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The objective of the study is to thoroughly analyze the flow and heat transfer of Bingham plastic fluids through an array of uniformly gapped rough surface cylinders embedded between two confined boundaries. Radial notches are used as the surface roughness in the model, evenly distributed. Due to the formation of front vortices in uniformly gapped cylinders, a negative pressure gradient is developed. The results of the numerical simulation analysis have shown that, when compared to the averaged Nusselt number, roughness has a minimal effect on the drag coefficient and pressure drop. As the degree of roughness increases, the size of the vortices decreases, resulting in a drop in heat transfer. Moreover, the analysis of each column shows that the first column array of cylinders has a higher total drag coefficient and average Nusselt number. [ABSTRACT FROM AUTHOR]

The work presented here aims to provide design guidelines to create vortex-damping structures. A design of experiment was developed to investigate the individual and combined effects of the geometrical properties of planar regular grid structures, i.e., the wire diameter, the porosity, and the inter-grid spacing, on their vortex-breakdown performance. The simulations were carried out using a commercial unsteady RANS solver. The model relies on the Von Karman street effect to generate vortices in a pipe which are convected downstream, where they interact with an array of grids. The vortex-breakdown efficiency is characterized by the pressure drop, the residual turbulent kinetic energy, the flow homogeneity, and the size of the transmitted vortices. The wire diameter is shown to be an important design lever as it affects the level of distortion of the transmitted vortices. Increasing the number of grids augments the pressure loss, but their contribution to vortex breakdown is otherwise limited when the wire diameter is small. The influence of grid spacing strongly depends on the wire diameter and grid alignment. For instance, minimizing this gap reduces the pressure drop for the inline configurations, but increases the pressure drop for the offset configurations. [ABSTRACT FROM AUTHOR]

Unsteady laminar confined and unconfined fluid flow and mixed (forced and free) convection heat transfer around equilateral triangular cylinders are investigated numerically. The computation model is a two-dimensional domain with blockage ratios of BR=0.5, 0.25, 0.2, 0.1, 0.05, and 0.0333, with the Reynolds numbers ranging from 100 to 200. The working fluid is water (Pr = 7). The effects of aiding and opposing thermal buoyancy are incorporated into the Navier-Stokes equations using the Boussinesq approximation. The Richardson number, which is a relative measure of free convection, is varied in the range -2 = Ri = 2. The governing equations are solved by using the Finite Volume Method with a second-order upwind scheme used for differencing of the convection terms, and the SIMPLE algorithm is used for the velocity-pressure coupling. A discussion of the effect of the blockage ratio on the mean drag, mean rms lift coefficients, the Strouhal number, and the mean Nusselt number is also presented. The iso-vorticity contours and dimensionless temperature field are generated to interpret and understand the underlying physical mechanisms. The results reveal that, in addition to the Richardson and Reynolds numbers, the blockage rate is effective in the vortex distribution in the channel. It has been determined that the vortices formed behind the cylinder spread to the channel with a decreasing blockage rate. Especially at high Reynolds numbers, both the drag coefficient and the mean Nusselt number are significantly affected by the blockage ratio. For Ri=0, the drag coefficients for BR=0.25 in comparison to the BR=0.05 case are about 9% and 29% larger for Re= 100 and 200, respectively. For BR<0.1, two-column vortex formation at the back of the cylinder gave way to single vortexes in the aiding thermal buoyancy condition (Ri=2) compared to Ri=0 and -2. Also, useful correlations for flow characteristics and heat transfer are derived using the computed data. [ABSTRACT FROM AUTHOR]

A direct numerical simulation of a three-dimensional diffuser at Reynolds number Re = 10,000 (based on inlet bulk velocity) has been performed using a low-dissipation finite element code. The geometry chosen for this work is the Stanford diffuser, introduced by Cherry et al. (Int. J. Heat Fluid Flow 29:803–811, 2008). Results have been exhaustively compared with the published data with a quite good agreement. Additionally, further turbulent statistics have been provided such as the Reynolds stresses or the turbulent kinetic energy. A proper orthogonal decomposition and a dynamic mode decomposition analyses of the main flow variables have been performed to identify the main characteristics of the large-scale motions. A combined, self-induced movement of the large-scales has been found to originate in the top-right expansion corner with two clear features. A low-frequency diagonal cross-stream travelling wave first reported by Malm et al. (J. Fluid Mech. 699:320–351, 2012), has been clearly identified in the spatial modes of the stream-wise velocity components and the pressure, associated with the narrow band frequency of S t ∈ [ 0.083 , 0.01 ] . This movement is caused by the geometrical expansion of the diffuser in the cross-stream direction. A second low-frequency trait has been identified associated with the persisting secondary flows and acting as a back and forth global accelerating-decelerating motion located on the straight area of the diffuser, with associated frequencies of S t < 0.005 . The smallest frequency observed in this work has been S t = 0.0013 . This low-frequency observed in the Stanford diffuser points out the need for longer simulations in order to obtain further turbulent statistics. [ABSTRACT FROM AUTHOR]

The propagation of an isobaric premixed flame into a quiescent gas mixture of fuel and oxidizer contained between two parallel plates is investigated numerically. The plates are separated by a small distance h and considered as adiabatic. The mixture is assumed to be lean in fuel and the combustion model includes a single-step Arrhenius-type reaction, constant heat capacity and unity fuel Lewis number. Transport properties are considered to be temperature dependent or constant, which allows us to decouple two different instability mechanisms of hydrodynamic nature: (i) Darrieus-Landau (associated with the density change due to thermal expansion) and (ii) Saffman-Taylor (associated with the viscosity contrast). By performing three-dimensional (3D) simulations, the propagation rate and the flame front shape is analyzed as a function of the dimensionless parameter a = h / δ T , where δ T is the thermal thickness of the planar flame. The parameter a ranges from very small values to large enough ones so that flame curvature between the plates manifests itself. Results show that, as the distance between the plates decreases, loss of momentum enhances the hydrodynamic instability in comparison with that of a freely (unconfined) propagating flame. Likewise, viscosity contrast across the flame brings about an additional destabilizing mechanism. When distance between the plates increases, flame curvature can become important and contribute significantly to the overall propagation rate. Finally, by comparison with the 3D simulations, we show that confinement effects can be effectively described by a two-dimensional formulation written in the limit a → 0 , in which momentum conservation is reduced to a linear equation for the velocity similar to Darcy's law. [ABSTRACT FROM AUTHOR]

The work presented here aims to provide design guidelines to create vortex-damping structures. A design of experiment was developed to investigate the individual and combined effects of the geometrical properties of planar regular grid structures, i.e., the wire diameter, the porosity, and the inter-grid spacing, on their vortex-breakdown performance. The simulations were carried out using a commercial unsteady RANS solver. The model relies on the Von Karman street effect to generate vortices in a pipe which are convected downstream, where they interact with an array of grids. The vortex-breakdown efficiency is characterized by the pressure drop, the residual turbulent kinetic energy, the flow homogeneity, and the size of the transmitted vortices. The wire diameter is shown to be an important design lever as it affects the level of distortion of the transmitted vortices. Increasing the number of grids augments the pressure loss, but their contribution to vortex breakdown is otherwise limited when the wire diameter is small. The influence of grid spacing strongly depends on the wire diameter and grid alignment. For instance, minimizing this gap reduces the pressure drop for the inline configurations, but increases the pressure drop for the offset configurations.

A temperature‐controlled submillimeter‐gap (500 μm) rheo‐magnetic resonance imaging (MRI) Couette cell has been developed to measure confined flow of soft structured materials under controlled temperature. The proposed setup enables performing rheo‐MRI measurements using (i) a spatially uniform temperature control over the range 15°C to 40°C and (ii) a high spatial resolution up to 10 μm, as a consequence of the improved mechanical stability of the in‐house developed rotating elements. Here, we demonstrate the performance of the cell for the rheo‐MRI velocimetry study of a thixotropic fat crystal dispersion, a complex fluid commonly used in food manufacturing. The submillimeter‐gap geometry and variable temperature capability of the cell enable observing the effects of shear‐ and temperature‐induced fat recrystallization on both wall slip and shear banding under strongly confined flow. Our improved rheo‐MRI setup opens new perspectives for the fundamental study of strongly confined flow, cooperative effects, and the underlying interparticle interactions and for ultimately aiding optimization of products involved in spreading/extrusion, such as cosmetics and foods. [ABSTRACT FROM AUTHOR]

Sınırlandırılmış bir kanal içerisinde yer alan tek kare silindir (KS) üzerinden iki boyutlu laminer sürekli akış için blokaj (=B/H) oranının ısı transferi ve akış karakteristiklerine olan etkisi incelenmiştir. Çalışmada Reynolds sayısı Re=40 değerinde sabit tutulurken blokaj oranı =0.125-0.8 değerleri arasında değiştirilmiştir. Hesaplamalarda ANSYS CFX 14.0 kullanılmıştır. Blokaj oranı etkisinin KS yüzeyleri üzerindeki sürükleme katsayısı (Cd), sürtünme katsayısı (Cf), boyutsuz yeniden birleşme uzunluğu (Lr/B) ve ortalama Nusselt sayısı (Nu) üzerine olan etkileri incelenmiştir. Blokaj oranı arttıkça sürükleme katsayısı (Cd), sürtünme katsayısı (Cf) ve ortalama Nusselt sayısı (Nu) değerlerinin arttığı ancak boyutsuz yeniden birleşme uzunluğu (Lr/B) değerinin azaldığı bulunmuştur. Sürükleme katsayısı (Cd), sürtünme faktörü (Cf), boyutsuz yeniden birleşme uzunluğu (Lr/B) ve ortalama Nusselt sayısı değerlerinin blokaj oranına göre değişimini veren bağıntılar elde edilmiştir.

We present a continuum-based approach to predict the structure and thermodynamic properties of confined fluids at multiple length-scales, ranging from a few angstroms to macro-meters. The continuum approach is based on the empirical potential-based quasi-continuum theory (EQT) and classical density functional theory (cDFT). EQT is a simple and fast approach to predict inhomogeneous density and potential profiles of confined fluids. We use EQT potentials to construct a grand potential functional for cDFT. The EQT-cDFT-based grand potential can be used to predict various thermodynamic properties of confined fluids. In this work, we demonstrate the EQT-cDFT approach by simulating Lennard-Jones fluids, namely, methane and argon, confined inside slit-like channels of graphene. We show that the EQT-cDFT can accurately predict the structure and thermodynamic properties, such as density profiles, adsorption, local pressure tensor, surface tension, and solvation force, of confined fluids as compared to the molecular dynamics simulation results. [ABSTRACT FROM AUTHOR]

The distribution of wall static and stagnation (CP and CPO) pressure coefficient on a flat rectangular element by impinging air jet from the hexagonal orifice is obtained from experimentation. The past research studies helped to identify key parameters such as orifice geometry, jet exit-to-plate-distance (Z/dj), test section inclination (θ), jet Reynold number (Re), lateral distance-to-jet diameter (X/dj), test surface type and geometry, for better and acceptable results. The experimental outcome helps to know the effect of identified key parameters on wall static and stagnation pressure on a rectangular test plate in a confined flow path. The independent nature of wall static pressure is observed for all jet Reynold number (10000 ≤ Re ≤ 50000). Higher pressure coefficient values were observed at lower Z/dj = 1, X/dj = 0 and θ = 0. A significant drop in CP values are seen with the increase in Z/dj, X/dj and θ. The experimental CP and CPO contribution of confined flow are compared against the unconfined flow, around 48% to 58% enhancement is observed when confinement is used. Experimental pressure drop measurements across orifice were made and pressure loss coefficient (PC) for hexagonal orifice of confined and unconfined condition are reported. [ABSTRACT FROM AUTHOR]

Smoothed particle hydrodynamics (SPH) is a meshfree, Lagrangian, particle method, which combines the advantages of Euler and Lagrangian method and has special advantages in simulating violent non-linear fluid–structure interaction problems. In this study, an improved SPH method is used to simulate four fluid–structure interaction cases. Firstly, the violent flow in the mould filling process is simulated to validate that the SPH model can obtain accurate flow patterns with any complex solid boundaries. Secondly, the dam-break flow against a vertical wall is simulated, and the pressure curves are compared with the experiment. Then liquid sloshing problems under different external excitations are simulated, and the complex coupling characteristics can be captured. Finally, the interactions between fluid and floating bodies are researched. The obtained numerical results show good agreement with the results from other sources, and clearly demonstrate the effectiveness of the presented SPH method in modelling fluid–structure interaction problems.

The hybrid aeroacoustic approach is an efficient way to address the issue of the disparity of scales in Computational AeroAcoustics (CAA) at low Mach numbers. In the present paper, three wave equations governing propagation of flow-induced sound of low Mach number flows, namely the Perturbed Convective Wave Equation (PCWE), Ribner’s Dilatation (RIB) equation, and Lighthill’s wave equation, are applied using the Finite Element Method (FEM). An airflow through a circular pipe with a half-moon-shaped orifice at three operating flow speeds is considered, where validation data from measurements on a dedicated test rig is available. An extensive analysis of the flow field is provided based on the results of the incompressible flow simulation. The resulting acoustic source terms are investigated, and the relevant source term contributions are determined. The results of the acoustic propagation simulations revealed that the PCWE and RIB are best suited for the present task. The overall deviation of the predicted pressure spectra from the measured mean values amounted to 2.26 and 2.13 times the standard deviation of the measurement compared to 3.55 for Lighthill’s wave equation. Besides reliably predicting the flow-induced sound, the numerical procedure of source term computation is straightforward for PCWE and RIB, where the source term contributions, shown to be relevant, solely consist of time derivatives of the incompressible pressure. In contrast, the Lighthill source term involves spatial derivatives and, thus, is strongly dependent on the spatial resolution and the numerical method actually used for approximating these terms.

Smoothed particle hydrodynamics (SPH) is a meshfree, Lagrangian, particle method, which combines the advantages of Euler and Lagrangian method and has special advantages in simulating violent non-linear fluid–structure interaction problems. In this study, an improved SPH method is used to simulate four fluid–structure interaction cases. Firstly, the violent flow in the mould filling process is simulated to validate that the SPH model can obtain accurate flow patterns with any complex solid boundaries. Secondly, the dam-break flow against a vertical wall is simulated, and the pressure curves are compared with the experiment. Then liquid sloshing problems under different external excitations are simulated, and the complex coupling characteristics can be captured. Finally, the interactions between fluid and floating bodies are researched. The obtained numerical results show good agreement with the results from other sources, and clearly demonstrate the effectiveness of the presented SPH method in modelling fluid–structure interaction problems. [ABSTRACT FROM AUTHOR]

Scalable synthesis of transfer-free graphene over insulators offers exciting opportunity for next-generation electronics and optoelectronics. However, rational design of synthetic protocols to harvest wafer-scale production of directly grown graphene still remains a daunting challenge. Herein we explore a batch synthesis of large-area graphene with wafer-scale uniformity by virtue of direct chemical vapor deposition (CVD) on quartz. Such a controllable CVD approach allows to synthesize 30 pieces of 4-inch graphene wafers in one batch, affording a low fluctuation of optical and electrical properties. Computational fluid dynamics simulations reveal the mechanism of uniform growth, indicating thermal field and confined flow field play leading roles in attaining the batch uniformity. The resulting wafer-scale graphene enables the direct utilization as key components in optical elements. Our method is applicable to other types of insulating substrates (e.g., sapphire, SiO2/Si, Si3N4), which may open a new avenue for direct manufacture of graphene wafers in an economic fashion. [ABSTRACT FROM AUTHOR]

In many industrial applications, fluid flow around bluff bodies is confined between two parallel walls. This paper provides a thorough review of the studies on the confined flow around bluff bodies of different cross-sections including circular, square, and rectangular. Particular emphasis is put on the effect of blockage ratio on the steady-unsteady wake transition, shear layer transition, boundary layer transition, Strouhal number, aerodynamic forces, vortex formation length, and vortex inversion length. An increase in blockage ratio postpones steady-unsteady wake transition, hastens the shear layer transition, and increases the drag and Strouhal number. With increasing blockage ratio, fluctuating lift force for circular cylinder reduces in shear layer transition I but enhances in shear layer transition III. A square cylinder undergoes a greater blockage effect than a circular cylinder. The research gap in the literature is pinpointed and put forward for future investigations. • This paper provides a thorough review of confined flow around various bluff bodies. • An increase in blockage ratio postpones steady-unsteady wake transition but hastens the shear layer transition. • Increasing BR for circular cylinder reduces and enhances fluctuating lift in shear layer transitions I and III, respectively. • A square cylinder undergoes a greater blockage effect than a circular cylinder. [ABSTRACT FROM AUTHOR]

Sınırlandırılmış bir kanal içerisinde yer alan tek kare silindir (KS) üzerinden iki boyutlu laminer sürekli akış için blokaj (β=B/H) oranının ısı transferi ve akış karakteristiklerine olan etkisi incelenmiştir. Çalışmada Reynolds sayısı Re=40 değerinde sabit tutulurken blokaj oranı β=0.125-0.8 değerleri arasında değiştirilmiştir. Hesaplamalarda ANSYS CFX 14.0 kullanılmıştır. Blokaj oranı etkisinin KS yüzeyleri üzerindeki sürükleme katsayısı (Cd), sürtünme katsayısı (Cf), boyutsuz yeniden birleşme uzunluğu (Lr/B) ve ortalama Nusselt sayısı (Nu) üzerine olan etkileri incelenmiştir. Blokaj oranı arttıkça sürükleme katsayısı (Cd), sürtünme katsayısı (Cf) ve ortalama Nusselt sayısı (Nu) değerlerinin arttığı ancak boyutsuz yeniden birleşme uzunluğu (Lr/B) değerinin azaldığı bulunmuştur. Sürükleme katsayısı (Cd), sürtünme faktörü (Cf), boyutsuz yeniden birleşme uzunluğu (Lr/B) ve ortalama Nusselt sayısı değerlerinin blokaj oranına göre değişimini veren bağıntılar elde edilmiştir.

In order to determine the significance and importance order of the jet performance caused by fan nozzle's geometric parameters of aero-engine online cleaning equipment, the orthogonal experiment method, standard k–ɛ turbulence model and volume of fluid (VOF) two-phase flow algorithm are used to research the jet of fan nozzle. The sensitivity of nozzle contraction angle, nozzle area ratio, nozzle incision angle, nozzle exit thickness and offset to the influence of fan nozzle spray angle was verified by range analysis and variance analysis. The results of the two analysis methods consistently show that the main order of influence of the geometric parameters of fan nozzle on spray angle is nozzle area ratio, nozzle incision angle, nozzle exit thickness, offset and nozzle convergence angle. The former two have more influence on the spray angle than the others. Finally, the simulation results and orthogonal test results are verified by the injection experiment of the fan nozzle. The jet shape is mainly determined by the nozzle area ratio and incision angle. The larger the incision angle and the nozzle area ratio are, the smaller the spray angle becomes.

Highlights • Experimental investigation on flow through confined metal foam passage. • Investigating any possible effect of confined passage on flow characteristics. • Comparing RUC model to the experimental result. • Proposing correction in the RUC model based on experimental outcomes. Abstract This paper deals with flow behaviour through a confined passage filled with metal foams (MFs). Two sets (four samples) of aluminium foams of 20 and 40 ppi pore densities with 9–11% and 12–16% relative densities (RD) individually (as per supplier’s specification) were used for the study. Former RD belonged to uncompressed metal foams whereas the later was achieved by unidirectional compression of metal foam of 6–8% RD. Measured porosities of the uncompressed 20 and 40 ppi MF samples were equal, which was 0.88. However, measured porosities were 0.83 and 0.86 for compressed 20 and 40 ppi MF samples. Flow characteristics of both compressed and uncompressed samples were experimentally measured. Fourie-Plessis’s representative unit cell (RUC) model was used as an analytical tool for estimating pressure drop gradient through MFs because of its simplistic approach and independency from flow characteristics. Possible encasing wall effect on pressure drop was theoretically conducted. Due to a significant difference in estimated and experimentally obtained pressure drop gradient, authors have proposed a modification in the RUC model. The modified version of the RUC model was able to estimate pressure drop gradient of the uncompressed samples with a considerably higher accuracy. [ABSTRACT FROM AUTHOR]

Experiments of nanoconfined water between graphene sheets at high pressure suggest that it forms a square ice structure (Algara‐Siller et al., Nature, 2015, 519, 443). Molecular dynamics (MD) simulations have been used to attempt to recreate this structure, but there have been discrepancies in the structure formed by the confined water depending on the simulation set‐up that was employed and particularly on the choice of water model. Here, using classical molecular dynamics simulations, we have systematically investigated the effect that three different water models (SPC/E, TIP4P/2005 and TIP5P) have on the structure of water confined between two rigid graphene sheets with a 0.9 nm separation. We show that the TIP4P/2005 and the TIP5P water models form a hexagonal AA‐stacked structure, whereas the SPC/E model forms a rhombic AB‐stacked structure. Our work demonstrates that the formation of these structures is driven by differences in the strength of hydrogen bonds predicted by the three water models, and that the nature of the graphene/water interaction only mildly affects the phase diagram. Considering the available experimental data and first‐principle simulations we conclude that, among the models tested, the TIP4P/2005 and TIP5P force fields are for now the most reliable when simulating water under confinement. © 2018 Wiley Periodicals, Inc. Water freezes under 2D confinement. But what is the symmetry of this 2D ice? Different water models predict different structures and phase transition pressures depending on the strength of the hydrogen bonds they form [ABSTRACT FROM AUTHOR]

A novel type of two-phase heat spreader based on a flat confined thermosyphon is proposed for electronics cooling applications. Two wickless flat copper-water heat pipes with an inner thickness of 3 mm were experimentally investigated for two-phase flow visualizations and characterization of thermal performance. The effects of heat input, filling ratio, inclination, and saturation temperature were studied. Experimental results show that the confinement of the fluid inside the heat spreader induces confined boiling phenomenon with a strong coupling between condensation and boiling mechanisms. They also highlight an enhancement of heat transfer and interesting performance such as high heat transfer capability (tested up to 10 W/cm 2 with a corresponding thermal resistance around 0.07 K/W at an optimum filling ratio), low sensitivity to inclination and higher degree of freedom on heat sources location compared to a classical thermosyphon. [ABSTRACT FROM AUTHOR]

In this paper a full nonlinear model 1 1 A model that encompasses nonlinear fluid dynamics as well as nonlinear structural dynamics. is presented for the dynamics of a cantilevered cylinder, terminated by an ogival free end, and subjected to confined, inverted axial flow. This system is also known as “a free-clamped cylinder in axial flow”, since the flow is directed from the free end towards the clamped one. All the fluid-related forces and the gravity-related terms are derived separately to third-order accuracy; the inviscid forces are modelled using an extension of Lighthill’s slender-body analysis to the same accuracy, and the viscous forces are obtained semi-empirically. The boundary conditions related to the free end are also derived separately, to first-order accuracy, and added to the model. The final equation of motion is obtained via Hamilton’s principle, then discretized and solved numerically using AUTO and MATLAB software. The stability of the system is investigated by means of bifurcation diagrams, time histories, phase-plane and power-spectral-density plots, and the dynamical behaviour is compared to theoretical predictions and experimental observations, from the literature, for systems that have the same parameters. The theory is in good qualitative agreement with the experiments, and also good quantitative agreement in terms of the critical flow velocity of instability. [ABSTRACT FROM AUTHOR]

The lithium ion recognition is receiving significant attention because of its application in pharmaceuticals, lubricants and, especially, in energy technology. We present a nanofluidic device for specific lithium ion recognition via host-guest complexation in a confined environment. A lithium-selective receptor molecule, the aminoethyl-benzo-12-crown-4 (BC12C4-NH2), is designed and functionalized on single conical nanopores in polyethylene terephthalate (PET) membranes. The native carboxylic acid groups on the pore walls are covalently linked with the crown ether moieties and the process is monitored from the changes in the current-voltage (I-V) curves. The B12-crown-4 moieties are known to specifically bind with lithium ions and when the modified pore is exposed to different alkali metal chloride solutions separately, significant changes in the ion current and rectification are only observed for lithium chloride. This fact suggests the generation of positively charged B12C4-Li+ complexes on the pore surface. Furthermore, the nanofluidic diode is able to recognize the lithium ion even in the presence of high concentrations of potassium ions in the external electrolyte solution. Thus, this nanodevice suggests a strategy to miniaturize nanofluidic porous systems for efficient recognition, extraction, and separation of lithium from raw materials. [ABSTRACT FROM AUTHOR]

The recent technological developments in electronics and product miniaturization demand high-performance thermal management to secure the reliability and the operating characteristics of the components. The present 2-D numerical study investigates heat transfer characteristics of different geometries of electronic components such as capacitors, resistors, condensers, and pin fin heat sinks and the additional effect of their optimized orientation in the confined forced convective air-cooling arrangement. The flow confinement effect is induced by preferring the blockage ratio (channel height/characteristic length of various geometries) to be 4. A finite volume-based flow solver is used to solve flow and energy equations. The resulting flow and heat transfer characteristics of various geometries studied from pressure contour, temperature contour, total drag coefficient, total pressure drop, Strouhal number, and volume goodness factor, and local and average Nusselt numbers for various Reynolds numbers such as 200, 400, 600, and 800. The average forced convection heat transfer enhancement depends on profoundly attached flow velocity, not on flow shedding. As per the thermal hydraulic performance, 45° tilted square front cum segment is the best among geometries considered for the analysis at this confinement. [ABSTRACT FROM PUBLISHER]

Recurrent neural network (RNN) and Long Short-term Memory (LSTM) networks are the common go-to architecture for exploiting sequential information where the output is dependent on a sequence of inputs. However, in most considered problems, the dependencies typically lie in the latent domain which may not be suitable for applications involving the prediction of a step-wise transformation sequence that is dependent on the previous states only in the visible domain with a known terminal state. We propose a hybrid architecture of convolution neural networks (CNN) and stacked autoencoders (SAE) to learn a sequence of causal actions that nonlinearly transform an input visual pattern or distribution into a target visual pattern or distribution with the same support and demonstrated its practicality in a real-world engineering problem involving the physics of fluids. We solved a high-dimensional one-to-many inverse mapping problem concerning microfluidic flow sculpting, where the use of deep learning methods as an inverse map is very seldom explored. This work serves as a fruitful use-case to applied scientists and engineers in how deep learning can be beneficial as a solution for high-dimensional physical problems, and potentially opening doors to impactful advance in fields such as material sciences and medical biology where multistep topological transformations is a key element. [ABSTRACT FROM AUTHOR]

Coastal sites with typical lagoon features are extremely vulnerable, often suffering from scarce circulation. Especially in the case of shallow basins subjected to strong anthropization and urban discharges, it is fundamental to monitor their hydrodynamics and water quality. The proper detection of events by high performance sensors and appropriate analysis of sensor signals has proved to be a necessary tool for local authorities and stakeholders, leading to early warning and preventive measures against environmental degradation and related hazards. At the same time, assessed datasets are not only essential to deepen the knowledge of the physical processes in the target basin, but are also necessary to calibrate and validate modelling systems providing forecasts. The present paper aims to show how long-term and continuous recordings of meteorological and hydrodynamic data, collected in a semi-enclosed sea, can be managed to rapidly provide fundamental insights on its hydrodynamic structure. The acquired signals have been analyzed in time domain, processed and finally, correlated. The adopted method is simple, feasible and easily replicable. Even if the results are site-dependent, the procedure is generic, and depends on having good quality available data. To show how this might be employed, a case study is examined. In fact, it has been applied to a coastal system, located in Southern Italy, where two monitoring stations are placed in two interconnected basins. The inferred results show that the system is not wind dominated, and that the annual trends in the wind regime, wave spreading and current circulation are not independent, but rather reiterate. These deductions are of great interest as a predictive perspective and for numerical modelling. [ABSTRACT FROM AUTHOR]

A hydropower conversion system based on vortex-induced vibrations is investigated experimentally. It consists in a cylinder immerged in a low-velocity flow in a channel (under 1 m/s), which is linked to a variable stiffness spring, so that the natural frequency of the system might be controlled. Current studies report investigations on marine applications. Although rivers or channels constitute a strong energy potential, they are not exploited enough. In this paper, we will investigate the feasibility of such a system implantation in a confined flow in a channel, with important edge effects. We propose a study of the effects of a confined flow on the efficiency of the system. We will highlight feasible improvements, particularly through automatic control strategies (generator behaviour, system's natural frequency). Moreover, we show the strong influence of confinement on the flow topology through velocity field measurements using pulse-pair method. [ABSTRACT FROM AUTHOR]

Fluid properties of five binary mixtures relevant to shale gas and light tight oil in confined nano-channels are studied. Canonical (NVT) Monte Carlo simulations are used to determine internal energies of departure of pure fluids using the RASPA software system (Dubbeldam et al., 2015). The linear mixing rule proposed by Lucia et al. (2012) is used to determine internal energies of departure for mixtures, U M D , in confined spaces and compared to U M D from direct NVT Monte Carlo simulation. The sensitivity of the mixture energy parameter, a M , for the Gibbs-Helmholtz constrained (GHC) equation, confined fluid molar volume, V M , and bubble point pressure are studied as a function of uncertainty in U M D . Results show that the sensitivity of confined fluid molar volume to 5% uncertainty in U M D is less than 1% and that the GHC equation predicts physically meaningful reductions in bubble point pressure for light tight oils. [ABSTRACT FROM AUTHOR]

Finding the mode, in which two component laser Doppler velocimetry can be applied to flows confined in cylindrical tubes or vessels, was the aim of this study. We have identified principle issues that influence the propagation of laser beams in laser Doppler velocimetry system, applied to flow confined in cylindrical tube. Among them, the most important are influences of fluid and wall refractive indices, wall thickness and internal radius ratio and beam intersection angle. In analysis of the degrees of these influences, we have applied mathematical model, based on geometrical optics. The separation of measurement volumes, that measure different velocity components, has been recognized as the main drawback. To overcome this, we propose a lens with dual focal length - primary focal length for the measurement of one velocity component and secondary focal length for the measurement of the other velocity component. We present here the procedure for calculating the optimal value of secondary focal length, depending on experimental set-up parameters. The mathematical simulation of the application of the dual focal length lens, for chosen cases presented here, confirmed the accuracy of the proposed procedure. [ABSTRACT FROM AUTHOR]

Predicting the rock fragmentation obtained in drawpoints or secondary fragmentation is crucial in Block caving application since many engineering decisions are based on this key variable. These can include drawpoints size and spacing, equipment selection; draw control procedures, production rates, dilution entry and operational blasting requirements. Secondary fragmentation depends on several variables including structures, rock mass strength, the vertical pressure acting on the column, the rate of draw, and the height of the ore column. In order to study these variables, 18 experiments on gravity flow under confinement were run to quantify the fragmentation occurring in a draw column. Based on these experiments, a mathematical model was developed the basis of which considers a modified comminution model fitted using the experimental data. Finally, the fitted model was scaled up to represent what is expected to occur in practice compared to field data with an estimated accuracy of around 1.5% of size distributions. An approximation of the secondary fragmentation expected as a function of draw height and vertical pressure, for example for 400 m of ore column (mean vertical pressure of 4.2 MPa), the mean size, d , could decrease from 0.82 to 0.47 m and the large size, d , from 1.08 to 0.62 m. [ABSTRACT FROM AUTHOR]

An abundance of empirical correlations between a dynamic contact angle and a capillary number representing a translational velocity of a contact line have been provided for the last decades. The experimentally obtained dynamic contact angles are inevitably apparent contact angles but often undistinguished from microscopic contact angles formed right on the wall. As Bonn et al. ["Wetting and spreading," Rev. Mod. Phys. 81, 739-805 (2009)] pointed out, however, most of the experimental studies simply report values of angles recorded at some length scale which is quantitatively unknown. It is therefore hard to evaluate or judge the physical validity and the generality of the empirical correlations. The present study is an attempt to clear this clutter regarding the dynamic contact angle by measuring both the apparent and the microscopic dynamic contact angles from the identical data sets in a well-controlled manner, by means of numerical simulation. The numerical method was constructed so that it reproduced the fine details of the flow with a moving contact line predicted by molecular dynamics simulations [T. Qian, X.Wang, and P. Sheng, "Molecular hydrodynamics of the moving contact line in two-phase immiscible flows," Commun. Comput. Phys. 1, 1-52 (2006)]. We show that the microscopic contact angle as a function of the capillary number has the same form as Blake's molecular-kinetic model [T. Blake and J. Haynes, "Kinetics of liquid/liquid displacement," J. Colloid Interface Sci. 30, 421-423 (1969)], regardless of the way the flow is driven, the channel width, the mechanical properties of the receding fluid, and the value of the equilibrium contact angle under the conditions where the Reynolds and capillary numbers are small. We have also found that the apparent contact angle obtained by the arc-fitting of the interface behaves surprisingly universally as claimed in experimental studies in the literature [e.g., X. Li et al., "An experimental study on dynamic pore wettability," Chem. Eng. Sci. 104, 988-997 (2013)], although the angle deviates significantly from the microscopic contact angle. It leads to a practically important point that it suffices to measure arc-fitted contact angles to make formulae to predict flow rates in capillary tubes. [ABSTRACT FROM AUTHOR]

The key concept of transient age distributions is that the age distribution of water at a fixed spatial location may change over time. Most previous studies involving groundwater age have relied on the assumption of a steady state age distribution over time. This assumption simplifies the problem and can also be used for predictive modeling, but few studies have addressed the validity of this assumption directly. We explore the variability of transient age distributions in simplified, confined groundwater flow systems. The main focus is determining when it is necessary to explicitly model the transience. A combination of analytical and numerical methods is used to demonstrate the transient variability of age for several hypothetical velocity fields and trial domains. Transient effects are strongest for samples taken near fluctuating interfaces or boundaries. Lastly, we investigate how heterogeneity and transient boundary signals interact in a 2-D domain. These impacts are difficult to generalize, so the article closes with a set of considerations to assist readers in determining whether or not transience is likely to play a significant role in other applications. The main conclusion is that high-frequency oscillations in flow boundaries have negligible impacts on confined age distributions but long-term trends in flow velocities (increasing or decreasing) can cause significant changes over time. [ABSTRACT FROM AUTHOR]

This study seeks to characterise the breakdown of the steady two-dimensional solution in the flow around a 180-degree sharp bend to infinitesimal three-dimensional disturbances using a linear stability analysis. The stability analysis predicts that three-dimensional transition is via a synchronous instability of the steady flows. A highly accurate global linear stability analysis of the flow was conducted with Reynolds number Re < 1150 and bend opening ratio (ratio of bend width to inlet height) 0:2≤β ≤5. This range of Re and β captures both steady-state two-dimensional flow solutions and the inception of unsteady two-dimensional flow. For 0:2 ≤ β ≤ 1, the two-dimensional base flow transitions from steady to unsteady at higher Reynolds number as β increases. The stability analysis shows that at the onset of instability, the base flow becomes three-dimensionally unstable in two different modes, namely a spanwise oscillating mode for β D 0:2 and a spanwise synchronous mode for β > 0:3. The critical Reynolds number and the spanwise wavelength of perturbations increase as β increases. For 1 < β ≤ 2 both the critical Reynolds number for onset of unsteadiness and the spanwise wavelength decrease as β increases. Finally, for 2 < β ≤ 5, the critical Reynolds number and spanwise wavelength remain almost constant. The linear stability analysis also shows that the base flow becomes unstable to different three-dimensional modes depending on the opening ratio. The modes are found to be localised near the reattachment point of the first recirculation bubble. [ABSTRACT FROM AUTHOR]

Recent field work at the Wax Lake Delta (WLD) in coastal Louisiana indicates lateral outflow from channels to islands upstream of the receiving basin; in this region of the delta the flow transitions from confined to unconfined (i.e., from a uniform discharge profile to a nonuniform discharge profile). The hydraulics of this transition zone and the controls exerted by vegetation, topography, and river discharge fluctuations are analyzed in this work. The shallow water equations are numerically solved in two model domains: an idealized channel-island complex and the full domain of the WLD. In both domains, a significant fraction of the river discharge flows laterally from the channels to deltaic islands before reaching the receiving basin. Vegetation roughness within the delta islands significantly impacts the fraction and rate of lateral outflow from the channel, while river discharge fluctuations have a limited effect, due to the backwater control on the subcritical flow. The presence of vegetation in the islands tends to increase velocities within the channel, except in the region upstream of significant lateral outflow, where the velocity increases with decreasing vegetation roughness due to the lowered water level that reduces the flow cross-section. The topography establishes a lateral water surface gradient between the channel and the islands even with low vegetation roughness, which drives lateral flow. A velocity spreading angle is used to mark the transition from confined to unconfined flow; the angle generally increases up to the onset of unconfined flow and then decreases as the flow approaches the receiving basin and the flow in the island tends to align with that of the channel. The lateral outflow from the primary channels influences the hydraulics of the flow throughout the backwater length. The transition between confined and unconfined flow in coastal river deltas has a significant impact on the flow hydraulics and the resulting transport dynamics of solids and solutes. [ABSTRACT FROM AUTHOR]

The application of lateral jets into a confined flow in industry is a common method for mixing of reagents. However, there is limited understanding of the fundamentals surrounding the flow structures, flow evolution and their respective effects on a downstream outflow of a round jet nozzle when there are multiple jets inside the nozzle. To address this, an experimental study of the near-field outflow of a turbulent round water jet affected by multiple side-jet injected laterally into the round flow upstream of the nozzle exit has been conducted. Planar Laser Induced Fluorescence and Particles Image Velocimetry were used to investigate the fluid mixing and velocity in the near-field, respectively. The influence of jet to cross-flow momentum ratio on flow characteristics; including mixing and turbulence intensity, were assessed by varying the primary jet and side injection flow rates. Results indicated that side injection has major effects on the resulting near-field region flow. Flow cases with side-jets show an increase in shear layer roll-ups and spread. Velocity decay rates and turbulence intensity within the jet core increase with increasing jet to cross-flow momentum ratio. However, these effects extend only to the near-field region, as no significant perturbations beyond two primary jet diameters downstream are observed. This indicates that the side injection has significant effect on the flow and mixing in the near-field region, but minor influences further downstream. [ABSTRACT FROM AUTHOR]

The flow field surrounding an axisymmetric, confined, impinging jet was investigated with a focus on the early development of the triple-layered wall jet structure. Experiments were conducted using stereo particle image velocimetry at three different confinement gap heights (2,4, and 8 jet diameters) across Reynolds numbers ranging from 1000 to 9000. The rotating flow structures within the confinement region and their interaction with the surrounding flow were dependent on the confinement gap height and Reynolds number. The recirculation core shifted downstream as the Reynolds number increased. For the smallest confinement gap height investigated, the strong recirculation caused a disruption of the wall jet development. The radial position of the recirculation core observed at this small gap height was found to coincide with the location where the maximum wall jet velocity had decayed to 15% of the impinging jet exit velocity. After this point, the self-similarity hypothesis failed to predict the evolution of the wall jet further downstream. A reduction in confinement gap height increased the growth rates of the wall jet thickness but did not affect the decay rate of the wall jet maximum velocity. For jet Reynolds numbers above 2500, the decay rate of the maximum velocity in the developing region of the wall jet was approximately -1.1, which is close to previous results reported for the fully developed region of radial wall jets. A much higher decay rate of -1.5 was found for the wall jet formed by a laminar impinging jet at Re = 1000. [ABSTRACT FROM AUTHOR]

When confined flow is oscillated from and into a quiescent volume, a periodic coherent flow that resembles that of a conventional jet can be generated. Such a jet, termed a synthetic jet, has been investigated for thermal management by causing it to impinge onto a heated surface. Because of its fluctuating nature, the impinging jet thus formed is dominated by vortices that are advected towards the surface. This surface-vortex interaction is key to understanding the fundamental mechanisms of convective heat transfer by the impinging synthetic jet, which motivates this work along with the search for the improvement of the system thermal efficiency. A canonical geometry was developed to investigate the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner by which it is produced. The unsteady two-dimensional Navier-Stokes equations and the convection-diffusion equation were solved using a finite volume approach in order to capture the complex time dependent flow field. The Q-criterion (Hunt et al., 1988), which defines vortices as connected fluid regions with positive second invariant of the velocity gradient tensor was utilized to identify vortices without ambiguity. A definition of the jet characteristic velocity was developed rigorously based on the vortex dynamics produced by the jet. It is equivalent to the common definition accepted in the literature, which has been successfully used to match the dynamics of synthetic and steady jets, but which was developed using heuristic reasoning. When the primary vortex advects in a direction parallel to the target surface it gives rise to a secondary vortex with opposite net vorticity. This secondary vortex is largely responsible for enhancement of the heat transfer within the wall jet region. Under certain conditions, vortex coalescence occurs, leading to degradation in the heat transfer enhancement due to the reduction in the number of secondary vortices interacting with the heated surface. By understanding, quantifying and predicting the mechanisms that drive the phenomenon of vortex merging, optimum conditions of operation are demonstrated, ultimately leading to higher efficiencies by maximizing the heat transfer at similar pumping costs. [ABSTRACT FROM AUTHOR]

Extraordinarily rapid mixing has surprisingly been achieved in confined mixing layers by active forcing within an optimal narrow frequency band (Wang, PhD Dissertation, 1999; Wang, Chem. Eng. Sci., 2003; Wang, AIChE J., 2006). However, the corresponding mechanism of the large receptivity and rapid mixing is still unclear so far. Since it is found that the optimal frequency that corresponds to the rapid mixing is independent of Reynolds number or bulk flow velocity, we postulate that some type of resonance of the experimental setup or fluid-structure interaction might be the cause. In this manuscript, the possible influence of acoustic resonance and fluid-structure interactions are investigated by both parametric study through flow visualization and finite element method (FEM). The results reveal the optimal forcing frequency of the fast mixing is tightly related to a complicated acoustic resonance mechanism of the entire water tunnel system, and when the confined mixing layers are forced under the resonance frequency, ultrafast mixing can be observed. This finding may lead to new technology for mixing and heat transfer enhancement. [ABSTRACT FROM AUTHOR]