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4 pages, 182 KiB

Open AccessEditorial

Recent Advances in Fluid Mechanics: Feature Papers

by Mehrdad Massoudi

Fluids 2021, 6(4), 143; https://doi.org/10.3390/fluids6040143 - 7 Apr 2021

Viewed by 3472

Abstract

This Special Issue is a collection of top-quality papers from some of the Editorial Board Members of Fluids, Guest Editors, and leading researchers discussing new knowledge or new cutting-edge developments on all aspects of fluid mechanics [...] Full article

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

Research

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15 pages, 588 KiB

Open AccessArticle

Analysis and Modelling of the Commutation Error

by Markus Klein and Massimo Germano

Fluids 2021, 6(1), 15; https://doi.org/10.3390/fluids6010015 - 31 Dec 2020

Cited by 5 | Viewed by 2245

Abstract

A multiscale dynamic analysis of the commutation error, based on the filtering approach is performed. The similarity multiscale hypothesis proposed by Bardina (1983) and extended by Geurts and Holm (2006) to the commutation error is examined in detail and an extension of the [...] Read more.

A multiscale dynamic analysis of the commutation error, based on the filtering approach is performed. The similarity multiscale hypothesis proposed by Bardina (1983) and extended by Geurts and Holm (2006) to the commutation error is examined in detail and an extension of the Germano identity to the analysis and the modelling of the commutation error is proposed. For a detailed analysis under controlled condition the method is first applied to synthetic turbulence and subsequently to the a-priori analysis of a turbulent channel flow at

R e_{τ} = 590

. The results illustrate the flexibility of the dynamic modelling approach. Combined with a scale similarity assumption for the commutation error very satisfactory results have been obtained for first order derivatives and reasonable results for second order derivatives. In all cases the modelling of the commutation error resulted in smaller errors than the error obtained by neglecting the commutation error. Full article

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

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23 pages, 11467 KiB

Open AccessArticle

Assessing Eulerian Indicators for Predicting Mixing in a Blinking Vortex System with Varying Degrees of Continuous Transition

by Hyekyung Ryu and Andrew N. Cookson

Fluids 2021, 6(1), 10; https://doi.org/10.3390/fluids6010010 - 30 Dec 2020

Cited by 1 | Viewed by 1797

Abstract

A discontinuous change in sequential velocity fields is known to generate laminar flow mixing through the mechanism of streamline crossing. However, previous research has suggested that a small degree of continuous transition between velocity fields may not necessarily be detrimental. This study therefore [...] Read more.

A discontinuous change in sequential velocity fields is known to generate laminar flow mixing through the mechanism of streamline crossing. However, previous research has suggested that a small degree of continuous transition between velocity fields may not necessarily be detrimental. This study therefore used a modified blinking vortex system with varying degree of continuous transition to assess the precise effect that this continuous transition has on mixing performance. This system was studied for the parameters: blinking period, vortex spacing, and the fraction of time spent in transition. Continuous Eulerian indicators were computed to investigate their correspondence with Lagrangian-based metrics, such as Intensity of Segregation, under such conditions. The results showed that up to 30% transition time yielded improvements in mixing, most notably when vortex spacing was large, and this was consistent across different time periods. The mixing prediction by the Eulerian indicators, particularly mobility, showed good agreement with actual mixing quality, albeit not perfectly, suggesting room for refinement in these metrics. Overall, the findings imply that mixing systems, such as continuous pipe flow-based devices, which are designed assuming a discontinuous change in velocity fields, might benefit from the presence of a small degree of continuous transition between discrete states. Full article

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25 pages, 4881 KiB

Open AccessArticle

Modeling Heavy-Gas Dispersion in Air with Two-Layer Shallow Water Equations

by Alexandre Chiapolino, Sébastien Courtiaud, Emmanuel Lapébie and Richard Saurel

Fluids 2021, 6(1), 2; https://doi.org/10.3390/fluids6010002 - 22 Dec 2020

Cited by 2 | Viewed by 2766

Abstract

Computation of gas dispersal in urban places or hilly grounds requires a large amount of computer time when addressed with conventional multidimensional models. Those are usually based on two-phase flow or Navier-Stokes equations. Different classes of simplified models exist. Among them, two-layer shallow [...] Read more.

Computation of gas dispersal in urban places or hilly grounds requires a large amount of computer time when addressed with conventional multidimensional models. Those are usually based on two-phase flow or Navier-Stokes equations. Different classes of simplified models exist. Among them, two-layer shallow water models are interesting to address large-scale dispersion. Indeed, compared to conventional multidimensional approaches, 2D simulations are carried out to mimic 3D effects. The computational gain in CPU time is consequently expected to be tremendous. However, such models involve at least three fundamental difficulties. The first one is related to the lack of hyperbolicity of most existing formulations, yielding serious consequences regarding wave propagation. The second is related to the non-conservative terms in the momentum equations. Those terms account for interactions between fluid layers. Recently, these two difficulties have been addressed in Chiapolino and Saurel (2018) and an unconditional hyperbolic model has been proposed along with a Harten-Lax-van Leer (HLL) type Riemann solver dealing with the non-conservative terms. In the same reference, numerical experiments showed robustness and accuracy of the formulation. In the present paper, a third difficulty is addressed. It consists of the determination of appropriate drag effect formulation. Such effects also account for interactions between fluid layers and become of particular importance when dealing with heavy-gas dispersion. With this aim, the model is compared to laboratory experiments in the context of heavy gas dispersal in quiescent air. It is shown that the model accurately reproduces experimental results thanks to an appropriate drag force correlation. This function expresses drag effects between the heavy and light gas layers. It is determined thanks to various experimental configurations of dam-break test problems. Full article

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16 pages, 352 KiB

Open AccessArticle

Stokes Equation in a Semi-Infinite Region: Generalization of the Lamb Solution and Applications to Marangoni Flows

by Goce Koleski and Thomas Bickel

Fluids 2020, 5(4), 249; https://doi.org/10.3390/fluids5040249 - 18 Dec 2020

Cited by 4 | Viewed by 2668

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

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17 pages, 4499 KiB

Open AccessArticle

Lightning Solvers for Potential Flows

by Peter J. Baddoo

Fluids 2020, 5(4), 227; https://doi.org/10.3390/fluids5040227 - 30 Nov 2020

Cited by 8 | Viewed by 4121

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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10 pages, 4352 KiB

Open AccessCommunication

Polymers and Plastrons in Parallel Yield Enhanced Turbulent Drag Reduction

by Anoop Rajappan and Gareth H. McKinley

Fluids 2020, 5(4), 197; https://doi.org/10.3390/fluids5040197 - 1 Nov 2020

Cited by 4 | Viewed by 2736

Abstract

Despite polymer additives and superhydrophobic walls being well known as stand-alone methods for frictional drag reduction in turbulent flows, the possibility of employing them simultaneously in an additive fashion has remained essentially unexplored. Through experimental friction measurements in turbulent Taylor–Couette flow, we show [...] Read more.

Despite polymer additives and superhydrophobic walls being well known as stand-alone methods for frictional drag reduction in turbulent flows, the possibility of employing them simultaneously in an additive fashion has remained essentially unexplored. Through experimental friction measurements in turbulent Taylor–Couette flow, we show that the two techniques may indeed be combined favorably to generate enhanced levels of frictional drag reduction in wall-bounded turbulence. We further propose an additive expression in Prandtl–von Kármán variables that enables us to quantitatively estimate the magnitude of this cooperative drag reduction effect for small concentrations of dissolved polymer. Full article

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

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15 pages, 257 KiB

Open AccessArticle

On the Foundations of Eddy Viscosity Models of Turbulence

by Nan Jiang, William Layton, Michael McLaughlin, Yao Rong and Haiyun Zhao

Fluids 2020, 5(4), 167; https://doi.org/10.3390/fluids5040167 - 29 Sep 2020

Cited by 12 | Viewed by 2654

Abstract

This report gives a summary of some recent developments in the mathematical foundations of eddy viscosity models of turbulence. Full article

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

20 pages, 7073 KiB

Open AccessArticle

Influence of Wind Buffers on the Aero-Thermal Performance of Skygardens

by Murtaza Mohammadi, Paige Wenbin Tien and John Kaiser Calautit

Fluids 2020, 5(3), 160; https://doi.org/10.3390/fluids5030160 - 18 Sep 2020

Cited by 11 | Viewed by 3089

Abstract

Many high-rise buildings have semi-enclosed landscaped spaces, which act as design elements to improve the social and environmental aspects of the building. Designs such as skygardens are open to outdoor airflow and allow occupants to observe the city skyline from a height. Due [...] Read more.

Many high-rise buildings have semi-enclosed landscaped spaces, which act as design elements to improve the social and environmental aspects of the building. Designs such as skygardens are open to outdoor airflow and allow occupants to observe the city skyline from a height. Due to their often high location, they are subjected to strong wind speeds and extreme environmental conditions. The current study investigates the effects of three common wind buffers (railing, hedges, and trees) located at a height of 92 m on the performance of a skygarden, in terms of occupants’ wind comfort. Computational fluid dynamics (CFD) simulations were carried out using the realisable k-epsilon method, where the vegetation was modelled as a porous zone with cooling capacity. The computational modelling of the high-rise building and vegetation were validated using previous works. The quality class (QC) of the Lawson comfort criteria was used for the evaluation of the wind comfort across the skygarden. The results indicate that, although the three wind buffers offer varying levels of wind reduction in the skygarden, the overall wind conditions generated are suitable for occupancy. Furthermore, vegetation is also able to offer slight temperature reductions in its wake. The right combination and dimension of these elements can greatly assist in generating aero-thermal comfort across skygardens. Full article

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

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29 pages, 499 KiB

Open AccessArticle

A Simple Construction of a Thermodynamically Consistent Mathematical Model for Non-Isothermal Flows of Dilute Compressible Polymeric Fluids

by Mark Dostalík, Josef Málek, Vít Průša and Endre Süli

Fluids 2020, 5(3), 133; https://doi.org/10.3390/fluids5030133 - 11 Aug 2020

Cited by 4 | Viewed by 2830

Abstract

We revisit some classical models for dilute polymeric fluids, and we show that thermodynamically consistent models for non-isothermal flows of these fluids can be derived in a very elementary manner. Our approach is based on the identification of energy storage mechanisms and entropy [...] Read more.

We revisit some classical models for dilute polymeric fluids, and we show that thermodynamically consistent models for non-isothermal flows of these fluids can be derived in a very elementary manner. Our approach is based on the identification of energy storage mechanisms and entropy production mechanisms in the fluid of interest, which, in turn, leads to explicit formulae for the Cauchy stress tensor and for all of the fluxes involved. Having identified these mechanisms and derived the governing equations, we document the potential use of the thermodynamic basis of the model in a rudimentary stability analysis. In particular, we focus on finite amplitude (nonlinear) stability of a stationary spatially homogeneous state in a thermodynamically isolated system. Full article

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

30 pages, 13398 KiB

Open AccessFeature PaperArticle

Numerical Simulation of Fountain Formation due to Normal and Inclined Twin-Jet Impingement on Ground

by Xiang Zhang and Ramesh K. Agarwal

Fluids 2020, 5(3), 132; https://doi.org/10.3390/fluids5030132 - 8 Aug 2020

Cited by 4 | Viewed by 3820

Abstract

The goal of this paper is to study numerically the flow physics of a fountain formed by twin-jet impingement on ground. The incompressible Reynolds-Averaged Navier-Stokes (RANS) equations with realizable k-ε and WA (Wray-Agarwal) turbulence model are employed in the numerical simulations with ANSYS [...] Read more.

The goal of this paper is to study numerically the flow physics of a fountain formed by twin-jet impingement on ground. The incompressible Reynolds-Averaged Navier-Stokes (RANS) equations with realizable k-ε and WA (Wray-Agarwal) turbulence model are employed in the numerical simulations with ANSYS Fluent. A series of numerical simulations for straight and inclined fountain formations are conducted by changing the geometric and flow parameters of twin jets and distance between them. The changes in parameters include variations in the jet Reynolds number from 2 × 10⁴ to 8 × 10⁴, impingement height, distance between the centerlines of the two jets from 1.4D to 16D where D is the jet diameter, and ratio of the Reynolds number of the two jets from 1 to 4. It is shown that different Reynolds numbers of the two jets can result in a fountain that inclines towards the jet with smaller Reynolds number. Detailed flow field simulations for a large number of cases are presented, and the flow physics of fountain formation is analyzed for the first time in the literature. Full article

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

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16 pages, 29959 KiB

Open AccessArticle

Controlled Synthetic Freestream Turbulence Intensity Introduced by a Local Volume Force

by Eike Tangermann and Markus Klein

Fluids 2020, 5(3), 130; https://doi.org/10.3390/fluids5030130 - 7 Aug 2020

Cited by 7 | Viewed by 2155

Abstract

Generating freestream turbulence within the computational domain instead of applying it as a boundary condition requires a method to introduce the turbulent fluctuations at a specific location. A method based on applying local volume forces has been adapted and supplemented with a control [...] Read more.

Generating freestream turbulence within the computational domain instead of applying it as a boundary condition requires a method to introduce the turbulent fluctuations at a specific location. A method based on applying local volume forces has been adapted and supplemented with a control loop in order to compensate for alterations of the turbulence structure resulting from the numerical treatment and physical reasons. The criteria for the tuning of the controller have been developed and the performance of the approach has been assessed. The capabilities of the method are demonstrated for the flow around an airfoil at high angle of attack and with massive flow separation. Full article

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

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56 pages, 10574 KiB

Open AccessArticle

NARMAX Identification Based Closed-Loop Control of Flow Separation over NACA 0015 Airfoil

by Sohaib Obeid, Goodarz Ahmadi and Ratneshwar Jha

Fluids 2020, 5(3), 100; https://doi.org/10.3390/fluids5030100 - 29 Jun 2020

Cited by 11 | Viewed by 3865

Abstract

A closed-loop control algorithm for the reduction of turbulent flow separation over NACA 0015 airfoil equipped with leading-edge synthetic jet actuators (SJAs) is presented. A system identification approach based on Nonlinear Auto-Regressive Moving Average with eXogenous inputs (NARMAX) technique [...] Read more.

A closed-loop control algorithm for the reduction of turbulent flow separation over NACA 0015 airfoil equipped with leading-edge synthetic jet actuators (SJAs) is presented. A system identification approach based on Nonlinear Auto-Regressive Moving Average with eXogenous inputs (NARMAX) technique was used to predict nonlinear dynamics of the fluid flow and for the design of the controller system. Numerical simulations based on URANS equations are performed at Reynolds number of 10⁶ for various airfoil incidences with and without closed-loop control. The NARMAX model for flow over an airfoil is based on the static pressure data, and the synthetic jet actuator is developed using an incompressible flow model. The corresponding NARMAX identification model developed for the pressure data is nonlinear; therefore, the describing function technique is used to linearize the system within its frequency range. Low-pass filtering is used to obtain quasi-linear state values, which assist in the application of linear control techniques. The reference signal signifies the condition of a fully re-attached flow, and it is determined based on the linearization of the original signal during open-loop control. The controller design follows the standard proportional-integral (PI) technique for the single-input single-output system. The resulting closed-loop response tracks the reference value and leads to significant improvements in the transient response over the open-loop system. The NARMAX controller enhances the lift coefficient from 0.787 for the uncontrolled case to 1.315 for the controlled case with an increase of 67.1%. Full article

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

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27 pages, 40295 KiB

Open AccessArticle

Experiments on Flexible Filaments in Air Flow for Aeroelasticity and Fluid-Structure Interaction Models Validation

by Jorge Silva-Leon and Andrea Cioncolini

Fluids 2020, 5(2), 90; https://doi.org/10.3390/fluids5020090 - 5 Jun 2020

Cited by 6 | Viewed by 3075

Abstract

Several problems in science and engineering are characterized by the interaction between fluid flows and deformable structures. Due to their complex and multidisciplinary nature, these problems cannot normally be solved analytically and experiments are frequently of limited scope, so that numerical simulations represent [...] Read more.

Several problems in science and engineering are characterized by the interaction between fluid flows and deformable structures. Due to their complex and multidisciplinary nature, these problems cannot normally be solved analytically and experiments are frequently of limited scope, so that numerical simulations represent the main analysis tool. Key to the advancement of numerical methods is the availability of experimental test cases for validation. This paper presents results of an experiment specifically designed for the validation of numerical methods for aeroelasticity and fluid-structure interaction problems. Flexible filaments of rectangular cross-section and various lengths were exposed to air flow of moderate Reynolds number, corresponding to laminar and mildly turbulent flow conditions. Experiments were conducted in a wind tunnel, and the flexible filaments dynamics was recorded via fast video imaging. The structural response of the filaments included static reconfiguration, small-amplitude vibration, large-amplitude limit-cycle periodic oscillation, and large-amplitude non-periodic motion. The present experimental setup was designed to incorporate a rich fluid-structure interaction physics within a relatively simple configuration without mimicking any specific structure, so that the results presented herein can be valuable for models validation in aeroelasticity and also fluid-structure interaction applications. Full article

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

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12 pages, 2302 KiB

Open AccessFeature PaperArticle

Scaling of Second-Order Structure Functions in Turbulent Premixed Flames in the Flamelet Combustion Regime

by Peter Brearley, Umair Ahmed, Nilanjan Chakraborty and Markus Klein

Fluids 2020, 5(2), 89; https://doi.org/10.3390/fluids5020089 - 2 Jun 2020

Cited by 6 | Viewed by 2925

Abstract

The second-order velocity structure function statistics have been analysed using a DNS database of statistically planar turbulent premixed flames subjected to unburned gas forcing. The flames considered here represent combustion for moderate values of Karlovitz number from the wrinkled flamelets to the thin [...] Read more.

The second-order velocity structure function statistics have been analysed using a DNS database of statistically planar turbulent premixed flames subjected to unburned gas forcing. The flames considered here represent combustion for moderate values of Karlovitz number from the wrinkled flamelets to the thin reaction zones regimes of turbulent premixed combustion. It has been found that the second-order structure functions exhibit the theoretical asymptotic scalings in the dissipative and (relatively short) inertial ranges. However, the constant of proportionality for the theoretical asymptotic variation for the inertial range changes from one case to another, and this value also changes with structure function orientation. The variation of the structure functions for small length scale separation remains proportional to the square of the separation distance. However, the constant of proportionality for the limiting behaviour according to the separation distance square remains significantly different from the theoretical value obtained in isotropic turbulence. The disagreement increases with increasing turbulence intensity. It has been found that turbulent velocity fluctuations within the flame brush remain anisotropic for all cases considered here and this tendency strengthens towards the trailing edge of the flame brush. It indicates that the turbulence models derived based on the assumptions of homogeneous isotropic turbulence may not be fully valid for turbulent premixed flames. Full article

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

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17 pages, 3360 KiB

Open AccessArticle

Numerical Study of the Magnetic Damping Effect on the Sloshing of Liquid Oxygen in a Propellant Tank

by Yutaro Furuichi and Toshio Tagawa

Fluids 2020, 5(2), 88; https://doi.org/10.3390/fluids5020088 - 1 Jun 2020

Cited by 9 | Viewed by 3694

Abstract

Nowadays, the use of baffle plates is anticipated to be one of potential devices used to dampen the sloshing of propellant in rocket tanks. However, some of previous studies reported that the use of a baffle plate may cause larger pressure fluctuations in [...] Read more.

Nowadays, the use of baffle plates is anticipated to be one of potential devices used to dampen the sloshing of propellant in rocket tanks. However, some of previous studies reported that the use of a baffle plate may cause larger pressure fluctuations in the tank. In this study, aiming at damping the sloshing without a baffle plate, we paid attention to the characteristic that liquid oxygen is paramagnetic and numerically investigated damping effect of a magnetic field when liquid oxygen sloshing occurs. An incompressible gas–liquid two-phase flow of gaseous oxygen and liquid oxygen was assumed in a spherical spacecraft tank with a diameter of 1 m in a non-gravitational field, and a triangular impact force was assumed to be imposed as the excitation force. In addition, an electric circular coil was placed outside the spherical tank to generate a static magnetic field. For the sake of simplicity, the effect of heat was not taken into consideration. As a result of computation, the sloshing was damped to a certain extent when the magnetic flux density at the coil center was 1.0 T, and a sufficient damping effect was obtained by setting it to 3.0 T. In fact, it is anticipated that less than 3.0 T is sufficient if the coil is placed on the tank surface. This may contribute to damping of the movement of the center of gravity of a spacecraft and prevention of mixing of ullage gas into the piping. Full article

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

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21 pages, 4624 KiB

Open AccessFeature PaperArticle

Hydrodynamic Dispersion in Porous Media and the Significance of Lagrangian Time and Space Scales

by Vi Nguyen and Dimitrios V. Papavassiliou

Fluids 2020, 5(2), 79; https://doi.org/10.3390/fluids5020079 - 21 May 2020

Cited by 22 | Viewed by 7178

Abstract

Transport in porous media is critical for many applications in the environment and in the chemical process industry. A key parameter for modeling this transport is the hydrodynamic dispersion coefficient for particles and scalars in a porous medium, which has been found to [...] Read more.

Transport in porous media is critical for many applications in the environment and in the chemical process industry. A key parameter for modeling this transport is the hydrodynamic dispersion coefficient for particles and scalars in a porous medium, which has been found to depend on properties of the medium structure, on the dispersing compound, and on the flow field characteristics. Previous studies have resulted in suggestions of different equation forms, showing the relationship between the hydrodynamic dispersion coefficient for various types of porous media in various flow regimes and the Peclet number. The Peclet number is calculated based on a Eulerian length scale, such as the diameter of the spheres in packed beds, or the pore diameter. However, the nature of hydrodynamic dispersion is Lagrangian, and it should take the molecular diffusion effects, as well as the convection effects, into account. This work shifts attention to the Lagrangian time and length scales for the definition of the Peclet number. It is focused on the dependence of the longitudinal hydrodynamic dispersion coefficient on the effective Lagrangian Peclet number by using a Lagrangian length scale and the effective molecular diffusivity. The lattice Boltzmann method (LBM) was employed to simulate flow in porous media that were constituted by packed spheres, and Lagrangian particle tracking (LPT) was used to track the movement of individual dispersing particles. It was found that the hydrodynamic dispersion coefficient linearly depends on the effective Lagrangian Peclet number for packed beds with different types of packing. This linear equation describing the dependence of the dispersion coefficient on the effective Lagrangian Peclet number is both simpler and more accurate than the one formed using the effective Eulerian Peclet number. In addition, the slope of the line is a characteristic coefficient for a given medium. Full article

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

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15 pages, 337 KiB

Open AccessArticle

The Heat Flux Vector(s) in a Two Component Fluid Mixture

by A. D. Kirwan, Jr. and Mehrdad Massoudi

Fluids 2020, 5(2), 77; https://doi.org/10.3390/fluids5020077 - 20 May 2020

Cited by 5 | Viewed by 2457

Abstract

Bulk kinematic properties of mixtures such as velocity are known to be the density weighed averages of the constituent velocities. No such paradigm exists for the heat flux of mixtures when the constituents have different temperatures. Using standard principles such as frame indifference, [...] Read more.

Bulk kinematic properties of mixtures such as velocity are known to be the density weighed averages of the constituent velocities. No such paradigm exists for the heat flux of mixtures when the constituents have different temperatures. Using standard principles such as frame indifference, we address this topic by developing linear constitutive equations for the constituent heat fluxes, the interaction force between constituents, and the stresses for a mixture of two fluids. Although these equations contain 18 phenomenological coefficients, we are able to use the Clausius-Duhem inequality to obtain inequalities involving the principal and cross flux coefficients. The theory is applied to some special cases and shown to reduce to standard results when the constituents have the same temperature. Full article

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

16 pages, 21654 KiB

Open AccessFeature PaperArticle

Investigation of High Lift Force Generation of Dragonfly Wing by a Novel Advanced Mode in Hover

by Xiaohui Su, Kaixuan Zhang, Juan Zheng, Yong Zhao, Ruiqi Han and Jiantao Zhang

Fluids 2020, 5(2), 59; https://doi.org/10.3390/fluids5020059 - 24 Apr 2020

Cited by 3 | Viewed by 2593

Abstract

In the paper, a novel flapping mode is presented that can generate high lift force by a dragonfly wing in hover. The new mode, named partial advanced mode (PAM), starts pitching earlier than symmetric rotation during the downstroke cycle of the hovering motion. [...] Read more.

In the paper, a novel flapping mode is presented that can generate high lift force by a dragonfly wing in hover. The new mode, named partial advanced mode (PAM), starts pitching earlier than symmetric rotation during the downstroke cycle of the hovering motion. As a result, high lift force can be generated due to rapid pitching coupling with high flapping velocity in the stroke plane. Aerodynamic performance of the new mode is investigated thoroughly using numerical simulation. The results obtained show that the period-averaged lift coefficient, C_L, increases up to 16% compared with that of the traditional symmetrical mode when an earlier pitching time is set to 8% of the flapping period. The reason for the high lift force generation mechanism is explained in detail using not only force investigation, but also by analyzing vortices produced around the wing. The proposed PAM is believed to lengthen the dynamic stall mechanism and enhance the LEV generated during the downstroke. The improvement of lift force could be considered as a result of a combination of the dynamic stall mechanism and rapid pitch mechanism. Finally, the energy expenditure of the new mode is also analyzed. Full article

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

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24 pages, 16609 KiB

Open AccessFeature PaperArticle

Flow in Fractured Porous Media Modeled in Closed-Form: Augmentation of Prior Solution and Side-Stepping Inconvenient Branch Cut Locations

by Ruud Weijermars and Aadi Khanal

Fluids 2020, 5(2), 51; https://doi.org/10.3390/fluids5020051 - 16 Apr 2020

Cited by 4 | Viewed by 2385

Abstract

Carefully chosen complex variable formulations can solve flow in fractured porous media. Such a calculus approach is attractive, because the gridless method allows for fast, high-resolution model results. Previously developed complex potentials to describe flow in porous media with discrete heterogeneities such as [...] Read more.

Carefully chosen complex variable formulations can solve flow in fractured porous media. Such a calculus approach is attractive, because the gridless method allows for fast, high-resolution model results. Previously developed complex potentials to describe flow in porous media with discrete heterogeneities such as natural fractures can be modified to expand the accuracy of the solution range. The prior solution became increasingly inaccurate for flows with fractures oriented at larger angles with respect to the far-field flow. The modified solution, presented here, based on complex analysis methods (CAM), removes the limitation of the earlier solution. Benefits of the CAM model are (1) infinite resolution, and (2) speed of use, as no gridding is required. Being gridless and meshless, the CAM model is computationally faster than integration methods based on solutions across discrete volumes. However, branch cut effects may occur in impractical locations due to mathematical singularities. This paper demonstrates how the augmented formulation corrects physically unfeasible refraction of streamlines across high-permeability bands (natural fractures) oriented at high angles with respect to a far-field flow. The current solution is an important repair. An application shows how a drained rock volume in hydraulically fractured hydrocarbon wells will be affected by the presence of natural fractures. Full article

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

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22 pages, 8135 KiB

Open AccessFeature PaperArticle

Modeling of Sedimentation and Creaming in Suspensions and Pickering Emulsions

by Rajinder Pal

Fluids 2019, 4(4), 186; https://doi.org/10.3390/fluids4040186 - 22 Oct 2019

Cited by 19 | Viewed by 10684

Abstract

Suspensions and emulsions are prone to kinetic instabilities of sedimentation and creaming, wherein the suspended particles and droplets fall or rise through a matrix fluid. It is important to understand and quantify sedimentation and creaming in such dispersed systems as they affect the [...] Read more.

Suspensions and emulsions are prone to kinetic instabilities of sedimentation and creaming, wherein the suspended particles and droplets fall or rise through a matrix fluid. It is important to understand and quantify sedimentation and creaming in such dispersed systems as they affect the shelf-life of products manufactured in the form of suspensions and emulsions. In this article, the unhindered and hindered settling/creaming behaviors of conventional emulsions and suspensions are first reviewed briefly. The available experimental data on settling/creaming of concentrated emulsions and suspensions are interpreted in terms of the drift flux theory. Modeling and simulation of nanoparticle-stabilized Pickering emulsions are carried out next. The presence of nanoparticles at the oil/water interface has a strong influence on the creaming/sedimentation behaviors of single droplets and swarm of droplets. Simulation results clearly demonstrate the strong influence of three-phase contact angle of nanoparticles present at the oil/water interface. This is the first definitive study dealing with modeling and simulation of unhindered and hindered creaming and sedimentation behaviors of nanoparticle-stabilized Pickering emulsions. Full article

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

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13 pages, 1149 KiB

Open AccessArticle

Turbulence Intensity Scaling: A Fugue

by Nils T. Basse

Fluids 2019, 4(4), 180; https://doi.org/10.3390/fluids4040180 - 9 Oct 2019

Cited by 17 | Viewed by 6130

Abstract

We study streamwise turbulence intensity definitions using smooth- and rough-wall pipe flow measurements made in the Princeton Superpipe. Scaling of turbulence intensity with the bulk (and friction) Reynolds number is provided for the definitions. The turbulence intensity scales with the friction factor for [...] Read more.

We study streamwise turbulence intensity definitions using smooth- and rough-wall pipe flow measurements made in the Princeton Superpipe. Scaling of turbulence intensity with the bulk (and friction) Reynolds number is provided for the definitions. The turbulence intensity scales with the friction factor for both smooth- and rough-wall pipe flow. Turbulence intensity definitions providing the best description of the measurements are identified. A procedure to calculate the turbulence intensity based on the bulk Reynolds number (and the sand-grain roughness for rough-wall pipe flow) is outlined. Full article

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

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Review

Jump to: Editorial, Research

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 - 4 Feb 2021

Cited by 58 | Viewed by 11794

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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25 pages, 4287 KiB

Open AccessFeature PaperReview

Theoretical Foundation of Rapid Distortion Theory on Transversely Sheared Mean Flows

by Marvin E. Goldstein

Fluids 2020, 5(2), 62; https://doi.org/10.3390/fluids5020062 - 27 Apr 2020

Cited by 2 | Viewed by 2517

Abstract

The focus of this paper is on Rapid Distortion Theory on transversely sheared mean flows, which is often used to investigate turbulence-solid surface interactions. The main purpose of the paper is to bring together and present in a consistent fashion a general theory [...] Read more.

The focus of this paper is on Rapid Distortion Theory on transversely sheared mean flows, which is often used to investigate turbulence-solid surface interactions. The main purpose of the paper is to bring together and present in a consistent fashion a general theory that has been developed in several different papers that have been published in the Journal of Fluid Mechanics. The equations for the unsteady pressure and velocity flections (which decouple from the entropy fluctuations) are rewritten in terms of a gauge function in order to obtain expressions that involve two arbitrarily convected quantities. A pair of very general conservation laws are used to derive upstream boundary conditions that relate these quantities to the actual physical variables. The entropy fluctuations can be determined after the fact once the solutions for the pressure and velocity fluctuations are known. The result involves a third arbitrary convected quantity that is equal to the entropy fluctuations at upstream infinity and can, therefore, be specified as an additional upstream boundary condition. A secondary purpose of the paper is to summarize a number of applications of the theory that have also appeared in the literature and show how they compare with an experiment. Full article

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

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19 pages, 9501 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Fast Models of Hydrocarbon Migration Paths and Pressure Depletion Based on Complex Analysis Methods (CAM): Mini-Review and Verification

by Ruud Weijermars, Aadi Khanal and Lihua Zuo

Fluids 2020, 5(1), 7; https://doi.org/10.3390/fluids5010007 - 5 Jan 2020

Cited by 13 | Viewed by 3580

Abstract

A recently developed code to model hydrocarbon migration and convective time of flight makes use of complex analysis methods (CAM) paired with Eulerian particle tracking. Because the method uses new algorithms that are uniquely developed by our research group, validation of the fast [...] Read more.

A recently developed code to model hydrocarbon migration and convective time of flight makes use of complex analysis methods (CAM) paired with Eulerian particle tracking. Because the method uses new algorithms that are uniquely developed by our research group, validation of the fast CAM solutions with independent methods is merited. Particle path solutions were compared with independent solutions methods (Eclipse). These prior and new benchmarks are briefly summarized here to further verify the results obtained with CAM codes. Pressure field solutions based on CAM are compared with independent embedded discrete fracture method (EDFM) solutions. The CAM method is particularly attractive because its grid-less nature offers fast computation times and unlimited resolution. The method is particularly well suited for solving a variety of practical field development problems. Examples are given for fast optimization of waterflood patterns. Another successful application area is the modeling of fluid withdrawal patterns in hydraulically fractured wells. Because no gridding is required, the CAM model can compute the evolution of the drained rock volume (DRV) for an unlimited (but finite) number of both hydraulic and natural fractures. Such computations of the DRV are based on the convective time of flight and show the fluid withdrawal zone in the reservoir. In contrast, pressure depletion models are based on the diffusive time of flight. In ultra-low permeability reservoirs, the pressure depletion zones do not correspond to the DRV, because the convective and diffusive displacement rates differ over an order of magnitude (diffusive time of flight being the fastest). Therefore, pressure depletion models vastly overestimate the drained volume in shale reservoirs, which is why fracture and well spacing decisions should be based on both pressure depletion and DRV models, not pressure only. Full article

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

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