Fluids

17 pages, 34712 KiB

Open AccessArticle

The Effect of Patterned Micro-Structure on the Apparent Contact Angle and Three-Dimensional Contact Line

by Patrick Foltyn, Ferdinand Restle, Markus Wissmann, Stefan Hengsbach and Bernhard Weigand

Fluids 2021, 6(2), 92; https://doi.org/10.3390/fluids6020092 - 23 Feb 2021

Cited by 3 | Viewed by 2927

The measurement of the apparent contact angle on structured surfaces is much more difficult to obtain than on smooth surfaces because the pinning of liquid to the roughness has a tremendous influence on the three phase contact line. The results presented here clearly [...] Read more.

The measurement of the apparent contact angle on structured surfaces is much more difficult to obtain than on smooth surfaces because the pinning of liquid to the roughness has a tremendous influence on the three phase contact line. The results presented here clearly show an apparent contact angle variation along the three phase contact line. Accordingly, not only one value for the apparent contact angle can be provided, but a contact angle distribution or an interval has to be given to characterize the wetting behavior. For measuring the apparent contact angle distribution on regularly structured surfaces, namely micrometric pillars and grooves, an experimental approach is presented and the results are provided. A short introduction into the manufacturing process of such structured surfaces, which is a combination of Direct LASER Writing (DLW) lithography, electroforming and hot embossing shows the high quality standard of the used surfaces. Full article

(This article belongs to the Special Issue Experimental Thermodynamics in Droplet Interactions)

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15 pages, 3146 KiB

Open AccessArticle

Simulation of Thermal Radiation and Turbulent Free Convection in an Enclosure with a Glass Wall and a Local Heater

by Igor V. Miroshnichenko, Aidar A. Toilibayev and Mikhail A. Sheremet

Fluids 2021, 6(2), 91; https://doi.org/10.3390/fluids6020091 - 23 Feb 2021

Cited by 6 | Viewed by 1997

Abstract

In this study, a numerical modelling of thermal radiation and turbulent thermogravitational convection in a large-scale chamber containing a thermally-generating element is conducted. The lower border of the cabinet is maintained under adiabatic conditions, while on the other walls the convective boundary conditions [...] Read more.

In this study, a numerical modelling of thermal radiation and turbulent thermogravitational convection in a large-scale chamber containing a thermally-generating element is conducted. The lower border of the cabinet is maintained under adiabatic conditions, while on the other walls the convective boundary conditions (Robin boundary condition) are used. The managing equations with corresponding restrictions are transformed using the stream function–vorticity formulation and then solved by employing a finite difference method. The influence of both the height and wall emissivity of the heated source on fluid motion and the heat transmission in a large-scale chamber is investigated. Our results of the calculations on non-uniform grids with algebraic transformation are in excellent agreement with other available experimental and numerical outcomes for turbulent thermal convection in enclosures. The computations indicate that the average total Nusselt number is enhanced up to 2 times with an increase in the heater height. The results show that the surface emissivity of the heat source has a great influence on the total thermal transference coefficient. Furthermore, a growth of the heater surface emissivity has no significant effect on the flow structure. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Dynamics in Energy Systems)

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21 pages, 32176 KiB

Open AccessArticle

Dynamic Behaviours of a Filament in a Viscoelastic Uniform Flow

by Jingtao Ma, Fang-Bao Tian, John Young and Joseph C. S. Lai

Fluids 2021, 6(2), 90; https://doi.org/10.3390/fluids6020090 - 22 Feb 2021

Cited by 6 | Viewed by 2185

Abstract

The dynamic behaviours of a filament in a viscoelastic uniform flow were investigated by an immersed boundary-lattice Boltzmann method. The effects of the Reynolds numbers (

R e

, ranging from 10 to 200) and the Weissenberg number (

W i

, ranging [...] Read more.

The dynamic behaviours of a filament in a viscoelastic uniform flow were investigated by an immersed boundary-lattice Boltzmann method. The effects of the Reynolds numbers (

R e

, ranging from 10 to 200) and the Weissenberg number (

W i

, ranging from 0 to 1.2) on the filament flapping motion and the drag and lift coefficients on the filament were studied. It was found that a higher inertial effect (larger

R e

) promotes the flapping motion of the filament. In addition, the major effect of the viscoelasticity of the Giesekus fluid is to decrease the critical Reynolds number for the flapping motion of the filament and to promote the flapping motion. The drag coefficient on the filament in a Giesekus uniform flow decreases with the increase of

W i

at low

R e

(

R e < 100

), and experiences oscillations with similar amplitudes at all

W i

at a sufficiently high

R e

(

R e > 100

). In contrast, the viscoelasticity of the FENE-CR fluid increases the critical Reynolds number at lower

W i

(

W i < 0.8

), and shows little influence on the critical Reynolds number at higher

W i

(

W i \geq 0.8

). In addition, the viscoelasticity of the FENE-CR fluid hinders the flapping motion of the filament, and increases the drag coefficient on the filament at low

R e

(

R e < 100

). Full article

(This article belongs to the Special Issue Fluid Structure Interaction: Methods and Applications)

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14 pages, 9871 KiB

Open AccessArticle

Modeling Immiscible Fluid Displacement in a Porous Medium Using Lattice Boltzmann Method

by Magzhan Atykhan, Bagdagul Kabdenova (Dauyeshova), Ernesto Monaco and Luis R. Rojas-Solórzano

Fluids 2021, 6(2), 89; https://doi.org/10.3390/fluids6020089 - 22 Feb 2021

Cited by 4 | Viewed by 2190

Abstract

The numerical investigation of the interpenetrating flow dynamics of a gas injected into a homogeneous porous media saturated with liquid is presented. The analysis is undertaken as a function of the inlet velocity, liquid–gas viscosity ratio (D) and physical properties of the porous [...] Read more.

The numerical investigation of the interpenetrating flow dynamics of a gas injected into a homogeneous porous media saturated with liquid is presented. The analysis is undertaken as a function of the inlet velocity, liquid–gas viscosity ratio (D) and physical properties of the porous medium, such as porous geometry and surface wettability. The study aims to improve understanding of the interaction between the physical parameters involved in complex multiphase flow in porous media (e.g., CO₂ sequestration in aquifers). The numerical simulation of a gaseous phase being introduced through a 2D porous medium constructed using seven staggered columns of either circular- or square-shaped micro-obstacles mimicking the solid walls of the pores is performed using the multiphase Lattice Boltzmann Method (LBM). The gas–liquid fingering phenomenon is triggered by a small geometrical asymmetry deliberately introduced in the first column of obstacles. Our study shows that the amount of gas penetration into the porous medium depends on surface wettability and on a set of parameters such as capillary number (C_a), liquid–gas viscosity ratio (D), pore geometry and surface wettability. The results demonstrate that increasing the capillary number and the surface wettability leads to an increase in the effective gas penetration rate, disregarding porous medium configuration, while increasing the viscosity ratio decreases the penetration rate, again disregarding porous medium configuration. Full article

(This article belongs to the Special Issue Convective Instability in Porous Media, Volume II)

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17 pages, 3926 KiB

Open AccessArticle

A Computational Model for Tail Undulation and Fluid Transport in the Giant Larvacean

by Alexander P. Hoover, Joost Daniels, Janna C. Nawroth and Kakani Katija

Fluids 2021, 6(2), 88; https://doi.org/10.3390/fluids6020088 - 20 Feb 2021

Cited by 4 | Viewed by 2324

Abstract

Flexible propulsors are ubiquitous in aquatic and flying organisms and are of great interest for bioinspired engineering. However, many animal models, especially those found in the deep sea, remain inaccessible to direct observation in the laboratory. We address this challenge by conducting an [...] Read more.

Flexible propulsors are ubiquitous in aquatic and flying organisms and are of great interest for bioinspired engineering. However, many animal models, especially those found in the deep sea, remain inaccessible to direct observation in the laboratory. We address this challenge by conducting an integrative study of the giant larvacean, an invertebrate swimmer and “fluid pump” of the mesopelagic zone. We demonstrate a workflow involving deep sea robots, advanced imaging tools, and numerical modeling to assess the kinematics and resulting fluid transport of the larvacean’s beating tail. A computational model of the tail was developed to simulate the local fluid environment and the tail kinematics using embedded passive (elastic) and active (muscular) material properties. The model examines how varying the extent of muscular activation affects the resulting kinematics and fluid transport rates. We find that muscle activation in two-thirds of the tail’s length, which corresponds to the observed kinematics in giant larvaceans, generates a greater average downstream flow speed than other designs with the same power input. Our results suggest that the active and passive material properties of the larvacean tail are tuned to produce efficient fluid transport for swimming and feeding, as well as provide new insight into the role of flexibility in biological propulsors. Full article

(This article belongs to the Special Issue Hydrodynamics of Swimming)

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15 pages, 4818 KiB

Open AccessEditor’s ChoiceArticle

Time-Periodic Cooling of Rayleigh–Bénard Convection

by Lyes Nasseri, Nabil Himrane, Djamel Eddine Ameziani, Abderrahmane Bourada and Rachid Bennacer

Fluids 2021, 6(2), 87; https://doi.org/10.3390/fluids6020087 - 16 Feb 2021

Cited by 5 | Viewed by 2233

Abstract

The problem of Rayleigh–Bénard’s natural convection subjected to a temporally periodic cooling condition is solved numerically by the Lattice Boltzmann method with multiple relaxation time (LBM-MRT). The study finds its interest in the field of thermal comfort where current knowledge has gaps in [...] Read more.

The problem of Rayleigh–Bénard’s natural convection subjected to a temporally periodic cooling condition is solved numerically by the Lattice Boltzmann method with multiple relaxation time (LBM-MRT). The study finds its interest in the field of thermal comfort where current knowledge has gaps in the fundamental phenomena requiring their exploration. The Boussinesq approximation is considered in the resolution of the physical problem studied for a Rayleigh number taken in the range 10³ ≤ Ra ≤ 10⁶ with a Prandtl number equal to 0.71 (air as working fluid). The physical phenomenon is also controlled by the amplitude of periodic cooling where, for small values of the latter, the results obtained follow a periodic evolution around an average corresponding to the formulation at a constant cold temperature. When the heating amplitude increases, the physical phenomenon is disturbed, the stream functions become mainly multicellular and an aperiodic evolution is obtained for the heat transfer illustrated by the average Nusselt number. Full article

(This article belongs to the Special Issue Thermal Flows)

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11 pages, 1697 KiB

Open AccessEditor’s ChoiceArticle

Velocity Profile and Turbulence Structure Measurement Corrections for Sediment Transport-Induced Water-Worked Bed

by Jaan H. Pu

Fluids 2021, 6(2), 86; https://doi.org/10.3390/fluids6020086 - 16 Feb 2021

Cited by 32 | Viewed by 2853

Abstract

When using point measurement for environmental or sediment laden flows, there is well-recognised risk for not having aligned measurements that causes misinterpretation of the measured velocity data. In reality, these kinds of mismeasurement mainly happen due to the misinterpretation of bed orientation caused [...] Read more.

When using point measurement for environmental or sediment laden flows, there is well-recognised risk for not having aligned measurements that causes misinterpretation of the measured velocity data. In reality, these kinds of mismeasurement mainly happen due to the misinterpretation of bed orientation caused by the complexity of its determination in natural flows, especially in bedload laden or rough bed flows. This study proposes a novel bed realignment method to improve the measured data benchmarking by three-dimensional (3D) bed profile orientation and implemented it into different sets of experimental data. More specifically, the effects of realignment on velocity profile and streamwise turbulence structure measurements were investigated. The proposed technique was tested against experimental data collected over a water-worked and an experimentally arranged well-packed beds. Different from the well-packed rough bed, the water-worked bed has been generated after long sediment transport and settling and hence can be used to verify the proposed bed-alignment technique thoroughly. During the flow analysis, the corrected velocity, turbulence intensity and Reynolds stress profiles were compared to the theoretical logarithmic law, exponential law and linear gravity (universal Reynolds stress distribution) profiles, respectively. It has been observed that the proposed method has improved the agreement of the measured velocity and turbulence structure data with their actual theoretical profiles, particularly in the near-bed region (where the ratio of the flow measurement vertical distance to the total water depth, z/h, is limited to ≤0.4). Full article

(This article belongs to the Special Issue Environmental Sediment Transport: Methods and Applications)

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36 pages, 1283 KiB

Open AccessArticle

A Unifying Perspective on Transfer Function Solutions to the Unsteady Ekman Problem

by Jonathan M. Lilly and Shane Elipot

Fluids 2021, 6(2), 85; https://doi.org/10.3390/fluids6020085 - 16 Feb 2021

Cited by 2 | Viewed by 2493

Abstract

The unsteady Ekman problem involves finding the response of the near-surface currents to wind stress forcing under linear dynamics. Its solution can be conveniently framed in the frequency domain in terms of a quantity that is known as the transfer function, the Fourier [...] Read more.

The unsteady Ekman problem involves finding the response of the near-surface currents to wind stress forcing under linear dynamics. Its solution can be conveniently framed in the frequency domain in terms of a quantity that is known as the transfer function, the Fourier transform of the impulse response function. In this paper, a theoretical investigation of a fairly general transfer function form is undertaken with the goal of paving the way for future observational studies. Building on earlier work, we consider in detail the transfer function arising from a linearly-varying profile of the vertical eddy viscosity, subject to a no-slip lower boundary condition at a finite depth. The horizontal momentum equations, rendered linear by the assumption of horizontally uniform motion, are shown to transform to a modified Bessel’s equation for the transfer function. Two self-similarities, or rescalings that each effectively eliminate one independent variable, are identified, enabling the dependence of the transfer function on its parameters to be more readily assessed. A systematic investigation of asymptotic behaviors of the transfer function is then undertaken, yielding expressions appropriate for eighteen different regimes, and unifying the results from numerous earlier studies. A solution to a numerical overflow problem that arises in the computation of the transfer function is also found. All numerical code associated with this paper is distributed freely for use by the community. Full article

(This article belongs to the Collection Geophysical Fluid Dynamics)

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16 pages, 8028 KiB

Open AccessEditor’s ChoiceArticle

Gas–Liquid Mass Transfer around a Rising Bubble: Combined Effect of Rheology and Surfactant

by Gaelle Lebrun, Feishi Xu, Claude Le Men, Gilles Hébrard and Nicolas Dietrich

Fluids 2021, 6(2), 84; https://doi.org/10.3390/fluids6020084 - 15 Feb 2021

Cited by 13 | Viewed by 4072

Abstract

The influence of viscosity and surface tension on oxygen transfer was investigated using planar laser-induced fluorescence with inhibition (PLIF-I). The surface tension and the viscosity were modified using Triton X-100 and polyacrylamide, respectively. Changes in the hydrodynamic parameters of millimetric bubbles were identified, [...] Read more.

The influence of viscosity and surface tension on oxygen transfer was investigated using planar laser-induced fluorescence with inhibition (PLIF-I). The surface tension and the viscosity were modified using Triton X-100 and polyacrylamide, respectively. Changes in the hydrodynamic parameters of millimetric bubbles were identified, and transfer parameters were calculated. The results revealed a decrease in the mass transferred in the presence of a contaminant. For modified viscosity, the decrease in mass transferred was allowed for by current correlations, but the presence of surfactant led to a sharp decrease in the liquid side mass transfer coefficient, which became even lower when polymer was added. An explanation for the gap between classical correlations and experimental values of k_L is discussed, and a hypothesis of the existence of an accumulation of contaminant in the diffusion layer is proposed. This led to the possibility of a decrease in the diffusion coefficient and oxygen saturation concentration in the liquid film, explaining the discrepancy between models and experience. Adapted values of D_O2 and [O₂] * in this layer were estimated. This original study unravels the complexity of mass transfer from an air bubble in a complex medium. Full article

(This article belongs to the Special Issue Flow and Heat Transfer Intensification in Chemical Engineering)

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14 pages, 10626 KiB

Open AccessEditor’s ChoiceReview

Hierarchical Adaptive Eddy-Capturing Approach for Modeling and Simulation of Turbulent Flows

by Giuliano De Stefano and Oleg V. Vasilyev

Fluids 2021, 6(2), 83; https://doi.org/10.3390/fluids6020083 - 13 Feb 2021

Cited by 12 | Viewed by 1973

Abstract

A short review of wavelet-based adaptive methods for modeling and simulation of incompressible turbulent flows is presented. Wavelet-based computational modeling approaches of different fidelities are recast into an integrated hierarchical adaptive eddy-capturing turbulence modeling framework. The wavelet threshold filtering procedure and the [...] Read more.

A short review of wavelet-based adaptive methods for modeling and simulation of incompressible turbulent flows is presented. Wavelet-based computational modeling approaches of different fidelities are recast into an integrated hierarchical adaptive eddy-capturing turbulence modeling framework. The wavelet threshold filtering procedure and the associated wavelet-filtered Navier–Stokes equations are briefly discussed, along with the adaptive wavelet collocation method that is used for numerical computations. Depending on the level of wavelet thresholding, the simulation is possibly supplemented with a localized closure model. The latest advancements in spatiotemporally varying wavelet thresholding procedures along with the adaptive-anisotropic wavelet-collocation method make the development of a fully adaptive approach feasible with potential applications for complex turbulent flows. Full article

(This article belongs to the Special Issue Wavelets and Fluid Dynamics)

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18 pages, 1048 KiB

Open AccessArticle

Cross-Flow-Induced Vibration of an Elastic Plate

by Efstathios Konstantinidis

Fluids 2021, 6(2), 82; https://doi.org/10.3390/fluids6020082 - 13 Feb 2021

Cited by 6 | Viewed by 2119

Abstract

The cross-flow over a surface-mounted elastic plate and its vibratory response are studied as a fundamental two-dimensional configuration to gain physical insight into the interaction of viscous flow with flexible structures. The governing equations are numerically solved on a deforming mesh using an [...] Read more.

The cross-flow over a surface-mounted elastic plate and its vibratory response are studied as a fundamental two-dimensional configuration to gain physical insight into the interaction of viscous flow with flexible structures. The governing equations are numerically solved on a deforming mesh using an arbitrary Lagrangian-Eulerian finite-element method. The turbulent flow is resolved using the unsteady Reynolds-averaged Navier–Stokes equations at a Reynolds number of

2.5 \times 10^{4}

based on the plate height. The material properties of the plate are selected so that the structural frequency is close to the frequency of vortex shedding from the free edge of a rigid plate, which is studied initially as the reference case. The results show that the plate tip oscillates back and forth in response to unsteady fluid loading at twice the frequency of vortex shedding, which is attributable to the sequential formation of a primary vortex from the free edge and a secondary vortex near the base of the plate. The effects of the plate elasticity and density on the structural response are considered, and results are compiled in terms of the reduced velocity

U^{*}

and the density ratio

ρ^{*}

. The standard deviation of tip displacement increases with reduced velocity in the range

7.1 ⩽ U^{*} ⩽ 18.4

, irrespective of whether the elasticity or the density of the plate is varied. However, the average deflection of the plate in the streamwise direction displays different scaling with

U^{*}

and

ρ^{*}

, but scales almost linearly with the Cauchy number ∼

U^{* 2} / ρ^{*}

. Interestingly, the synchronization between plate motion and vortex shedding ceases at

U^{*} = 18.4

, and the excitation mechanism in the latter case resembles flutter instability, rather than vortex-induced vibration found at lower

U^{*}

. Full article

(This article belongs to the Special Issue Fluid Structure Interaction: Methods and Applications)

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16 pages, 95290 KiB

Open AccessEditor’s ChoiceArticle

CFD and Experimental Study of Wind Pressure Distribution on the High-Rise Building in the Shape of an Equilateral Acute Triangle

by Norbert Jendzelovsky and Roland Antal

Fluids 2021, 6(2), 81; https://doi.org/10.3390/fluids6020081 - 12 Feb 2021

Cited by 7 | Viewed by 4559

Abstract

There is a lack of detailed information about wind flow and distribution of wind pressure around atypically shaped high-rise buildings. The national standard EN 1991-1-4 Eurocode 1 used to determine the effects of wind on the territory of Slovakia (and indeed other countries [...] Read more.

There is a lack of detailed information about wind flow and distribution of wind pressure around atypically shaped high-rise buildings. The national standard EN 1991-1-4 Eurocode 1 used to determine the effects of wind on the territory of Slovakia (and indeed other countries of the European Union) does not have a procedure for determining the effects of wind on objects of triangular shape. This presents a problem for designers and engineers, as there exist no generally binding/valid rules to follow when performing the wind effect analysis. This paper shows the procedure of identification and results of the external wind pressure coefficient for the triangularly shaped high-rise building. Two methods of calculation have been chosen for this purpose. First, experimental measurements were performed on a scaled model of the building cross-section in the wind tunnel. Subsequently, software simulations were performed on the same scaled model in the CFD (computational fluid dynamics) program ANSYS CFX. Results of wind pressure were obtained for two directions of wind flow measured in 16 sampling points distributed irregularly around the circumference of the model. Results were mutually compared and verified. At the end, the wind flow effects on a real-size triangular high-rise building in the built-up area performed by software simulation are shown. Full article

(This article belongs to the Special Issue Recent Advances in Computational Methods in Fluid Dynamics and Applications)

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18 pages, 22383 KiB

Open AccessArticle

Numerical Study on an Interface Compression Method for the Volume of Fluid Approach

by Yuria Okagaki, Taisuke Yonomoto, Masahiro Ishigaki and Yoshiyasu Hirose

Fluids 2021, 6(2), 80; https://doi.org/10.3390/fluids6020080 - 10 Feb 2021

Cited by 13 | Viewed by 2824

Abstract

Many thermohydraulic issues about the safety of light water reactors are related to complicated two-phase flow phenomena. In these phenomena, computational fluid dynamics (CFD) analysis using the volume of fluid (VOF) method causes numerical diffusion generated by the first-order upwind scheme used in [...] Read more.

Many thermohydraulic issues about the safety of light water reactors are related to complicated two-phase flow phenomena. In these phenomena, computational fluid dynamics (CFD) analysis using the volume of fluid (VOF) method causes numerical diffusion generated by the first-order upwind scheme used in the convection term of the volume fraction equation. Thus, in this study, we focused on an interface compression (IC) method for such a VOF approach; this technique prevents numerical diffusion issues and maintains boundedness and conservation with negative diffusion. First, on a sufficiently high mesh resolution and without the IC method, the validation process was considered by comparing the amplitude growth of the interfacial wave between a two-dimensional gas sheet and a quiescent liquid using the linear theory. The disturbance growth rates were consistent with the linear theory, and the validation process was considered appropriate. Then, this validation process confirmed the effects of the IC method on numerical diffusion, and we derived the optimum value of the IC coefficient, which is the parameter that controls the numerical diffusion. Full article

(This article belongs to the Special Issue Advances in Numerical Methods for Multiphase Flows)

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13 pages, 1076 KiB

Open AccessArticle

Statistical Mechanics-Based Surrogates for Scalar Transport in Channel Flow

by Molly Ross and Hitesh Bindra

Fluids 2021, 6(2), 79; https://doi.org/10.3390/fluids6020079 - 10 Feb 2021

Cited by 3 | Viewed by 2288

Abstract

Thermal hydraulics, in certain components of nuclear reactor systems, involve complex flow scenarios, such as flows assisted by free jets and stratified flows leading to turbulent mixing and thermal fluctuations. These complex flow patterns and thermal fluctuations can be extremely critical from a [...] Read more.

Thermal hydraulics, in certain components of nuclear reactor systems, involve complex flow scenarios, such as flows assisted by free jets and stratified flows leading to turbulent mixing and thermal fluctuations. These complex flow patterns and thermal fluctuations can be extremely critical from a reactor safety standpoint. The component-level lumped approximations (0D) or one-dimensional approximations (1D) models for such components and subsystems in safety analysis codes cannot capture the physics accurately, and may introduce a large degree of modeling uncertainty. On the other hand, high-fidelity computational fluid dynamics codes, which provide numerical solutions to the Navier–Stokes equations, are accurate but computationally intensive, and thus cannot be used for system-wide analysis. An alternate way to improve reactor safety analysis is by building reduced-order emulators from computational fluid dynamics (CFD) codes to improve system scale models. One of the key challenges in developing a reduced-order emulator is to preserve turbulent mixing and thermal fluctuations across different-length scales or time-scales. This paper presents the development of a reduced-order, non-linear, “Markovian” statistical surrogate for turbulent mixing and scalar transport. The method and its implementation are demonstrated on a canonical problem of differentially heated channel flow, and high-resolution direct numerical simulations (DNS) data are used for emulator or surrogate development. This statistical surrogate model relies on Kramers–Moyal expansion and emulates the turbulent velocity signal with a high degree of accuracy. Full article

(This article belongs to the Special Issue Application of Computational Fluid Dynamics in Nuclear Reactor Safety Analysis)

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17 pages, 16605 KiB

Open AccessArticle

Numerical Treatment of the Interface in Two Phase Flows Using a Compressible Framework in OpenFOAM: Demonstration on a High Velocity Droplet Impact Case

by Giovanni Tretola and Konstantina Vogiatzaki

Fluids 2021, 6(2), 78; https://doi.org/10.3390/fluids6020078 - 09 Feb 2021

Cited by 6 | Viewed by 3311

Abstract

The ability to accurately predict the dynamics of fast moving and deforming interfaces is of interest to a number of applications including ink printing, drug delivery and fuel injection. In the current work we present a new compressible framework within OpenFOAM which incorporates [...] Read more.

The ability to accurately predict the dynamics of fast moving and deforming interfaces is of interest to a number of applications including ink printing, drug delivery and fuel injection. In the current work we present a new compressible framework within OpenFOAM which incorporates mitigation strategies for the well known issue of spurious currents. The framework incorporates the compressible algebraic Volume-of-Fluid (VoF) method with additional interfacial treatment techniques including volume fraction smoothing and sharpening (for the calculation of the interface geometries and surface tension force, respectively) as well as filtering of the capillary forces. The framework is tested against different benchmarks: A 2D stationary droplet, a high velocity impact droplet case (500 m/s impact velocity) against a dry substrate and, with the same impact conditions, against a liquid film. For the 2D static droplet case, our results are consistent with what is observed in the literature when these strategies are implemented within incompressible frameworks. For the high impact droplet cases we find that accounting for both compressibility and correct representation of the interface is very important in numerical simulations, since pressure waves develop and propagate within the droplet interacting with the interface. While the implemented strategies do not alter the dynamics of the impact and the droplet shape, they have a considerable effect on the lamella formation. Our numerical method, although currently implemented for droplet cases, can also be used for any fast moving interface with or without considering the impact on a surface. Full article

(This article belongs to the Special Issue Modelling of Reactive and Non-reactive Multiphase Flows)

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16 pages, 7078 KiB

Open AccessReview

Fluidic Oscillators Mediating Generation of Microbubbles (Survey)

by Václav Tesař

Fluids 2021, 6(2), 77; https://doi.org/10.3390/fluids6020077 - 09 Feb 2021

Cited by 8 | Viewed by 2861

Abstract

If a gas volume is distributed into many microbubbles of a sub-millimetre size, the total gas/liquid surface becomes very large. This increases overall heat and/or mass transport across the sum of surfaces. The paper discusses several applications in which the use of microbubbles [...] Read more.

If a gas volume is distributed into many microbubbles of a sub-millimetre size, the total gas/liquid surface becomes very large. This increases overall heat and/or mass transport across the sum of surfaces. The paper discusses several applications in which the use of microbubbles increases efficiency of various processes, especially in wastewater treatment and in growing microorganisms such as algae, yeast, bacteria, or primitive fungi. The problem of microbubble generation by percolation in aerator is their coalescence into larger bubbles, whatever small are the pores in the aerator in which the microbubbles are generated. The solution of this size discrepancy question was found in agitating the gas flow by a fluidic oscillator prior to its injection through the aerator. The oscillator is a no-moving-part device, simple, inexpensive, resistant to external effects like acceleration or heat, and with long maintenance-free working life. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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14 pages, 3233 KiB

Open AccessArticle

A Systematic Approach to Predict the Behavior of Cough Droplets Using Feedforward Neural Networks Method

by Irfan Bahiuddin, Setyawan Bekti Wibowo, M. Syairaji, Jimmy Trio Putra, Cahyo Adi Pandito, Ahdiar Fikri Maulana, Rian Mantasa Salve Prastica and Nurhazimah Nazmi

Fluids 2021, 6(2), 76; https://doi.org/10.3390/fluids6020076 - 09 Feb 2021

Cited by 2 | Viewed by 2055

Abstract

Coronavirus disease 2019 (Covid-19) has been identified as being transmitted among humans with droplets from breath, cough, and sneezes. Understanding the droplets’ behavior can be critical information to avoid disease transmission, especially while designing a device deals with human air respiratory. Although various [...] Read more.

Coronavirus disease 2019 (Covid-19) has been identified as being transmitted among humans with droplets from breath, cough, and sneezes. Understanding the droplets’ behavior can be critical information to avoid disease transmission, especially while designing a device deals with human air respiratory. Although various studies have provided enormous computational fluid simulations, most cases are too specific and quite challenging to combine with other similar studies directly. Therefore, this paper proposes a systematic approach to predict the droplet behavior for coughing cases using machine learning. The approach consists of three models, which are droplet generator, mask model, and free droplet model modeled using feedforward neural network (FFNN). The evaluation has shown that the three FFNNs models’ accuracies are relatively high, with R-values of more than 0.990. The model has successfully predicted the evaporation effect on the diameter reduction and the completely evaporated state, which can be considered unlearned cases for machine learning models. The predicted horizontal distance pattern also agrees with the data in the literature. In summary, the proposed approach has demonstrated the capability to predict the diameter pattern according to the experimental or previous work data at various mask face types. Full article

(This article belongs to the Special Issue Deep Learning for Fluid Simulation)

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17 pages, 10108 KiB

Open AccessArticle

Development of an Algebraic Model of Empirical Parameterization of Near Wakes around a Vehicle

by Arata Kimura, Mitsufumi Asami, Hideyuki Oka and Yasushi Oka

Fluids 2021, 6(2), 75; https://doi.org/10.3390/fluids6020075 - 08 Feb 2021

Cited by 1 | Viewed by 1803

Abstract

In this paper, we describe and evaluate an algebraic model that has been adopted in a diagnostic wind flow model. Our numerical model is based on the Röckle’s wind modelling approach and we intend to reproduce the steady-state flow patterns of recirculation vortices [...] Read more.

In this paper, we describe and evaluate an algebraic model that has been adopted in a diagnostic wind flow model. Our numerical model is based on the Röckle’s wind modelling approach and we intend to reproduce the steady-state flow patterns of recirculation vortices that are generated in the near-wake region behind a vehicle. The evaluation of the practicality of our proposed model is performed by comparing the wind tunnel experiments of the flow around a vehicle conducted by the Loughborough University. We also compare the numerical results of our model with the CFD model. The proposed model reproduces the flow patterns behind a vehicle and it has significant advantages, such as low numerical costs. We expect that further improvements in the algebraic model when considering the vehicle’s shape will improve its practicality for the numerical analysis of flow fields around a vehicle. Full article

(This article belongs to the Special Issue Aerodynamics and Aeroacoustics of Vehicles)

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23 pages, 5787 KiB

Open AccessArticle

Performance Assessment of Fluidic Oscillators Tested on the NASA Hump Model

by Mehti Koklu

Fluids 2021, 6(2), 74; https://doi.org/10.3390/fluids6020074 - 07 Feb 2021

Cited by 4 | Viewed by 2302

Abstract

Flow separation control over a wall-mounted hump model was studied experimentally to assess the performance of fluidic oscillators (sweeping jet actuators). An array of fluidic oscillators was used to control flow separation. The results showed that the fluidic oscillators were able to achieve [...] Read more.

Flow separation control over a wall-mounted hump model was studied experimentally to assess the performance of fluidic oscillators (sweeping jet actuators). An array of fluidic oscillators was used to control flow separation. The results showed that the fluidic oscillators were able to achieve substantial control over the separated flow by increasing the upstream suction pressure and downstream pressure recovery. Using the data available in the literature, the performance of the fluidic oscillators was compared to other active flow control (AFC) methods such as steady blowing, steady suction, and zero-net-mass-flux (ZNMF) actuators. Several integral parameters, such as the inviscid flow comparison coefficient, pressure drag coefficient, and modified normal force coefficient, were used as quality metrics in the performance comparison of the AFC methods. These quality metrics indicated the superiority of the steady suction method, especially at lower excitation amplitudes that is followed by the fluidic oscillators, steady blowing, and the ZNMF actuators, respectively. An aerodynamic figure of merit (AFM) was also constructed using the integral parameters and AFC power usage. The AFM results revealed that, for this study, steady suction was the most efficient AFC method at lower excitation amplitudes. The steady suction loses its efficiency as the excitation amplitude increases, and the fluidic oscillators become the most efficient AFC method. Both the steady suction and the fluidic oscillators have an AFM > 1 for the range tested in this study, indicating that they provide a net benefit when the AFC power consumption is also considered. On the other hand, both the steady blowing and ZNMF actuators were found to be inefficient AFC methods (AFM < 1) for the current configuration. Although they improved the flow field by controlling flow separation, the power requirement was more than their benefit. Full article

(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)

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17 pages, 2204 KiB

Open AccessReview

Numerical Modeling of Sperm Swimming

by Fang-Bao Tian and Li Wang

Fluids 2021, 6(2), 73; https://doi.org/10.3390/fluids6020073 - 07 Feb 2021

Cited by 8 | Viewed by 4939

Abstract

Due to rising human infertility, sperm motility has been an important subject. Among the hundreds of millions of sperms on the journey up the oviducts, only a few excellent travelers will reach the eggs. This journey is affected by many factors, some of [...] Read more.

Due to rising human infertility, sperm motility has been an important subject. Among the hundreds of millions of sperms on the journey up the oviducts, only a few excellent travelers will reach the eggs. This journey is affected by many factors, some of which include sperm quality, sperm density, fluid rheology and chemotaxis. In addition, the sperm swimming through different body tracks and fluids involves complex sperm flagellar, complex fluid environment, and multi-sperm and sperm-wall interactions. Therefore, this topic has generated substantial research interest. In this paper, we present a review of computational studies on sperm swimming from an engineering perspective with focus on both simplified theoretical methods and fluid–structure interaction methods. Several open issues in this field are highlighted. Full article

(This article belongs to the Special Issue Computational Biofluid Mechanics)

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30 pages, 20153 KiB

Open AccessArticle

Two-Phase Turbulence Statistics from High Fidelity Dispersed Droplet Flow Simulations in a Pressurized Water Reactor (PWR) Sub-Channel with Mixing Vanes

by Nadish Saini and Igor A. Bolotnov

Fluids 2021, 6(2), 72; https://doi.org/10.3390/fluids6020072 - 06 Feb 2021

Cited by 3 | Viewed by 2119

Abstract

In the dispersed flow film boiling regime (DFFB), which exists under post-LOCA (loss-of-coolant accident) conditions in pressurized water reactors (PWRs), there is a complex interplay between droplet dynamics and turbulence in the surrounding steam. Experiments have accredited particular significance to droplet collision with [...] Read more.

In the dispersed flow film boiling regime (DFFB), which exists under post-LOCA (loss-of-coolant accident) conditions in pressurized water reactors (PWRs), there is a complex interplay between droplet dynamics and turbulence in the surrounding steam. Experiments have accredited particular significance to droplet collision with the spacer-grids and mixing vane structures and their consequent positive feedback to the heat transfer recorded in the immediate downstream vicinity. Enabled by high-performance computing (HPC) systems and a massively parallel finite element-based flow solver—PHASTA (Parallel Hierarchic Adaptive Stabilized Transient Analysis)—this work presents high fidelity interface capturing, two-phase, adiabatic simulations in a PWR sub-channel with spacer grids and mixing vanes. Selected flow conditions for the simulations are informed by the experimental data found in the literature, including the steam Reynolds number and collision Weber number (

W e_{c} = {40, 80}

), and are characteristic of the DFFB regime. Data were collected from the simulations at an unprecedented resolution, which provides detailed insights into the continuous phase turbulence statistics, highlighting the effects of the presence of droplets and the comparative effect of different Weber numbers on turbulence in the surrounding steam. Further, axial evolution of droplet dynamics was analyzed through cross-sectionally averaged quantities, including droplet volume, surface area and Sauter mean diameter (SMD). The downstream SMD values agree well with the existing empirical correlations for the selected range of

W e_{c}

. The high-resolution data repository from the simulations herein is expected to be of significance to guide model development for system-level thermal hydraulic codes. Full article

(This article belongs to the Special Issue Application of Computational Fluid Dynamics in Nuclear Reactor Safety Analysis)

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15 pages, 1445 KiB

Open AccessArticle

Numerical Study of Bloodstream Diffusion of the New Generation of Drug-Eluting Stents in Coronary Arteries

by Leandro Marques and Gustavo R. Anjos

Fluids 2021, 6(2), 71; https://doi.org/10.3390/fluids6020071 - 05 Feb 2021

Cited by 1 | Viewed by 2573

Abstract

The present work aims at developing a numerical study on the drug diffusion in the bloodstream in a coronary artery with drug-eluting stent implanted. The blood was modeled as a single-phase, incompressible and Newtonian fluid and the Navier–Stokes equation was approximated according to [...] Read more.

The present work aims at developing a numerical study on the drug diffusion in the bloodstream in a coronary artery with drug-eluting stent implanted. The blood was modeled as a single-phase, incompressible and Newtonian fluid and the Navier–Stokes equation was approximated according to the Finite Element Method (FEM). The dynamics of drug-eluting concentration in bloodstream was investigated using four drug-eluting stents with different mass diffusivities in microchannels with variable cross sections, including a real coronary artery geometry with atherosclerosis. The results reveal complex drug concentration patterns and accumulation in the vicinity of the fat buildup. Full article

(This article belongs to the Special Issue Recent Advances in Single and Multiphase Flows in Microchannels)

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20 pages, 5248 KiB

Open AccessArticle

Numerical Analysis of Air Vortex Interaction with Porous Screen

by Xudong An, Lin Jiang and Fatemeh Hassanipour

Fluids 2021, 6(2), 70; https://doi.org/10.3390/fluids6020070 - 05 Feb 2021

Cited by 6 | Viewed by 2338

Abstract

In many industrial applications, a permeable mesh (porous screen) is used to control the unsteady (most commonly vortex) flows. Vortex flows are known to display intriguing behavior while propagating through porous screens. This numerical study aims to investigate the effects of physical properties [...] Read more.

In many industrial applications, a permeable mesh (porous screen) is used to control the unsteady (most commonly vortex) flows. Vortex flows are known to display intriguing behavior while propagating through porous screens. This numerical study aims to investigate the effects of physical properties such as porosity, Reynolds number, inlet flow dimension, and distance to the screen on the flow behavior. The simulation model includes a piston-cylinder vortex ring generator and a permeable mesh constructed by evenly arranged rods. Two methods of user-defined function and moving mesh have been applied to model the vortex ring generation. The results show the formation, evolution, and characteristics of the vortical rings under various conditions. The results for vorticity contours and the kinetic energy dissipation indicate that the physical properties alter the flow behavior in various ways while propagating through the porous screens. The numerical model, cross-validated with the experimental results, provides a better understanding of the fluid–solid interactions of vortex flows and porous screens. Full article

(This article belongs to the Special Issue Fluid Structure Interaction: Methods and Applications)

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13 pages, 6524 KiB

Open AccessReview

Electroviscous Effects in Stationary Solid Phase Suspensions

by Francisco J. Rubio-Hernández

Fluids 2021, 6(2), 69; https://doi.org/10.3390/fluids6020069 - 05 Feb 2021

Viewed by 2080

Abstract

Flowing through porous media is a matter of interest in different research fields such as medicine, engineering and science. The spontaneous appearance of ionic distribution at the solid liquid interface gives place to a reduction in the flow rate, which is generally named [...] Read more.

Flowing through porous media is a matter of interest in different research fields such as medicine, engineering and science. The spontaneous appearance of ionic distribution at the solid liquid interface gives place to a reduction in the flow rate, which is generally named electroviscous effect. However, this should be differentiated in two more specific effects, the primary effect due to the distortion of ionic clouds, and the secondary effect due to the overlapping of ionic clouds. Theoretical and experimental works have not always been clearly conducted in order to separate both effects. Instead, they have been globally grouped. The purpose of this review is to revise theoretical and experimental bibliography on the electroviscous effect in stationary solid phase suspensions (porous plugs, membranes, microchannels, capillaries). The main conclusions of this brief revision are: (i) when ionic clouds are relatively small, it is possible to accept that only the primary effect is the cause for the apparent increase of the viscosity of the liquid phase when it is forced to flow relative to the stationary solid phase; (ii) although theory predicts a maximum for the variation of the overall electroviscous effect vs the relative size of the ionic cloud, it has been experimentally observed but not properly reasoned that its existence depends on the salt type; and (iii) it is necessary to justify why, if the fluid is non-Newtonian, electrokinetic parameters dominate the characteristics of the flow due to high pressure gradients, but the rheological parameters are more decisive when the flow is generated by low pressure gradients. Full article

(This article belongs to the Special Issue Fluid Mechanics of Suspensions and Emulsions)

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49 pages, 4163 KiB

Open AccessEditor’s ChoiceReview

A Review of Vortex Methods and Their Applications: From Creation to Recent Advances

by Chloé Mimeau and Iraj Mortazavi

Fluids 2021, 6(2), 68; https://doi.org/10.3390/fluids6020068 - 04 Feb 2021

Cited by 38 | Viewed by 9130

Abstract

This review paper presents an overview of Vortex Methods for flow simulation and their different sub-approaches, from their creation to the present. Particle methods distinguish themselves by their intuitive and natural description of the fluid flow as well as their low numerical dissipation [...] Read more.

This review paper presents an overview of Vortex Methods for flow simulation and their different sub-approaches, from their creation to the present. Particle methods distinguish themselves by their intuitive and natural description of the fluid flow as well as their low numerical dissipation and their stability. Vortex methods belong to Lagrangian approaches and allow us to solve the incompressible Navier-Stokes equations in their velocity-vorticity formulation. In the last three decades, the wide range of research works performed on these methods allowed us to highlight their robustness and accuracy while providing efficient computational algorithms and a solid mathematical framework. On the other hand, many efforts have been devoted to overcoming their main intrinsic difficulties, mostly relying on the treatment of the boundary conditions and the distortion of particle distribution. The present review aims to describe the Vortex methods by following their chronological evolution and provides for each step of their development the mathematical framework, the strengths and limits as well as references to applications and numerical simulations. The paper ends with a presentation of some challenging and very recent works based on Vortex methods and successfully applied to problems such as hydrodynamics, turbulent wake dynamics, sediment or porous flows. Full article

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

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10 pages, 1417 KiB

Open AccessEditor’s ChoiceArticle

Mechanical Forces Impacting Cleavage of Von Willebrand Factor in Laminar and Turbulent Blood Flow

by Alireza Sharifi and David Bark

Fluids 2021, 6(2), 67; https://doi.org/10.3390/fluids6020067 - 03 Feb 2021

Cited by 6 | Viewed by 2334

Abstract

Von Willebrand factor (VWF) is a large multimeric hemostatic protein. VWF is critical in arresting platelets in regions of high shear stress found in blood circulation. Excessive cleavage of VWF that leads to reduced VWF multimer size in plasma can cause acquired von [...] Read more.

Von Willebrand factor (VWF) is a large multimeric hemostatic protein. VWF is critical in arresting platelets in regions of high shear stress found in blood circulation. Excessive cleavage of VWF that leads to reduced VWF multimer size in plasma can cause acquired von Willebrand syndrome, which is a bleeding disorder found in some heart valve diseases and in patients receiving mechanical circulatory support. It has been proposed that hemodynamics (blood flow) found in these environments ultimately leads to VWF cleavage. In the context of experiments reported in the literature, scission theory, developed for polymers, is applied here to provide insight into flow that can produce strong extensional forces on VWF that leads to domain unfolding and exposure of a cryptic site for cleavage through a metalloproteinase. Based on theoretical tensile forces, laminar flow only enables VWF cleavage when shear rate is large enough (>2800 s⁻¹) or when VWF is exposed to constant shear stress for nonphysiological exposure times (>20 min). Predicted forces increase in turbulence, increasing the chance for VWF cleavage. These findings can be used when designing blood-contacting medical devices by providing hemodynamic limits to these devices that can otherwise lead to acquired von Willebrand syndrome. Full article

(This article belongs to the Special Issue Turbulence in Blood Flow)

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23 pages, 5231 KiB

Open AccessArticle

Numerical Flow Characterization around a Type 209 Submarine Using OpenFOAM

by Ruben J. Paredes, Maria T. Quintuña, Mijail Arias-Hidalgo and Raju Datla

Fluids 2021, 6(2), 66; https://doi.org/10.3390/fluids6020066 - 03 Feb 2021

Cited by 8 | Viewed by 4463

Abstract

The safety of underwater operation depends on the accuracy of its speed logs which depends on the location of its probe and the calibration thoroughness. Thus, probes are placed in areas where the flow of water is smooth, continuous, without high velocity gradients, [...] Read more.

The safety of underwater operation depends on the accuracy of its speed logs which depends on the location of its probe and the calibration thoroughness. Thus, probes are placed in areas where the flow of water is smooth, continuous, without high velocity gradients, air bubbles, or vortical structures. In the present work, the flow around two different submarines is numerically described in deep-water and near-surface conditions to identify hull zones where probes could be installed. First, the numerical setup of a multiphase solver supplied with OpenFOAM v7 was verified and validated using the DARPA SUBOFF-5470 submarine at scaled model including the hull and sail configuration at

H / D = 5.4

and

F r = 0.466

. Later, the grid sensitivity of the resistance was assessed for the full-scale Type 209/1300 submarine at

H / D = 0.347

and

F r = 0.194

. Free-surface effect on resistance and flow characteristics was evaluated by comparing different operational conditions. Results shows that the bow and near free-surface regions should be avoided due to high flow velocity gradient, pressure fluctuations, and large turbulent vortical structures. Moreover, free-surface effect is stronger close to the bow nose. In conclusion, the probe could be installed in the acceleration region where the local flow velocity is

15 %

higher than the navigation speed at surface condition. A 4% correction factor should be applied to the probe readings to compensate free-surface effect. Full article

(This article belongs to the Special Issue Selected Papers from the 15th OpenFOAM Workshop)

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22 pages, 2843 KiB

Open AccessReview

Analysis of the Parameters Required to Properly Define Nanofluids for Heat Transfer Applications

by Sergio Bobbo, Bernardo Buonomo, Oronzio Manca, Silvio Vigna and Laura Fedele

Fluids 2021, 6(2), 65; https://doi.org/10.3390/fluids6020065 - 02 Feb 2021

Cited by 8 | Viewed by 3133

Abstract

Nanofluids are obtained by dispersing nanoparticles and dispersant, when present, in a base fluid. Their properties, in particular their stability, however, are strictly related to several other parameters, knowledge of which is important to reproduce the nanofluids and correctly interpret their behavior. Due [...] Read more.

Nanofluids are obtained by dispersing nanoparticles and dispersant, when present, in a base fluid. Their properties, in particular their stability, however, are strictly related to several other parameters, knowledge of which is important to reproduce the nanofluids and correctly interpret their behavior. Due to this complexity, the results appear to be frequently unreliable, contradictory, not comparable and/or not repeatable, in particular for the scarcity of information on their preparation. Thus, it is essential to define what is the minimum amount of information necessary to fully describe the nanofluid, so as to ensure the possibility of reproduction of both their formulation and the measurements of their properties. In this paper, a literature analysis is performed to highlight what are the most important parameters necessary to describe the configuration of each nanofluid and their influence on the nanofluid’s properties. A case study is discussed, analyzing the information reported and the results obtained for the thermophysical properties of nanofluids formed by water and TiO₂ nanoparticles. The aim is to highlight the differences in the amount of information given by the different authors and exemplify how results can be contradictory. A final discussion gives some suggestions on the minimum amount of information that should be given on a nanofluid to have the possibility to compare results obtained for similar nanofluids and to reproduce the same nanofluid in other laboratories. Full article

(This article belongs to the Special Issue Thermophysical Properties of Nanofluids - From Measurement to Interpretation)

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Graphical abstract

20 pages, 6632 KiB

Open AccessArticle

An Investigation of Aerodynamic Effects of Body Morphing for Passenger Cars in Close-Proximity

by Geoffrey Le Good, Max Resnick, Peter Boardman and Brian Clough

Fluids 2021, 6(2), 64; https://doi.org/10.3390/fluids6020064 - 01 Feb 2021

Cited by 8 | Viewed by 2688

Abstract

The potential energy-saving benefit for vehicles when travelling in a ‘platoon’ formation results from the reduction in total aerodynamic drag which may result from the interaction of bluff bodies in close-proximity. Early investigations of platooning, prompted by problems of congestion, had shown the [...] Read more.

The potential energy-saving benefit for vehicles when travelling in a ‘platoon’ formation results from the reduction in total aerodynamic drag which may result from the interaction of bluff bodies in close-proximity. Early investigations of platooning, prompted by problems of congestion, had shown the potential for drag reduction but was not pursued. More recently, technologies developed for connected-autonomous vehicle control have provided a renewed interest in platooning particularly within the commercial vehicle industry. To date, most aerodynamics-based considerations of platooning have been conducted to assess the sensitivity of drag-saving to vehicle spacing and were based on formations of identically shaped constituents. In this study, the interest was the sensitivity of drag-saving to the shape of the individual platoon constituents. A new reference car, the Resnick model, was specially designed to include front and rear-end add-on sections to make distinct changes in profile form and simulate large-scale body morphing. The results of wind tunnel tests on small-scale models suggested that current trends in low-drag styling may not provide the ideal shape for platoon constituent members and that optimised forms are likely to be dependent upon position in the platoon. Full article

(This article belongs to the Special Issue Aerodynamics and Aeroacoustics of Vehicles)

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18 pages, 10660 KiB

Open AccessArticle

Numerical Modeling of Jet at the Bottom of Tank at Moderate Reynolds Number Using Compact Hermitian Finite Differences Method

by Mohammed Loukili, Kamila Kotrasova and Denys Dutykh

Fluids 2021, 6(2), 63; https://doi.org/10.3390/fluids6020063 - 01 Feb 2021

Cited by 1 | Viewed by 2133

Abstract

In this manuscript, the injection of a homogeneous jet in a numerical tank is considered to revolve around discussing the limitation of the direct numerical simulation (DNS), to resolve the equations governing the problem of a jet emitted from the bottom of a [...] Read more.

In this manuscript, the injection of a homogeneous jet in a numerical tank is considered to revolve around discussing the limitation of the direct numerical simulation (DNS), to resolve the equations governing the problem of a jet emitted from the bottom of a numerical tank. The investigation has been made in the context of an unsteady, viscous, and incompressible fluid. The numerical resolution of the equations governing the problem is made by the compact Hermitian finite differences method (HFDM) high accuracy

O (h^{2}, h^{4})

First, the numerical code used in this work is validated by comparing the profiles of the velocity components at the median of the lid-driven cavity with the results of the literature. Furthermore, to confirm the validity of the present numerical code, an evaluation of mesh domain sensitivity is assessed by comparing the numerical vertical velocity profiles for different steps of y-direction (flow direction) with the analytical solution. Afterward, the aim is to perform the nonlinear simulations of the Navier–Stokes equations in a large computational domain. Next, the goal is to characterize the instabilities associated with high Reynolds numbers when a jet is emitted from the bottom of the numerical tank. Full article

(This article belongs to the Special Issue Flow and Heat Transfer in Non-linear Fluids)

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