performed on some of the most commonly employed RK schemes. The proposed methodology includes, as special cases, some fast-projection schemes already presented in the literature. An order-of-accuracy analysis of the family of interpolations here presented reveals that the method generally has second-order accuracy, though it is able to attain third-order accuracy only for specific interpolation schemes. Applications to wall-bounded 2D (driven cavity) and 3D (turbulent channel flow) cases are presented to assess the performances of the schemes in more realistic configurations. Full article
(This article belongs to the Special Issue Recent Numerical Advances in Fluid Mechanics, Volume II)
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Article
Numerical Simulation of an Air-Core Vortex and Its Suppression at an Intake Using OpenFOAM
Fluids 2020, 5(4), 221; https://doi.org/10.3390/fluids5040221 - 26 Nov 2020
Cited by 1 | Viewed by 665
Abstract
A common challenge faced by engineers in the hydraulic industry is the formation of free surface vortices at pump and power intakes. This undesirable phenomenon which sometimes entrains air could result in several operational problems: noise, vibration, cavitation, surging, structural damage to turbines [...] Read more.
A common challenge faced by engineers in the hydraulic industry is the formation of free surface vortices at pump and power intakes. This undesirable phenomenon which sometimes entrains air could result in several operational problems: noise, vibration, cavitation, surging, structural damage to turbines and pumps, energy losses, efficiency losses, etc. This paper investigates the numerical simulation of an experimentally observed air-core vortex at an intake using the LTSInterFoam solver in OpenFOAM. The solver uses local time-stepping integration. In simulating the air-core vortex, the standard kε, realizable kε, renormalization group (RNG) kε and the shear stress transport (SST) kω models were used. The free surface was modelled using the volume of fluid (VOF) model. The simulation was validated using a set of analytical models and experimental data. The SST kω model provided the best results compared to the other turbulence models. The study was extended to simulate the effect of installing an anti-vortex device on the formation of a free surface vortex. The LTSInterFoam solver proved to be a reliable solver for the steady state simulation of a free surface vortex in OpenFOAM. Full article
(This article belongs to the Special Issue Selected Papers from the 15th OpenFOAM Workshop)
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