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Fluids, Volume 4, Issue 4 (December 2019) – 30 articles

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Open AccessArticle
Numerical Investigation of Air-Side Heat Transfer and Pressure Drop Characteristics of a New Triangular Finned Microchannel Evaporator with Water Drainage Slits
Fluids 2019, 4(4), 205; https://doi.org/10.3390/fluids4040205 - 11 Dec 2019
Viewed by 323
Abstract
The present study investigated a new microchannel profile design encompassing condensate drainage slits for improved moisture removal with use of triangular shaped plain fins. Heat transfer and pressure drop correlations were developed using computational fluid dynamics (CFD) and defined in terms of Colburn [...] Read more.
The present study investigated a new microchannel profile design encompassing condensate drainage slits for improved moisture removal with use of triangular shaped plain fins. Heat transfer and pressure drop correlations were developed using computational fluid dynamics (CFD) and defined in terms of Colburn j-factor and Fanning f-factor. The microchannels were square 2.00 × 2.00 mm and placed with 4.50 mm longitudinal tube pitch. The transverse tube pitch and the triangular fin pitch were varied from 9.00 to 21.00 mm and 2.50 to 10.00 mm, respectively. Frontal velocity ranged from 1.47 to 4.40 m·s−1. The chosen evaporator geometry corresponds to evaporators for industrial refrigeration systems with long frosting periods. Furthermore, the CFD simulations covered the complete thermal entrance and developed regions, and made it possible to extract virtually infinite longitudinal heat transfer and pressure drop characteristics. The developed Colburn j-factor and Fanning f-factor correlations are able to predict the numerical results with 3.41% and 3.95% deviation, respectively. Full article
(This article belongs to the Special Issue Flow-Based Optimization of Products or Devices)
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Open AccessArticle
Inertial Micromixing in Curved Serpentine Micromixers with Different Curve Angles
Fluids 2019, 4(4), 204; https://doi.org/10.3390/fluids4040204 - 08 Dec 2019
Viewed by 369
Abstract
Micromixers are of considerable significance in many microfluidics system applications, from chemical reactions to biological analysis processes. Passive micromixers, which rely solely on their geometry, have the advantages of low cost and a less-complex fabrication process. Dean vortices seen in curved microchannels are [...] Read more.
Micromixers are of considerable significance in many microfluidics system applications, from chemical reactions to biological analysis processes. Passive micromixers, which rely solely on their geometry, have the advantages of low cost and a less-complex fabrication process. Dean vortices seen in curved microchannels are one of the useful tools to enhance micromixing. In this study, the effects of curve angle on micromixing were experimentally investigated in three curved serpentine micromixers consisting of ten segments with curve angles of 180 ° , 230 ° and 280 ° , at Dean numbers between 12 and 87. To characterize and compare the performance of the micromixers, fluorescence intensity maps and mixing indices were utilized. Accordingly, the micromixer having segments with 280 ° curve angle had significantly higher mixing index values up to the Dean number 60 and outperformed the other two micromixers. This was due to the severe distortion of flow streamlines by Dean vortices and the occurrence of chaotic advection at lower Dean numbers. Beyond the Dean number of 70, no difference was observed in the performance of the micromixers and the mixing index at their outlets had the asymptotic value of 0.93 ± 0.02. Furthermore, the flow behavior of the micromixers was numerically simulated to provide further insight about the mixing phenomena. Full article
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Open AccessArticle
Using Legitimation Code Theory to Conceptualize Learning Opportunities in Fluid Mechanics
Fluids 2019, 4(4), 203; https://doi.org/10.3390/fluids4040203 - 06 Dec 2019
Viewed by 276
Abstract
With widespread industry feedback on engineering graduates’ lack of technical skills and research demonstrating that higher education does not effectively facilitate the development of open-ended problem-solving competencies, many educators are attempting to implement measures that address these concerns. In order to properly formulate [...] Read more.
With widespread industry feedback on engineering graduates’ lack of technical skills and research demonstrating that higher education does not effectively facilitate the development of open-ended problem-solving competencies, many educators are attempting to implement measures that address these concerns. In order to properly formulate sensible interventions that result in meaningful improvements in student outcomes, useful educational measurement and analysis approaches are needed. Legitimation Code Theory (LCT) has rapidly emerged as an effective, theoretically informed ‘toolkit’ offering a suite of dimensions through which to observe, analyze, interpret, and design teaching and learning practices. LCT Semantics has been used to help engineering educators unpack both levels of engineering knowledge abstraction and the complexity of engineering terms, while LCT Specialization focuses on knowledge practices (using the epistemic plane) and enables a visualization and differentiation between kinds of phenomena and the fixed versus open-ended methods with which to approach a particular phenomenon. Drawing on a range of initiatives to enable an improved practical grasp of fluid mechanics concepts, this paper presents a description and graphic LCT analysis of student learning that has been designed to anchor the ‘purist’ principles underpinning applied fluid mechanics concepts (such as in piping and pump network design) by way of concerted ‘doctrinal’ practices, and the exposure to more open-ended practical situations involving peer learning/group work, allowing educators to visualize the code clash between the curriculum and the world of work. Full article
(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics)
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Open AccessArticle
Analysis of Transition for a Flow in a Channel via Reduced Basis Methods
Fluids 2019, 4(4), 202; https://doi.org/10.3390/fluids4040202 - 05 Dec 2019
Viewed by 234
Abstract
The study of the flow mechanisms leading to transition in a planar channel flow is investigated by means of a reduced basis method known as Dynamic Mode Decomposition (DMD). The problem of identification of the most relevant DMD modes is addressed in terms [...] Read more.
The study of the flow mechanisms leading to transition in a planar channel flow is investigated by means of a reduced basis method known as Dynamic Mode Decomposition (DMD). The problem of identification of the most relevant DMD modes is addressed in terms of the ability to (i) provide a fairly accurate reconstruction of the flow field, and (ii) match the most relevant flow structures at the beginning of the transition region. A comparative study between a natural method of selection based on the energetic content of the modes and a new one based on the temporal dynamics of the modes is here presented. Full article
(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)
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Open AccessArticle
Numerical Study of Flow and Particle Deposition in Wall-Flow Filters with Intact or Damaged Exit
Fluids 2019, 4(4), 201; https://doi.org/10.3390/fluids4040201 - 02 Dec 2019
Viewed by 278
Abstract
We examine the time-dependent three-dimensional gas-particle flow in an intact wall-flow filter consisting of channels alternatively plugged at each end and a partially damaged filter in which the rear plugs are removed. Our focus is placed on highlighting the differences in the flow [...] Read more.
We examine the time-dependent three-dimensional gas-particle flow in an intact wall-flow filter consisting of channels alternatively plugged at each end and a partially damaged filter in which the rear plugs are removed. Our focus is placed on highlighting the differences in the flow pattern and the deposition process between the two geometries. The Navier–Stokes equations are solved for the fluid flow coupled with a Brinkman/Forchheimmer model in order to simulate the flow in the porous walls and plugs. Discrete particle simulation is utilized to determine the nanoparticle trajectories. Using this scheme, we are able to characterize the main features of the flow fields developing in the intact and damaged filters with respect to the Reynolds number and identify those affecting the transport and deposition of particles that have three representative response times. We present fluid velocity iso-contours, which describe the flow regimes inside the channels, as well as in regions upstream and downstream of them. We provide evidence of local recirculating bubbles at the entrance of the channels and after their exit, whereas back-flow occurs in front of the rear plugs of the intact channels. We show that the flow leaves the channels as strong jets that may break up for certain flow parameters, leading to turbulence with features that depend on the presence of the rear plugs. The removal of the rear plugs affects the flow distribution, which, in turn influences the flow rates along the channels and through the walls. We describe the particle trajectories and the topology of deposited particles and show that particles follow closely the streamlines, which may cross the surface of permeable walls for both flow configurations. The distribution of deposited particles resembles the spatial variation of the through-wall flow rate, exhibiting two peak values at both ends of the intact filter channel, and one local maximum near the entrance of the damaged filter channel that is diminished at the exit. We also investigate in detail the particle deposition on the frontal face and indicate that particle accumulation at the edges of the entrance is favored for particles with low response times in flows with high fluid mass rates for both intact and damaged filters. Finally, we examine the filtration efficiency for the defective channels without rear plugs and show that fewer particles are captured as the Reynolds number is increased. A smaller reduction of the filtration efficiency is also predicted with increasing particle size. Full article
(This article belongs to the Special Issue Particle-Based Simulation of Fluid Dynamics)
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Open AccessArticle
Dynamics of an Ellipse-Shaped Meniscus on a Substrate-Supported Drop under an Electric Field
Fluids 2019, 4(4), 200; https://doi.org/10.3390/fluids4040200 - 29 Nov 2019
Cited by 1 | Viewed by 337
Abstract
The behavior of a conducting droplet and a dielectric droplet placed under an electric potential is analyzed. Expressions for drop height based on electrode separation and the applied voltage are found, and problem parameters associated with breakup and droplet ejection are classified. Similar [...] Read more.
The behavior of a conducting droplet and a dielectric droplet placed under an electric potential is analyzed. Expressions for drop height based on electrode separation and the applied voltage are found, and problem parameters associated with breakup and droplet ejection are classified. Similar to previous theoretical work, the droplet interface is restricted to an ellipse shape. However, contrary to previous work, the added complexity of the boundary condition at the electrode is taken into account. To gain insight into this problem, a two-dimensional droplet is addressed. This allows for conformal maps to be used to solve for the potential surrounding the drop, which gives the total upward electrical force on the drop that is then balanced by surface tension and gravitational forces. For the conducting case, the maximum droplet height is attained when the distance between the electrode and the drop becomes sufficiently large, in which case, the droplet can stably grow to about 2.31 times its initial height before instabilities occur. In the dielectric case, hysteresis can occur for certain values of electrode separation and relative permittivity. Full article
(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids)
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Open AccessArticle
Forty Years’ Experience in Teaching Fluid Mechanics at Strasbourg University
Fluids 2019, 4(4), 199; https://doi.org/10.3390/fluids4040199 - 29 Nov 2019
Viewed by 257
Abstract
A summary of the personal investment in teaching fluid mechanics over 40 years in a French university is presented. Learning and Teaching Science and Engineering has never been easy, and in recent years it has become a crucial challenge for curriculum developers and [...] Read more.
A summary of the personal investment in teaching fluid mechanics over 40 years in a French university is presented. Learning and Teaching Science and Engineering has never been easy, and in recent years it has become a crucial challenge for curriculum developers and teaching staff to offer attractive courses and optimized assessments. One objective is to ensure that students acquire competitive skills in higher science education that enable them to compete in the employment market, as the mechanical field is a privileged sector in industry. During the last decade, classical learning and teaching methods have been coupled with hands-on practice for future schoolteachers in a specific course on subjects including fluid mechanics. The hands-on/minds-on/hearts-on approach has demonstrated its effectiveness in training primary school teachers, and fluids are certainly a nice source of motivation for pupils in science learning. In mechanical engineering, for undergraduate and graduate students, the development of teaching material and the learning and teaching experience covers up to 40 years, mostly on fluid dynamics and related topics. Two periods are identified, those prior to and after the Bologna Process. Most recently, teaching instruction has focused on the Fluid Mechanics Concept Inventory (FMCI). This inventory has been recently introduced in France, with some modifications, and remedial tools have been developed and are proposed to students to remove misconceptions and misunderstandings of key concepts in fluid mechanics. The FMCI has yet to be tested in French higher education institutions, as are the innovative teaching methods that are emerging in fluid mechanics. Full article
(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics)
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Open AccessArticle
Dynamics of Thin Film Under a Volatile Solvent Source Driven by a Constant Pressure Gradient Flow
Fluids 2019, 4(4), 198; https://doi.org/10.3390/fluids4040198 - 29 Nov 2019
Viewed by 263
Abstract
The evolution of a thin liquid film subject to a volatile solvent source and an air-blow effect which modifies locally the surface tension and leads to Marangoni-induced flow is shown to be governed by a degenerate fourth order nonlinear parabolic h-evolution equation [...] Read more.
The evolution of a thin liquid film subject to a volatile solvent source and an air-blow effect which modifies locally the surface tension and leads to Marangoni-induced flow is shown to be governed by a degenerate fourth order nonlinear parabolic h-evolution equation of the type given by t h = div x M 1 h x 3 h + M 2 h x h + M 3 h , where the mobility terms M 1 h and M 2 h result from the presence of the source and M 3 h results from the air-blow effect. Various authors assume M 2 h 0 and exclude the air-blow effect into M 3 h . In this paper, the authors show that such assumption is not necessarily correct, and the inclusion of such effect does disturb the dynamics of the thin film. These emphasize the importance of the full definition t · grad γ = grad x γ + x h grad y γ of the surface tension gradient at the free surface in contrast to the truncated expression t · grad γ grad x γ employed by those authors and the effect of the air-blow flowing over the surface. Full article
(This article belongs to the Special Issue Free surface flows)
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Open AccessArticle
A New Wall Model for Large Eddy Simulation of Separated Flows
Fluids 2019, 4(4), 197; https://doi.org/10.3390/fluids4040197 - 28 Nov 2019
Viewed by 327
Abstract
The aim of this work is to propose a new wall model for separated flows which is combined with large eddy simulation (LES) of the flow field in the whole domain. The model is designed to give reasonably good results for engineering applications [...] Read more.
The aim of this work is to propose a new wall model for separated flows which is combined with large eddy simulation (LES) of the flow field in the whole domain. The model is designed to give reasonably good results for engineering applications where the grid resolution is generally coarse. Since in practical applications a geometry can share body fitted and immersed boundaries, two different methodologies are introduced, one for body fitted grids, and one designed for immersed boundaries. The starting point of the models is the well known equilibrium stress model. The model for body fitted grid uses the dynamic evaluation of the von Kármán constant κ of Cabot and Moin (Flow, Turbulence and Combustion, 2000, 63, pp. 269–291) in a new fashion to modify the computation of shear velocity which is needed for evaluation of the wall shear stress and the near-wall velocity gradients based on the law of the wall to obtain strain rate tensors. The wall layer model for immersed boundaries is an extension of the work of Roman et al. (Physics of Fluids, 2009, 21, p. 101701) and uses a criteria based on the sign of the pressure gradient, instead of one based on the friction velocity at the projection point, to construct the velocity under an adverse pressure gradient and where the near-wall computational node is in the log region, in order to capture flow separation. The performance of the models is tested over two well-studied geometries, the isolated two-dimensional hill and the periodic two-dimensional hill, respectively. Sensitivity analysis of the models is also performed. Overall, the models are able to predict the first and second order statistics in a reasonable way, including the position and extension of the downward separation region. Full article
(This article belongs to the Special Issue Recent Numerical Advances in Fluid Mechanics)
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Open AccessReview
Fluid Flow and Mass Transport in Brain Tissue
Fluids 2019, 4(4), 196; https://doi.org/10.3390/fluids4040196 - 26 Nov 2019
Viewed by 296
Abstract
Despite its small size, the brain consumes 25% of the body’s energy, generating its own weight in potentially toxic proteins and biological debris each year. The brain is also the only organ lacking lymph vessels to assist in removal of interstitial waste. Over [...] Read more.
Despite its small size, the brain consumes 25% of the body’s energy, generating its own weight in potentially toxic proteins and biological debris each year. The brain is also the only organ lacking lymph vessels to assist in removal of interstitial waste. Over the past 50 years, a picture has been developing of the brain’s unique waste removal system. Experimental observations show cerebrospinal fluid, which surrounds the brain, enters the brain along discrete pathways, crosses a barrier into the spaces between brain cells, and flushes the tissue, carrying wastes to routes exiting the brain. Dysfunction of this cerebral waste clearance system has been demonstrated in Alzheimer’s disease, traumatic brain injury, diabetes, and stroke. The activity of the system is observed to increase during sleep. In addition to waste clearance, this circuit of flow may also deliver nutrients and neurotransmitters. Here, we review the relevant literature with a focus on transport processes, especially the potential role of diffusion and advective flows. Full article
(This article belongs to the Special Issue Coupled Flow and Heat or Mass Transport)
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Open AccessCommunication
Changes in Vertical Distribution of Zooplankton under Wind-Induced Turbulence: A 36-Year Record
Fluids 2019, 4(4), 195; https://doi.org/10.3390/fluids4040195 - 25 Nov 2019
Viewed by 229
Abstract
A multidecadal record of a local zooplankton community, stored in an open-access database, was analyzed with wind data to examine the impact of wind-induced turbulence on vertical distribution of zooplankton. Two major findings were made. First, the abundance of zooplankton assemblage (composed of [...] Read more.
A multidecadal record of a local zooplankton community, stored in an open-access database, was analyzed with wind data to examine the impact of wind-induced turbulence on vertical distribution of zooplankton. Two major findings were made. First, the abundance of zooplankton assemblage (composed of copepods, cladocerans, etc.) in the upper layer (<10 m deep) decreased with increasing turbulence intensity, suggesting turbulence avoidance by zooplankton. Second, when focusing on each species, it was found that ambush (sit-and-wait) feeders showed statistically significant changes in response to turbulence, whereas suspension (filter) feeders did not. This is the first clear evidence that ambush feeders change vertical distribution in response to turbulence. Full article
(This article belongs to the Special Issue Fluid Mechanics of Plankton)
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Open AccessEssay
Teaching and Learning Pressure and Fluids
Fluids 2019, 4(4), 194; https://doi.org/10.3390/fluids4040194 - 25 Nov 2019
Viewed by 220
Abstract
This essay is a synthesis of more than twenty years of research, already published, on teaching and learning fluids and pressure. We examine teaching fluids globally, i.e., the content to be taught and its transformations, students’ alternative conceptions and their remediation, the sequence [...] Read more.
This essay is a synthesis of more than twenty years of research, already published, on teaching and learning fluids and pressure. We examine teaching fluids globally, i.e., the content to be taught and its transformations, students’ alternative conceptions and their remediation, the sequence of educational activities, being right for students’ understanding, as well as tasks for evaluating their conceptual evolution. Our samples are junior high school students and primary school student-teachers. This long-term study combines research and development concerning teaching and learning fluids and has evolved through iteratively based design application and reflective feedback related to empirical data. The results of our research include several publications. Full article
(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics)
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Open AccessArticle
Comparative Studies of Hyaluronic Acid Concentration in Normal and Osteoarthritic Equine Joints
Fluids 2019, 4(4), 193; https://doi.org/10.3390/fluids4040193 - 05 Nov 2019
Viewed by 478
Abstract
Osteoarthritis (OA) is the most common major disabling disease in humans and horses. Hyaluronic acid (HA), naturally abundantly present in synovial fluid (SF), is thought to have crucial impact on the functional rheological and biochemical features of SF in healthy and osteoarthritic joints. [...] Read more.
Osteoarthritis (OA) is the most common major disabling disease in humans and horses. Hyaluronic acid (HA), naturally abundantly present in synovial fluid (SF), is thought to have crucial impact on the functional rheological and biochemical features of SF in healthy and osteoarthritic joints. Here we present comparative measurements of HA concentration in SF from 35 normal and osteoarthritic equine joints, between two different approaches. On the one hand, an established biochemical HA-specific Enzyme–Linked Immunosorbent Assay (ELISA) assay was employed, which determined that SF in healthy and osteoarthritic equine joints is characterized by HA concentration of ca 0.3–2 mg/mL and 0.1–0.7 mg/mL respectively. On the other hand the same SF samples were also examined with a new exploratory approach of finding out HA concentration, which is based on SF rheology. This was done following “calibration” using appropriate model HA solutions. Comparative analysis of the results obtained by both the biochemical and the rheological approaches, revealed that in most cases the rheological approach greatly overestimates HA concentration in SF, by ca 3 to 8 times and 6 to 11 times, in healthy and diseased SF respectively. Overall these findings support the notion that, contrary to the established view, HA may not be the major contributor of equine SF rheology. This should be taken into account for the development of new more effective preventive strategies, as well as more effective early-stage interventions in osteoarthritis. Full article
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Open AccessArticle
Natural Convection in a Non-Newtonian Fluid: Effects of Particle Concentration
Fluids 2019, 4(4), 192; https://doi.org/10.3390/fluids4040192 - 01 Nov 2019
Cited by 1 | Viewed by 307
Abstract
In this paper we study the buoyancy driven flow of a particulate suspension between two inclined walls. The suspension is modeled as a non-linear fluid, where the (shear) viscosity depends on the concentration (volume fraction of particles) and the shear rate. The motion [...] Read more.
In this paper we study the buoyancy driven flow of a particulate suspension between two inclined walls. The suspension is modeled as a non-linear fluid, where the (shear) viscosity depends on the concentration (volume fraction of particles) and the shear rate. The motion of the particles is determined by a convection-diffusion equation. The equations are made dimensionless and the boundary value problem is solved numerically. A parametric study is performed, and velocity, concentration and temperature profiles are obtained for various values of the dimensionless numbers. The numerical results indicate that due to the non-uniform shear rate, the particles tend to concentrate near the centerline; however, for a small Lewis number (Le) related to the size of the particles, a uniform concentration distribution can be achieved. Full article
(This article belongs to the Special Issue Recent Advances in Mechanics of Non-Newtonian Fluids)
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Open AccessArticle
Instability of Vertical Throughflows in Porous Media under the Action of a Magnetic Field
Fluids 2019, 4(4), 191; https://doi.org/10.3390/fluids4040191 - 01 Nov 2019
Viewed by 217
Abstract
The instability of a vertical fluid motion (throughflow) in a binary mixture saturating a horizontal porous layer, uniformly heated from below, uniformly salted from below by one salt and permeated by an imposed uniform magnetic field H , normal to the layer, is [...] Read more.
The instability of a vertical fluid motion (throughflow) in a binary mixture saturating a horizontal porous layer, uniformly heated from below, uniformly salted from below by one salt and permeated by an imposed uniform magnetic field H , normal to the layer, is analyzed. By employing the order-1 Galerkin weighted residuals method, the critical Rayleigh numbers for the onset of steady or oscillatory instability, have been determined. Full article
(This article belongs to the Special Issue Convective Instability in Porous Media, Volume II)
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Open AccessArticle
Blood Flow Dynamics at the Pulmonary Artery Bifurcation
Fluids 2019, 4(4), 190; https://doi.org/10.3390/fluids4040190 - 01 Nov 2019
Viewed by 290
Abstract
Knowledge of physiologic hemodynamics is a fundamental requirement to establish pathological findings. However, little is known about the normal flow fields in the pulmonary arteries, especially for children. The purpose of this study is to characterize flow patterns in the pulmonary artery bifurcation [...] Read more.
Knowledge of physiologic hemodynamics is a fundamental requirement to establish pathological findings. However, little is known about the normal flow fields in the pulmonary arteries, especially for children. The purpose of this study is to characterize flow patterns in the pulmonary artery bifurcation of healthy pediatric subjects using direct numerical simulations. A realistic geometry is obtained via statistical shape modeling, by averaging five subject-specific digital models extracted from cardiovascular magnetic resonance datasets of healthy volunteers. Boundary conditions are assigned to mimic physiological conditions at rest, corresponding to a peak Reynolds number equal to 3400 and a Womersley number equal to 15. Results show that the normal bifurcation is highly hemodynamically efficient, as measured by an energy dissipation index. The curvature of the pulmonary arteries is sufficiently small to prevent flow separation along the inner walls, and no signs of a turbulent-like state are found. In line with previous imaging studies, a helical structure protruding into the right pulmonary artery is detected, and its formation mechanism is elucidated in the paper. These findings might help to identify abnormal flow features in patients with altered anatomic and physiologic states, particularly those with repaired congenital heart disease. Full article
(This article belongs to the Special Issue Cardiovascular Flows)
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Open AccessArticle
Energy Transport by Kelvin-Helmholtz Instability at the Magnetopause
Fluids 2019, 4(4), 189; https://doi.org/10.3390/fluids4040189 - 01 Nov 2019
Viewed by 255
Abstract
By means of the formation of vortices in the nonlinear phase, the Kelvin Helmholtz instability is able to redistribute the flux of energy of the solar wind that flows parallel to the magnetopause. The energy transport associated with the Kelvin Helmholtz instability contributes [...] Read more.
By means of the formation of vortices in the nonlinear phase, the Kelvin Helmholtz instability is able to redistribute the flux of energy of the solar wind that flows parallel to the magnetopause. The energy transport associated with the Kelvin Helmholtz instability contributes significantly to the magnetosphere and magnetosheath dynamics, in particular at the flanks of the magnetopause where the presence of a magnetic field perpendicular to the velocity flow does not inhibit the instability development. By means of a 2D two-fluid simulation code, the behavior of the Kelvin Helmholtz instability is investigated in the presence of typical conditions observed at the magnetopause. In particular, the energy penetration in the magnetosphere is studied as a function of an important parameter such as the solar wind velocity. The influence of the density jump at the magnetopause is also discussed. Full article
(This article belongs to the Special Issue Modelling of Plasma Flow)
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Open AccessArticle
Influence of Fluidic Control in a Staged Lean Jet Engine Burner on Combustor Performance
Fluids 2019, 4(4), 188; https://doi.org/10.3390/fluids4040188 - 01 Nov 2019
Viewed by 239
Abstract
To improve the turn-down ratio of a lean combustor, which has the greatest potential for reducing NOx emissions from jet engines, fuel staging is commonly employed. To further extend the stable operation range, air staging with a fluidic element is also considered. [...] Read more.
To improve the turn-down ratio of a lean combustor, which has the greatest potential for reducing NOx emissions from jet engines, fuel staging is commonly employed. To further extend the stable operation range, air staging with a fluidic element is also considered. The influence of fluidic control on combustion was analyzed to better understand fluidic element-burner interactions. The pressure loss of each fluidic element was determined by measuring the pressure at the element exits. The effect of fluidic control on the atomization, fuel distribution, and flow field was investigated using optical, noninvasive techniques. The combustion performance of the burner with the fluidic element was evaluated using exhaust gas analyses. The pressure losses of the swirlers and fuel mixers were varied depending on the bleed air from the fluidic element. Under the idle condition, the reduction of pressure loss in the pilot fuel mixer resulted in inferior atomization due to the reduced gas velocity around the fuel film, which had a positive effect on lean blowout. Under the cruise condition and the staged mode, the reduction of the pilot air flow increased the equivalence ratio of the lean pilot stage and resulted in higher combustion efficiency. Full article
(This article belongs to the Special Issue Turbomachinery Flow Analysis)
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Open AccessArticle
Investigations of Evaporative Cooling and Turbulence Flame Interaction Modeling in Ethanol Turbulent Spray Combustion Using Tabulated Chemistry
Fluids 2019, 4(4), 187; https://doi.org/10.3390/fluids4040187 - 31 Oct 2019
Viewed by 291
Abstract
Evaporative cooling effects and turbulence flame interaction are analyzed in the large eddy simulation (LES) context for an ethanol turbulent spray flame. Investigations are conducted with the artificially thickened flame (ATF) approach coupled with an extension of the mixture adaptive thickening procedure to [...] Read more.
Evaporative cooling effects and turbulence flame interaction are analyzed in the large eddy simulation (LES) context for an ethanol turbulent spray flame. Investigations are conducted with the artificially thickened flame (ATF) approach coupled with an extension of the mixture adaptive thickening procedure to account for variations of enthalpy. Droplets are tracked in a Euler–Lagrangian framework, in which an evaporation model accounting for the inter-phase non-equilibrium is applied. The chemistry is tabulated following the flamelet generated manifold (FGM) method. Enthalpy variations are incorporated in the resulting FGM database in a universal fashion, which is not limited to the heat losses caused by evaporative cooling effects. The relevance of the evaporative cooling is evaluated with a typically applied setting for a flame surface wrinkling model. Using one of the resulting cases from the evaporative cooling analysis as a reference, the importance of the flame wrinkling modeling is studied. Besides its novelty, the completeness of the proposed modeling strategy allows a significant contribution to the understanding of the most relevant phenomena for the turbulent spray combustion modeling. Full article
(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)
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Open AccessFeature PaperArticle
Modeling of Sedimentation and Creaming in Suspensions and Pickering Emulsions
Fluids 2019, 4(4), 186; https://doi.org/10.3390/fluids4040186 - 22 Oct 2019
Cited by 1 | Viewed by 736
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 Feature Papers in Fluids)
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Open AccessEditorial
Editorial for Special Issue “Multiscale Turbulent Transport”
Fluids 2019, 4(4), 185; https://doi.org/10.3390/fluids4040185 - 18 Oct 2019
Viewed by 260
Abstract
Turbulent transport is currently a great subject of ongoing investigation at the interface of methodologies running from theory to numerical simulations and experiments, and covering several spatio-temporal scales [...] Full article
(This article belongs to the Special Issue Multiscale Turbulent Transport)
Open AccessArticle
Study of Leading-Edge Dimple Effects on Airfoil Flow Using Tomographic PIV and Temperature Sensitive Paint
Fluids 2019, 4(4), 184; https://doi.org/10.3390/fluids4040184 - 14 Oct 2019
Viewed by 351
Abstract
Airfoil blades can experience a significant change of angle of attack during operation cycles, which may lead to static or dynamic stall in various applications. It is unclear how elements distributed at the leading edge would affect the aerodynamic performance and stall behaviors. [...] Read more.
Airfoil blades can experience a significant change of angle of attack during operation cycles, which may lead to static or dynamic stall in various applications. It is unclear how elements distributed at the leading edge would affect the aerodynamic performance and stall behaviors. In the present study, a distributed dimples configuration was investigated and compared to a baseline smooth NACA0015 airfoil at low Reynolds numbers. Two- and four-camera, tomographic particle image velocimetry (PIV), and temperature sensitive paint (TSP) techniques were set up to gather flow and surface information near the curved leading-edge surface and to study flow separation. Results suggest that distributed dimples configuration create abrupt separation leading to stall and induce a similar stall compared to the smooth model. However, the stall is induced more abruptly and with different flow patterns. Results show that patterns of separated shear layer at stalled conditions were enhanced by the current configuration. Effect of these structures on the boundary layer transition were also analyzed based on combined tomographic PIV and TSP measurement techniques. Full article
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Open AccessFeature PaperArticle
Bottle Emptying: A Fluid Mechanics and Measurements Exercise for Engineering Undergraduate Students
Fluids 2019, 4(4), 183; https://doi.org/10.3390/fluids4040183 - 11 Oct 2019
Viewed by 310
Abstract
A comprehensive exercise, suitable for an undergraduate engineering audience studying fluid mechanics, is presented, in which participants were tasked with emptying a bottle. That simple request yielded data collected by students and the author for N = 454 commercially available bottles, spanning nearly [...] Read more.
A comprehensive exercise, suitable for an undergraduate engineering audience studying fluid mechanics, is presented, in which participants were tasked with emptying a bottle. That simple request yielded data collected by students and the author for N = 454 commercially available bottles, spanning nearly four orders of magnitude for volume V , and representing the largest experimental dataset available in the literature. Fundamental statistics are used to describe the emptying time, T ¯ e , for any single bottle. Dimensional analysis is used to transform the raw data to yield a predictive trend, and a method of least-squares regression analysis is performed to find an empirical correlation relating dimensionless time T ¯ e g / d and dimensionless volume V / d 3 . We find that volume, V , and neck diameter, d, can be used to estimate the emptying time for any bottle, although the data suggests that the shape of the neck plays a role. Furthermore, two basic analytical models found in the literature compare favorably to our data and empirical correlation when recast using our dimensionless groups. The documented exercise provides students with the opportunity to use basic engineering statistics and to see the utility of dimensional analysis. Full article
(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics)
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Open AccessArticle
Bathymetry Development and Flow Analyses Using Two-Dimensional Numerical Modeling Approach for Lake Victoria
Fluids 2019, 4(4), 182; https://doi.org/10.3390/fluids4040182 - 10 Oct 2019
Viewed by 290
Abstract
This study explored two-dimensional (2D) numerical hydrodynamic model simulations of Lake Victoria. Several methods were developed in Matlab to build the lake topography. Old depth soundings taken in smaller parts of the lake were combined with more recent extensive data to produce a [...] Read more.
This study explored two-dimensional (2D) numerical hydrodynamic model simulations of Lake Victoria. Several methods were developed in Matlab to build the lake topography. Old depth soundings taken in smaller parts of the lake were combined with more recent extensive data to produce a smooth topographical model. The lake free surface numerical model in the COMSOL Multiphysics (CM) software was implemented using bathymetry and vertically integrated 2D shallow water equations. Validated by measurements of mean lake water level, the model predicted very low mean flow speeds and was thus close to being linear and time invariant, allowing long-time simulations with low-pass filtered inflow data. An outflow boundary condition allowed an accurate simulation to achieve the lake’s steady state level. The numerical accuracy of the linear measurement of lake water level was excellent. Full article
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Open AccessArticle
Fluids in Music: The Mathematics of Pan’s Flutes
Fluids 2019, 4(4), 181; https://doi.org/10.3390/fluids4040181 - 10 Oct 2019
Viewed by 312
Abstract
We discuss the mathematics behind the Pan’s flute. We analyze how the sound is created, the relationship between the notes that the pipes produce, their frequencies and the length of the pipes. We find an equation which models the curve that appears at [...] Read more.
We discuss the mathematics behind the Pan’s flute. We analyze how the sound is created, the relationship between the notes that the pipes produce, their frequencies and the length of the pipes. We find an equation which models the curve that appears at the bottom of any Pan’s flute due to the different pipe lengths. Full article
(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics)
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Open AccessArticle
Turbulence Intensity Scaling: A Fugue
Fluids 2019, 4(4), 180; https://doi.org/10.3390/fluids4040180 - 09 Oct 2019
Viewed by 382
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 Feature Papers in Fluids)
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Open AccessArticle
Approximating Isoneutral Ocean Transport via the Temporal Residual Mean
Fluids 2019, 4(4), 179; https://doi.org/10.3390/fluids4040179 - 02 Oct 2019
Viewed by 358
Abstract
Ocean volume and tracer transports are commonly computed on density surfaces because doing so approximates the semi-Lagrangian mean advective transport. The resulting density-averaged transport can be related approximately to Eulerian-averaged quantities via the Temporal Residual Mean (TRM), valid in the limit of small [...] Read more.
Ocean volume and tracer transports are commonly computed on density surfaces because doing so approximates the semi-Lagrangian mean advective transport. The resulting density-averaged transport can be related approximately to Eulerian-averaged quantities via the Temporal Residual Mean (TRM), valid in the limit of small isopycnal height fluctuations. This article builds on a formulation of the TRM for volume fluxes within Neutral Density surfaces, (the “NDTRM”), selected because Neutral Density surfaces are constructed to be as neutral as possible while still forming well-defined surfaces. This article derives a TRM, referred to as the “Neutral TRM” (NTRM), that approximates volume fluxes within surfaces whose vertical fluctuations are defined directly by the neutral relation. The purpose of the NTRM is to more closely approximate the semi-Lagrangian mean transport than the NDTRM, because the latter introduces errors associated with differences between the instantaneous state of the modeled/observed ocean and the reference climatology used to assign the Neutral Density variable. It is shown that the NDTRM collapses to the NTRM in the limiting case of a Neutral Density variable defined with reference to the Eulerian-mean salinity, potential temperature and pressure, rather than an external reference climatology, and therefore that the NTRM approximately advects this density variable. This prediction is verified directly using output from an idealized eddy-resolving numerical model. The NTRM therefore offers an efficient and accurate estimate of modeled semi-Lagrangian mean transports without reference to an external reference climatology, but requires that a Neutral Density variable be computed once from the model’s time-mean state in order to estimate isopycnal and diapycnal components of the transport. Full article
(This article belongs to the collection Geophysical Fluid Dynamics)
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Open AccessArticle
An Efficient Strategy to Deliver Understanding of Both Numerical and Practical Aspects When Using Navier-Stokes Equations to Solve Fluid Mechanics Problems
Fluids 2019, 4(4), 178; https://doi.org/10.3390/fluids4040178 - 01 Oct 2019
Viewed by 313
Abstract
An efficient and thorough strategy to introduce undergraduate students to a numerical approach of calculating flow is outlined. First, the basic steps, especially discretization, involved when solving Navier-Stokes equations using a finite-volume method for incompressible steady-state flow are developed with the main aim [...] Read more.
An efficient and thorough strategy to introduce undergraduate students to a numerical approach of calculating flow is outlined. First, the basic steps, especially discretization, involved when solving Navier-Stokes equations using a finite-volume method for incompressible steady-state flow are developed with the main aim being for the students to follow through from the mathematical description of a given problem to the final solution of the governing equations in a transparent way. The well-known ‘driven-cavity’ problem is used as the problem for testing coding written by the students, and the Navier-Stokes equations are initially cast in the vorticity-streamfunction form. This is followed by moving on to a solution method using the primitive variables and discussion of details such as, closure of the Navier-Stokes equations using turbulence modelling, appropriate meshing within the computation domain, various boundary conditions, properties of fluids, and the important methods for determining that a convergence solution has been reached. Such a course is found to be an efficient and transparent approach for introducing students to computational fluid dynamics. Full article
(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics)
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Open AccessArticle
Linear Stability Analysis of Liquid Metal Flow in an Insulating Rectangular Duct under External Uniform Magnetic Field
Fluids 2019, 4(4), 177; https://doi.org/10.3390/fluids4040177 - 01 Oct 2019
Viewed by 315
Abstract
Linear stability analysis of liquid metal flow driven by a constant pressure gradient in an insulating rectangular duct under an external uniform magnetic field was carried out. In the present analysis, since the Joule heating and induced magnetic field were neglected, the governing [...] Read more.
Linear stability analysis of liquid metal flow driven by a constant pressure gradient in an insulating rectangular duct under an external uniform magnetic field was carried out. In the present analysis, since the Joule heating and induced magnetic field were neglected, the governing equations consisted of the continuity of mass, momentum equation, Ohm’s law, and conservation of electric charge. A set of linearized disturbance equations for the complex amplitude was decomposed into real and imaginary parts and solved numerically with a finite difference method using the highly simplified marker and cell (HSMAC) algorithm on a two-dimensional staggered mesh system. The difficulty of the complex eigenvalue problem was circumvented with a Newton—Raphson method during which its corresponding eigenfunction was simultaneously obtained by using an iterative procedure. The relation among the Reynolds number, the wavenumber, the growth rate, and the angular frequency was successfully obtained for a given value of the Hartmann number as well as for a direction of external uniform magnetic field. Full article
(This article belongs to the Special Issue Numerical Analysis of Magnetohydrodynamics Flows)
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Open AccessReview
Molecular Dynamics Simulation of the Superspreading of Surfactant-Laden Droplets. A Review
Fluids 2019, 4(4), 176; https://doi.org/10.3390/fluids4040176 - 01 Oct 2019
Viewed by 511
Abstract
Superspreading is the rapid and complete spreading of surfactant-laden droplets on hydrophobic substrates. This phenomenon has been studied for many decades by experiment, theory, and simulation, but it has been only recently that molecular-level simulation has provided significant insights into the underlying mechanisms [...] Read more.
Superspreading is the rapid and complete spreading of surfactant-laden droplets on hydrophobic substrates. This phenomenon has been studied for many decades by experiment, theory, and simulation, but it has been only recently that molecular-level simulation has provided significant insights into the underlying mechanisms of superspreading thanks to the development of accurate force-fields and the increase of computational capabilities. Here, we review the main advances in this area that have surfaced from Molecular Dynamics simulation of all-atom and coarse-grained models highlighting and contrasting the main results and discussing various elements of the proposed mechanisms for superspreading. We anticipate that this review will stimulate further research on the interpretation of experimental results and the design of surfactants for applications requiring efficient spreading, such as coating technology. Full article
(This article belongs to the Special Issue Coupled Flow and Heat or Mass Transport)
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