Fluids

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15 pages, 313 KiB

Open AccessArticle

Oscillatory Bifurcations in Porous Layers with Stratified Porosity, Driven by Each Coefficient of the Spectrum Equation

by Salvatore Rionero

Fluids 2021, 6(2), 57; https://doi.org/10.3390/fluids6020057 - 1 Feb 2021

Cited by 3 | Viewed by 1365

The onset of oscillatory bifurcations in a porous horizontal layer L, uniformly rotating about a vertical axis, with vertically stratified porosity, heated from below and salted from above and below, is investigated. Denoting by [...] Read more.

The onset of oscillatory bifurcations in a porous horizontal layer L, uniformly rotating about a vertical axis, with vertically stratified porosity, heated from below and salted from above and below, is investigated. Denoting by

P_{i}, (i = 1, 2)

, the Prandtl numbers of the salt

S_{i}

salting L from below

(i = 1)

and above

(i = 2)

respectively, it is shown that: (i) in L the oscillatory bifurcations can occur only if one of the structural conditions

P_{1} > 1, P_{2} < 1

or

P_{1} = 1, P_{2} < 1

or

P_{1} > 1, P_{2} = 1

is verified; (ii) exists a bound

{\bar{R}}_{2}

for the Rayleigh number

R_{2}

of

S_{2}

such that

R_{2} < {\bar{R}}_{2}

guarantees the absence of cold convection; (iii) via a new approach based on the instability power of each coefficient of the spectrum equation, criteria of existence, location and frequency of oscillatory (Hopf) bifurcations are furnished for any porosity stratification law. These criteria, as far as we know are, for the case at stake, the first criteria of Hopf bifurcations appearing in literature. We are confident that, via experimental results, will be validated. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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14 pages, 526 KiB

Open AccessEditor’s ChoiceArticle

The Effect of the Vadasz Number on the Onset of Thermal Convection in Rotating Bidispersive Porous Media

by Florinda Capone and Roberta De Luca

Fluids 2020, 5(4), 173; https://doi.org/10.3390/fluids5040173 - 6 Oct 2020

Cited by 16 | Viewed by 2252

Abstract

The onset of thermal convection in uniformly rotating bidispersive horizontal porous layer, uniformly heated from below, is analyzed. A generalized Darcy equation for the macro-phase is considered to take the Vadasz number into account. It is proved that the presence of the Vadasz [...] Read more.

The onset of thermal convection in uniformly rotating bidispersive horizontal porous layer, uniformly heated from below, is analyzed. A generalized Darcy equation for the macro-phase is considered to take the Vadasz number into account. It is proved that the presence of the Vadasz number can give rise to oscillatory motion at the loss of stability of thermal conduction solution. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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18 pages, 4306 KiB

Open AccessFeature PaperArticle

Flow and Convection in Metal Foams: A Survey and New CFD Results

by Beatrice Pulvirenti, Michele Celli and Antonio Barletta

Fluids 2020, 5(3), 155; https://doi.org/10.3390/fluids5030155 - 6 Sep 2020

Cited by 13 | Viewed by 4381

Abstract

Metal foams are widely studied as possible tools for the enhancement of heat transfer from hot bodies. The basic idea is that a metal foam tends to significantly increase the heat exchange area between the hot solid body and the external cooling fluid. [...] Read more.

Metal foams are widely studied as possible tools for the enhancement of heat transfer from hot bodies. The basic idea is that a metal foam tends to significantly increase the heat exchange area between the hot solid body and the external cooling fluid. For this reason, this class of porous materials is considered as a good candidate for an alternative to finned surfaces, with different pros and cons. Among the pros, we mention the generally wider area of contact per unit volume between solid and fluid. Among the cons is the difficulty to produce different specimens with the same inner structure, with the consequence that their performance may be significantly variable. This paper will offer a survey of the literature with a focus on the main heat transfer characteristics of the metal foams and the energy balance model based on Local Thermal Non-Equilibrium (LTNE). Then, a numerical simulation of the heat transfer at the pore-scale level for an artificial foam with a spatially periodic structure will be discussed. Finally, these numerical results will be employed to assess the macroscopic modeling of the flow and heat transfer in a metal foam. More precisely, the Darcy–Forchheimer model and the LTNE model adopted to describe the momentum and energy transfer in metal foams have been validated for metallic periodic structures. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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20 pages, 30615 KiB

Open AccessArticle

Mass Transport and Turbulent Statistics within Two Branching Coral Colonies

by Md Monir Hossain and Anne E. Staples

Fluids 2020, 5(3), 153; https://doi.org/10.3390/fluids5030153 - 4 Sep 2020

Cited by 7 | Viewed by 3151

Abstract

Large eddy simulations were performed to characterize the flow and mass transport mechanisms in the interior of two Pocillopora coral colonies with different geometries, one with a relatively loosely branched morphology (P. eydouxi), and the other with a relatively densely branched [...] Read more.

Large eddy simulations were performed to characterize the flow and mass transport mechanisms in the interior of two Pocillopora coral colonies with different geometries, one with a relatively loosely branched morphology (P. eydouxi), and the other with a relatively densely branched structure (P. meandrina). Detailed velocity vector and streamline fields were obtained inside both corals for the same unidirectional oncoming flow, and significant differences were found between their flow profiles and mass transport mechanisms. For the densely branched P. meandrina colony, a significant number of vortices were shed from individual branches, which passively stirred the water column and enhanced the mass transport rate inside the colony. In contrast, vortices were mostly absent within the more loosely branched P. eydouxi colony. To further understand the impact of the branch density on internal mass transport processes, the non-dimensional Stanton number for mass transfer, St, was calculated based on the local flow time scale and compared between the colonies. The results showed up to a

219 %

increase in St when the mean vortex diameter was used to calculate St, compared to calculations based on the mean branch diameter. Turbulent flow statistics, including the fluctuating velocity components, the mean Reynolds stress, and the variance of the velocity components were calculated and compared along the height of the flow domain. The comparison of turbulent flow statistics showed similar Reynolds stress profiles for both corals, but higher velocity variations, in the interior of the densely branched coral, P. meandrina. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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19 pages, 4415 KiB

Open AccessEditor’s ChoiceArticle

On the Optimal Control of Stationary Fluid–Structure Interaction Systems

by Leonardo Chirco and Sandro Manservisi

Fluids 2020, 5(3), 144; https://doi.org/10.3390/fluids5030144 - 28 Aug 2020

Cited by 11 | Viewed by 2710

Abstract

Fluid–structure interaction (FSI) systems consist of a fluid which flows and deforms one or more solid surrounding structures. In this paper, we study inverse FSI problems, where the goal is to find the optimal value of some control parameters, such that the FSI [...] Read more.

Fluid–structure interaction (FSI) systems consist of a fluid which flows and deforms one or more solid surrounding structures. In this paper, we study inverse FSI problems, where the goal is to find the optimal value of some control parameters, such that the FSI solution is close to a desired one. Optimal control problems are formulated with Lagrange multipliers and adjoint variables formalism. In order to recover the symmetry of the stationary state-adjoint system an auxiliary displacement field is introduced and used to extend the velocity field from the fluid into the structure domain. As a consequence, the adjoint interface forces are balanced automatically. We present three different FSI optimal controls: inverse parameter estimation, boundary control and distributed control. The optimality system is derived from the first order necessary condition by taking the Fréchet derivatives of the augmented Lagrangian with respect to all the variables involved. The optimal solution is obtained through a gradient-based algorithm applied to the optimality system. In order to support the proposed approach and compare these three optimal control approaches numerical tests are performed. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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14 pages, 354 KiB

Open AccessArticle

Stability of the Plane Bingham–Poiseuille Flow in an Inclined Channel

by Paolo Falsaperla, Andrea Giacobbe and Giuseppe Mulone

Fluids 2020, 5(3), 141; https://doi.org/10.3390/fluids5030141 - 27 Aug 2020

Cited by 7 | Viewed by 2689

Abstract

We study the stability of laminar Bingham–Poiseuille flows in a sheet of fluid (open channel) down an incline with constant slope angle

β \in (0, π / 2)

. This problem has geophysical applications to the evolution of landslides. In [...] Read more.

We study the stability of laminar Bingham–Poiseuille flows in a sheet of fluid (open channel) down an incline with constant slope angle

β \in (0, π / 2)

. This problem has geophysical applications to the evolution of landslides. In this article, we apply to this problem recent results of Falsaperla et al. for laminar Couette and Poiseuille flows of Newtonian fluids in inclined channels. The stability of the basic motion of the generalised Navier–Stokes system for a Bingham fluid in a horizontal channel against linear perturbations has been studied. In this article, we study the flows of a Bingham fluid when the channel is oblique and we prove a stabilizing effect of the Bingham parameter B. We also study the stability of the linear system with an energy method (Lyapunov functions) and prove that the streamwise perturbations are always stable, while the spanwise perturbations are energy-stable if the Reynolds number

Re

is less than the critical Reynolds number

R_{c}

obtained solving a generalised Orr equation of a maximum variational problem. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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22 pages, 314 KiB

Open AccessArticle

Acceleration Waves in Rational Extended Thermodynamics of Rarefied Monatomic Gases

by Francesca Brini and Leonardo Seccia

Fluids 2020, 5(3), 139; https://doi.org/10.3390/fluids5030139 - 25 Aug 2020

Cited by 4 | Viewed by 3099

Abstract

Rational Extended Thermodynamics theories with different number of moments are usually introduced to study non-equilibrium phenomena in rarefied gases. Here, we use them to describe one-dimensional acceleration waves in a rarefied monatomic gas. In particular, we focus on the degeneracy of the acceleration [...] Read more.

Rational Extended Thermodynamics theories with different number of moments are usually introduced to study non-equilibrium phenomena in rarefied gases. Here, we use them to describe one-dimensional acceleration waves in a rarefied monatomic gas. In particular, we focus on the degeneracy of the acceleration wave to a shock wave, in order to test the validity of the models and the role played by an increasing number of moments. As a byproduct, some peculiarities of the characteristic velocities at equilibrium are analyzed as well. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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16 pages, 2737 KiB

Open AccessArticle

Effect of Micropolar Fluid Properties on the Blood Flow in a Human Carotid Model

by Evangelos Karvelas, Giorgos Sofiadis, Thanasis Papathanasiou and Ioannis Sarris

Fluids 2020, 5(3), 125; https://doi.org/10.3390/fluids5030125 - 29 Jul 2020

Cited by 27 | Viewed by 3782

Abstract

Blood is a non-homogeneous fluid that flows inside the human artery system and provides the cells with nutrients. In this study the auto rotation effect of blood’s microstructure on its flow inside a human carotid model is studied by using a micropolar fluid [...] Read more.

Blood is a non-homogeneous fluid that flows inside the human artery system and provides the cells with nutrients. In this study the auto rotation effect of blood’s microstructure on its flow inside a human carotid model is studied by using a micropolar fluid model. The study aims to investigate the flow differences that occur due to its microstructure as compared to a Newtonian fluid. We focus on the vortex viscosity effect, i.e., the ratio of microrotation viscosity to the total one, because this is the only parameter that affects directly the fluid flow. Simulations in a range of vortex viscosities, are carried out in a

3 D

human carotid model that is computationally reconstructed. All of the simulations are conducted at the diastolic Reynolds number that occurs in the human carotid. Results indicate that micropolarity affects blood velocity in the range of parameters studied by

4 %

. As micropolarity is increased, higher velocities in the center of vessels and lower near the boundaries are found as compared to a Newtonian fluid consideration. This is an indication that the increase of the fluid’s micropolarity leads to an increase of the boundary layer thickness. More importantly, an increase in vortex viscosity and the resulting increase in microrotation result in decreased shear stress in the carotid’s walls; this finding can be significant in regards to the onset and the development of atherosclerosis. Finally, the flow distribution at the carotid seems to heavily be affected by the geometry and the micropolarity of the fluid. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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12 pages, 1306 KiB

Open AccessArticle

Vadasz Number Effects on Convection in a Horizontal Porous Layer Subjected to Internal Heat Generation and G-Jitter

by Saneshan Govender

Fluids 2020, 5(3), 124; https://doi.org/10.3390/fluids5030124 - 27 Jul 2020

Cited by 1 | Viewed by 2086

Abstract

Flow and heat transfer in a horizontal porous layer subjected to internal heat generation and g-jitter is considered for the Dirichlet thermal boundary condition. A linear stability analysis is used to determine the convection threshold in terms of the critical Rayleigh number. For [...] Read more.

Flow and heat transfer in a horizontal porous layer subjected to internal heat generation and g-jitter is considered for the Dirichlet thermal boundary condition. A linear stability analysis is used to determine the convection threshold in terms of the critical Rayleigh number. For the low amplitude, high frequency approximation, the results show that vibration has a stabilizing effect on the onset of convection when the porous layer is heated from below. When the porous layer is cooled from below and heated from above, the vibration has a destabilizing effect in the presence of internal heat generation. It is also demonstrated that when the top and bottoms walls are cooled and rigid/impermeable, the critical Rayleigh number is infinitely large and conduction is the only possible mode of heat transfer. The impact of increasing the Vadasz number is to stabilize the convection, in addition to reducing the transition point from synchronous to subharmonic solutions. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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18 pages, 1517 KiB

Open AccessArticle

Effects of Particles Diffusion on Membrane Filters Performance

by Shi Yue Liu, Zhengyi Chen and Pejman Sanaei

Fluids 2020, 5(3), 121; https://doi.org/10.3390/fluids5030121 - 24 Jul 2020

Cited by 11 | Viewed by 3248

Abstract

Membrane filtration fouling is a very complex process and is determined by many properties such as the membrane internal morphology, membrane pore structure, flow rate and contaminant properties. In a very slow filtration process or during the late stage of filtration, when the [...] Read more.

Membrane filtration fouling is a very complex process and is determined by many properties such as the membrane internal morphology, membrane pore structure, flow rate and contaminant properties. In a very slow filtration process or during the late stage of filtration, when the flow rate is naturally low and Péclet number is small, particle diffusion is essential and cannot be neglected, while in typical filtration models, especially in moderate and fast filtration process, the main contribution stems from the particle advection. The objectives of this study is to formulate mathematical models that can (i) investigate how filtration process varies under possible effects of particles diffusion; and (ii) describe how membrane morphology evolves and investigate the filtration performance during the filtration process. We also compare the results with the case that diffusion is less important and make a prediction about what kind of membrane filter pore structure should be employed to achieve a particular optimum filtration performance. According to our results, the filtrate and efficiency of particle separation are found to be under the trade-off relationship, and the selection of the membrane properties depends on the requirement of the filtration. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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19 pages, 14668 KiB

Open AccessEditor’s ChoiceArticle

Data-Driven Pulsatile Blood Flow Physics with Dynamic Mode Decomposition

by Milad Habibi, Scott T. M. Dawson and Amirhossein Arzani

Fluids 2020, 5(3), 111; https://doi.org/10.3390/fluids5030111 - 14 Jul 2020

Cited by 32 | Viewed by 6417

Abstract

Dynamic mode decomposition (DMD) is a purely data-driven and equation-free technique for reduced-order modeling of dynamical systems and fluid flow. DMD finds a best fit linear reduced-order model that represents any given spatiotemporal data. In DMD, each mode evolves with a fixed frequency [...] Read more.

Dynamic mode decomposition (DMD) is a purely data-driven and equation-free technique for reduced-order modeling of dynamical systems and fluid flow. DMD finds a best fit linear reduced-order model that represents any given spatiotemporal data. In DMD, each mode evolves with a fixed frequency and therefore DMD modes represent physically meaningful structures that are ranked based on their dynamics. The application of DMD to patient-specific cardiovascular flow data is challenging. First, the input flow rate is unsteady and pulsatile. Second, the flow topology can change significantly in different phases of the cardiac cycle. Finally, blood flow in patient-specific diseased arteries is complex and often chaotic. The objective of this study was to overcome these challenges using our proposed multistage dynamic mode decomposition with control (mDMDc) method and use this technique to study patient-specific blood flow physics. The inlet flow rate was considered as the controller input to the systems. Blood flow data were divided into different stages based on the inlet flow waveform and DMD with control was applied to each stage. The system was augmented to consider both velocity and wall shear stress (WSS) vector data, and therefore study the interaction between the coherent structures in velocity and near-wall coherent structures in WSS. First, it was shown that DMD modes can exactly represent the analytical Womersley solution for incompressible pulsatile flow in tubes. Next, our method was applied to image-based coronary artery stenosis and cerebral aneurysm models where complex blood flow patterns are anticipated. The flow patterns were studied using the mDMDc modes and the reconstruction errors were reported. Our augmented mDMDc framework could capture coherent structures in velocity and WSS with a fewer number of modes compared to the traditional DMD approach and demonstrated a close connection between the velocity and WSS modes. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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18 pages, 294 KiB

Open AccessArticle

A New Mathematical Framework for Describing Thin-Reaction-Zone Regime of Turbulent Reacting Flows at Low Damköhler Number

by Vladimir A. Sabelnikov and Andrei N. Lipatnikov

Fluids 2020, 5(3), 109; https://doi.org/10.3390/fluids5030109 - 9 Jul 2020

Cited by 4 | Viewed by 1964

Abstract

Recently, Sabelnikov et al. (2019) developed a phenomenological theory of propagation of an infinitely thin reaction sheet, which is adjacent to a mixing layer, in a constant-density turbulent flow in the case of a low Damköhler number. In the cited paper, the theory [...] Read more.

Recently, Sabelnikov et al. (2019) developed a phenomenological theory of propagation of an infinitely thin reaction sheet, which is adjacent to a mixing layer, in a constant-density turbulent flow in the case of a low Damköhler number. In the cited paper, the theory is also supported by Direct Numerical Simulation data and relevance of such a physical scenario to highly turbulent premixed combustion is argued. The present work aims at complementing the theory with a new mathematical framework that allows for appearance of thick mixing zones adjacent to an infinitely thin reaction sheet. For this purpose, the instantaneous reaction-progress-variable

c (x, t)

is considered to consist of two qualitatively different zones, that is, (i) mixture of products and reactants,

c (x, t) < 1

, where molecular transport plays an important role, and (ii) equilibrium products,

c (x, t) = 1

. The two zones are separated by an infinitely thin reaction sheet, where

c (x, t) = 1

and

| \nabla c |

is fixed in order for the molecular flux into the sheet to yield a constant local consumption velocity equal to the speed of the unperturbed laminar reaction wave. Exact local instantaneous field equations valid in the entire spaceare derived for the conditioned (to the former, mixing, zone) reaction progress variable, its second moment, and instantaneous characteristic functions. Averaging of these equations yields exact, unclosed transport equations for the conditioned reaction-progress-variable moments and Probability Density Function (PDF), as well as a boundary condition for the PDF at the reaction sheet. The closure problem for the derived equations is beyond the scope of the paper. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

19 pages, 886 KiB

Open AccessArticle

Stochastic Modelling of Turbulent Flows for Numerical Simulations

by Carlo Cintolesi and Etienne Mémin

Fluids 2020, 5(3), 108; https://doi.org/10.3390/fluids5030108 - 9 Jul 2020

Cited by 7 | Viewed by 3584

Abstract

Numerical simulations are a powerful tool to investigate turbulent flows, both for theoretical studies and practical applications. The reliability of a simulation is mainly dependent on the turbulence model adopted, and improving its accuracy is a crucial issue. In this study, we investigated [...] Read more.

Numerical simulations are a powerful tool to investigate turbulent flows, both for theoretical studies and practical applications. The reliability of a simulation is mainly dependent on the turbulence model adopted, and improving its accuracy is a crucial issue. In this study, we investigated the potential for an alternative formulation of the Navier–Stokes equations, based on the stochastic representation of the velocity field. The new approach, named pseudo-stochastic simulation (PSS), is a generalisation of the widespread classical eddy–viscosity model, where the contribution of the unresolved scales of motion is expressed by a variance tensor, modelled following different paradigms. The PSS models were compared with the classical ones mathematically and numerically in the turbulent channel flow at

R e_{τ} = 590

. The PSS and the classical models are equivalent when the variance tensor is shaped through a molecular dissipation analogy, while it is more accurate when the tensor is defined by the way of a local variance model. A near-wall damping function derived from recent advancement in the field is also proposed and was successfully validated. The analyses demonstrate the relevance of the approach proposed and provide a basis for the development of an alternative turbulence model. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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15 pages, 5957 KiB

Open AccessArticle

Experimental Study of Sidewall Pressure Induced by Ferroparticles in Fluid under a Pulsating Magnetic Field

by Or Werner, Asaf Azulay, Boris Mikhailovich and Avi Levy

Fluids 2020, 5(2), 98; https://doi.org/10.3390/fluids5020098 - 22 Jun 2020

Viewed by 2217

Abstract

For several decades, magnetic nano- and microparticles have been used in various applications, as they can be attracted and controlled using external magnetic fields. Recently, carbonyl iron microparticles were used in a feasibility study of a new cardiac pacing application. The particles were [...] Read more.

For several decades, magnetic nano- and microparticles have been used in various applications, as they can be attracted and controlled using external magnetic fields. Recently, carbonyl iron microparticles were used in a feasibility study of a new cardiac pacing application. The particles were inserted into a heart, attracted to its sidewall using a pulsating magnetic field, and applied pulsating pressure on its sidewall. The magnitude of the sidewall pressure is a critical parameter for the success and safety of the application, and it was evaluated analytically using a simplified model. In the present study, the behaviour of carbonyl iron microparticles in a water chamber was studied experimentally. Several masses of these particles were attracted to the sidewall of the chamber using an external pulsating magnetic field; the behaviours of the masses of particles, the particle–particle interaction, and the influence of fluid dynamics on them were examined during different periods of pulses. The sidewall pressure during their attraction was measured using an in-house piezoelectric polyvinylidene fluoride sensor. The relations between the measured sidewall pressure and the mass of the particles, their sizes, and the magnetic field exposure time were investigated. The obtained results suggest an asymptotic sidewall pressure value for the specified magnetic field. The measurements of the sidewall pressure are compared with evaluated results from the analytical model, showing that the model over-predicts the sidewall pressure. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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16 pages, 5341 KiB

Open AccessArticle

On the Formation and Accumulation of Solid Carbon Particles in High-Enthalpy Flows Mimicking Re-Entry in the Titan Atmosphere

by Antonio Esposito, Marcello Lappa, Gennaro Zuppardi, Christophe Allouis, Barbara Apicella, Mario Commodo, Patrizia Minutolo and Carmela Russo

Fluids 2020, 5(2), 93; https://doi.org/10.3390/fluids5020093 - 12 Jun 2020

Cited by 6 | Viewed by 2485

Abstract

The problem relating to the formation of solid particles enabled by hypersonic re-entry in methane-containing atmospheres (such as that of Titan) has been tackled in the framework of a combined experimental–numerical approach implemented via a three-level analysis hierarchy. First experimental tests have been [...] Read more.

The problem relating to the formation of solid particles enabled by hypersonic re-entry in methane-containing atmospheres (such as that of Titan) has been tackled in the framework of a combined experimental–numerical approach implemented via a three-level analysis hierarchy. First experimental tests have been conducted using a wind tunnel driven by an industrial arc-heated facility operating with nitrogen as working gas (the SPES, i.e., the Small Planetary Entry Simulator). The formation of solid phases as a result of the complex chemical reactions established in such conditions has been detected and quantitatively measured with high accuracy. In a second stage of the study, insights into the related formation process have been obtained by using multispecies models relying on the NASA CEA code and the Direct Simulation Monte Carlo (DSMC) method. Through this approach the range of flow enthalpies in which carbonaceous deposits can be formed has been identified, obtaining good agreement with the experimental findings. Finally, the deposited substance has been analyzed by means of a set of complementary diagnostic techniques, i.e., SEM, spectroscopy (Raman, FTIR, UV–visible absorption and fluorescence), GC–MS and TGA. It has been found that carbon produced by the interaction of the simulated Titan atmosphere with a solid probe at very high temperatures can be separated into two chemically different fractions, which also include “tholins”. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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20 pages, 1673 KiB

Open AccessFeature PaperArticle

Three-Dimensional Convective Planforms for Inclined Darcy-Bénard Convection

by D. Andrew S. Rees and Antonio Barletta

Fluids 2020, 5(2), 83; https://doi.org/10.3390/fluids5020083 - 27 May 2020

Cited by 2 | Viewed by 2234

Abstract

We investigate the onset of convection in an inclined Darcy-Bénard layer. When such a layer is unbounded in the spanwise direction it is generally known that longitudinal rolls comprise the most unstable planform. On the other hand, when a layer has a sufficiently [...] Read more.

We investigate the onset of convection in an inclined Darcy-Bénard layer. When such a layer is unbounded in the spanwise direction it is generally known that longitudinal rolls comprise the most unstable planform. On the other hand, when a layer has a sufficiently small spanwise width, then transverse rolls form the most unstable planform. However, the layer remains stable to transverse roll disturbances when the inclination is above roughly 31 degrees from the horizontal. This paper considers the transition between these two extreme cases where the spanwise width takes moderate values and where rectangular cells are considered. It is found that the most unstable planform is quite strongly sensitive to the magnitude of the spanwise width and that there are large regions of parameter space within which three-dimensional convection patterns have the smallest critical Darcy-Rayleigh number. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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28 pages, 18030 KiB

Open AccessEditor’s ChoiceArticle

Droplet Impact on Suspended Metallic Meshes: Effects of Wettability, Reynolds and Weber Numbers

by Konstantinos Vontas, Cristina Boscariol, Manolia Andredaki, Anastasios Georgoulas, Cyril Crua, Jens Honoré Walther and Marco Marengo

Fluids 2020, 5(2), 81; https://doi.org/10.3390/fluids5020081 - 22 May 2020

Cited by 29 | Viewed by 5342

Abstract

Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying [...] Read more.

Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying and quantifying impact characteristics that are difficult to measure experimentally. For this purpose, an enhanced Volume-Of-Fluid (VOF) numerical simulation framework is utilised, previously developed in the general context of the OpenFOAM CFD Toolbox. Droplet impacts on metallic meshes are performed both experimentally and numerically with satisfactory degree of agreement. From the experimental investigation three main outcomes are observed—deposition, partial imbibition, and penetration. The penetration into suspended meshes leads to spectacular multiple jetting below the mesh. A higher amount of liquid penetration is linked to higher impact velocity, lower viscosity and larger pore size dimension. An estimation of the liquid penetration is given in order to evaluate the impregnation properties of the meshes. From the parametric analysis it is shown that liquid viscosity affects the adhesion characteristics of the drops significantly, whereas droplet break-up after the impact is mostly controlled by surface tension. Additionally, wettability characteristics are found to play an important role in both liquid penetration and droplet break-up below the mesh. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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15 pages, 2772 KiB

Open AccessArticle

Electrical Conductivity of Field-Structured Emulsions

by Arthur R. Zakinyan, Ludmila M. Kulgina, Anastasia A. Zakinyan and Sergey D. Turkin

Fluids 2020, 5(2), 74; https://doi.org/10.3390/fluids5020074 - 16 May 2020

Cited by 5 | Viewed by 3645

Abstract

The structure formation influence on various macroscopic properties of fluid–fluid disperse systems is poorly investigated. The present work deals with the experimental study of the charge transfer in emulsions whose dispersed phase droplets are arranged into chainlike structures under the action of an [...] Read more.

The structure formation influence on various macroscopic properties of fluid–fluid disperse systems is poorly investigated. The present work deals with the experimental study of the charge transfer in emulsions whose dispersed phase droplets are arranged into chainlike structures under the action of an external force field. The emulsions studied are the fluid system in which water droplets are dispersed in a hydrocarbon-based magnetic fluid. Under the effect of an external uniform magnetic field, anisotropic aggregates form from the emulsion dispersed phase drops. The low-frequency electrical conductivity of emulsions has been measured. It is demonstrated that the emulsions’ conductivity grows several times under the effect of magnetic field parallel to the measuring electrical field. The anisotropic character of the emulsion electrical conductivity in the presence of magnetic field has been demonstrated. It is revealed that the maximal response of conductivity on the magnetic field action takes place at the dispersed phase volume fraction of about 20%. The dynamics of the conductivity variation is analyzed in dependence on the magnetic field strength and the dispersed phase volume fraction. The obtained results may be of interest in the development of potential applications of disperse systems with magnetic-field-controllable properties. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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