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One of the often employed flow structures in vertical pumping stations is the siphon outlet pipe. It has a complicated geometric structure and internal flow field, which directly affects how effectively, safely, and steadily pumping stations operate. The entire flow conduit of the vertical axial flow pump device was adopted as the study object in order to elucidate the mechanism of the internal biased flow of the siphon outlet pipe caused by the axial flow pump under different flow conditions, and the three-dimensional unsteady flow field of the vertical axial flow pump device was numerically solved using the numerical simulation technique. Physical model tests were used to confirm the results of the numerical simulation. The findings demonstrate that there is a horizontal bias in the flow of the inlet surface and outlet surface of the elbow pipe of the siphon outlet pipe, and that when the flow rate rises, the degree of the horizontal bias in the flow gradually diminishes, and the ratio of biased flow gradually decreases. The flow in hump segment presents up-and-down flow, and when the flow rate increases, the ratios of biased flow first rises before falling. The major causes of the production of biased flow in the outlet pipe are residual velocity circulation at the guide vane's outlet, wall constraint at the elbow pipe, and flow inertia; Under various flow conditions, the morphologies of the vortex structures in the outlet pipe vary; the characteristics of the energy conversion of each part of the pump device are disclosed, with the inlet pipe having the lowest proportion of energy conversion. Under the optimal flow condition, the elbow-inlet pipe's proportion of energy conversion is only around 1.04%, and the proportions of the guide vane, 60° elbow pipe, and siphon outlet pipe's energy conversion are significant and fluctuate with time. The proportion of energy conversion in the elbow-inlet pipe and siphon outlet pipe steadily increases as flow rate rises, but the proportion in the guide vane and 60° elbow pipe drops initially before increasing. [ABSTRACT FROM AUTHOR]

This paper describes a numerical simulation within the framework of a two-dimensional shallow water model of interaction of a solitary wave with a steady rectangular semi-submerged structure. The results of this study are compared with the results of calculations based on a model of irrotational three-dimensional flows, and it is shown that the simulation accuracy at small incident wave amplitudes is satisfactory. It is also revealed that, the solitary wave diffraction on the cylinder surface is neglected when using a one-dimensional shallow water model, then the maximum values of wave runup on the edge of the cylinder and the force loads on it become overestimated. [ABSTRACT FROM AUTHOR]

This work studies the three-dimensional flow dynamics around a rotating circular cylinder of finite length, whose axis is positioned perpendicular to the streamwise direction. Direct numerical simulations and global stability analyses are performed within a parameter range of Reynolds number $Re=DU_\infty /\nu (based on cylinder diameter $D$ , uniform incoming flow velocity $U_\infty$), length-to-diameter ratio ${\small \text{AR}}=L/D\leq 2$ and dimensionless rotation rate $\alpha =D\varOmega /2U_\infty \leq 2$ (where $\varOmega$ is rotation rate). By solving Navier–Stokes equations, we investigated the wake patterns and explored the phase diagrams of the lift and drag coefficients. For a cylinder with ${\small \text{AR}}=1$ , we found that when the rotation effect is weak ($0\leq \alpha \lesssim 0.3$), the wake pattern is similar to the unsteady wake past the non-rotating finite-length cylinder, but with a new linear unstable mode competing to dominate the saturation state of the wake. The flow becomes stable for $0.3\lesssim \alpha \lesssim 0.9$ when $Re. When the rotation effect is strong ($\alpha \gtrsim 0.9$), new low-frequency wake patterns with stronger oscillations emerge. Generally, the rotation effect first slightly decreases and then sharply increases the $Re$ threshold of the flow instability when $\alpha$ is relatively small, but significantly decreases the threshold at high $\alpha$ ($0.9). Furthermore, the stability analyses based on the time-averaged flows and on the steady solutions demonstrate the existence of multiple unstable modes undergoing Hopf bifurcation, greatly influenced by the rotation effect. The shapes of these global eigenmodes are presented and compared, as well as their structural sensitivity, visualising the flow region important for the disturbance development with rotation. This research contributes to our understanding of the complex bluff-body wake dynamics past this critical configuration. [ABSTRACT FROM AUTHOR]

The comparative numerical analysis of the butterfly valve is performed to simulate the fluid flow inside the pipe. In this study, the performance of the single-disc butterfly valve flow is investigated in the opening process. The flow separation for the turbulent case is also examined to observe the velocity and pressure distributions since the unsteady flow can be understood well with an examination of the velocity and pressure changes. The formations of the vortexes are considered. In addition, an optimization study is done to decrease the operational coefficient values. In the result of the study, the characteristic parameters of the conventional and optimized valves are compared including, flow coefficient; drag coefficients, and dynamic torque. The changes of the characteristic parameters at different opening angles are demonstrated as tables and graphs to evaluate the performance of the valves. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of the presented aileron modification analysis is the improvement of the flight handling by eliminating adverse phenomena in the aileron area, such as aileron shaking movements, without the risk of deterioration of flow characteristics during manoeuvres. It was also crucial to reduce aileron forces acting on the control stick. Design/methodology/approach: Numerical CFD analysis of the aileron system with modifications of sealing in the aileron gap area were performed. The effect of the caulking strip at the upper surface of the aileron gap was determined, as well as caulking at the entrance to the aileron gap on the bottom surface. A solution has also been proposed, consisting of completely closing the aileron gap by using a diaphragm. The three-dimensional flow analysis was carried out, allowing localization of the flow disturbances in the aileron gap at cruising speed. The result of the analysis are the aerodynamic and the hinge moment coefficients determining forces on the control stick, depending on the type of seals. Findings: It has been shown that the use of subsequent sealing means has a direct impact on the hinge moment value. The results of the CFD analysis showed that the more closed aileron gap is, the higher aileron forces are generated on the control stick. Completely closing the flow in the aileron gap changes the character of the force generated on the control stick. Practical implications: Through CFD analyses of the aileron gap sealing in the PZL-130 Orlik aircraft, the impact of successive aileron gap sealing on the aileron efficiency was determined. It has been shown that simple change of the aileron gap size by the slat sealing can significantly affect the value of the forces generated on the control stick. Originality/value: The research using CFD methods allowed to verify the impact of the particular type of aileron gap sealing on the hinge moment value and thus to determine proper sealing configuration for the PZL-130 Orlik aircraft at low computational cost. [ABSTRACT FROM AUTHOR]

The three-dimensional instability of the flow past a 5 : 1 rectangular cylinder is investigated via Floquet analysis and direct numerical simulations. A quasi-subharmonic (QS) unstable mode is detected, marking an important difference with the flow past bodies with lower aspect ratio and/or with smooth leading edge. The QS mode becomes unstable at Reynolds number (based on the cylinder thickness and free-stream velocity) $Re \approx 480$ ; its spanwise wavelength is approximately three times the cylinder thickness. The structural sensitivity locates the wavemaker region over the longitudinal sides of the cylinder, indicating that the instability is triggered by the mutual inviscid interaction of vortices generated by the leading edge shear layer. [ABSTRACT FROM AUTHOR]

Multi-structure and multi-regime flow in complex fractured system has always been a difficult problem in the study of fracture rock mass seepage theory. Based on the spatial structure and seepage characteristics of fractured rock mass, a three-dimensional seepage numerical equation was constructed according to the multi-structure medium composed of tubular pores, planar fractures, zonal faults, and massive rock mass. Focusing on the transition and evolution of linear flow, nonlinear flow, laminar flow-transition flow-turbulent flow in single structure, the multi-regime flow equation was derived, and numerical method of multi-structure and multi-regime seepage model was given. By comparing the results of pipeline flow test and three-dimensional fractured network seepage test with the numerical simulation analysis, the rationality and applicability of the model were discussed, and the practicability of large-scale discrete and continuous combined seepage model of rock mass was discussed, which provides a reference for fine simulation of fractured rock mass seepage problem. [ABSTRACT FROM AUTHOR]

We perform a thorough numerical analysis of the impact of inflow conditions on the aerodynamic performance of a tandem cascade. In particular, we investigate the effects of the incidence angle and the inlet boundary layer (IBL) thickness on the three-dimensional flow field structure and aerodynamic performance. Our results show that the gap flow strength of the tandem cascade decreases with the increase of incidence angle, and it can effectively reduce the mixing of the wakes of the forward blade (FB) and rear blade (RB). In turn, this prevents the passage vortex (PV) in the RB passage from developing along the circumferential direction. The occurrence of IBL does not modify the effects of the incidence angle on the tandem cascade, however, it reduces the load of the RB and the gap flow strength near the endwall. Under all incidence angles, IBL increases the total pressure loss of the tandem cascade, and decreases the static pressure rise (except for an incidence angle equal to -6°). The maximum loss increment is at 2° incidence angle, and the maximum static pressure rise decrement is at 6° incidence angle (Thick-IBL condition) or 7° incidence angle (Thin-IBL condition). Furthermore, we found that the presence of IBL changes the minimum loss condition from 0° (design condition) to -2° incidence angle. Our results thus indicate that in the practical engineering application of the tandem cascade, the reality that IBL degrades the tandem cascade performance in the full incidence angle range should be considered. And the strong endwall secondary flow effect caused by IBL should be considered in the tandem cascade threedimensional design, so that the tandem cascade two-dimensional performance advantage can be better played. [ABSTRACT FROM AUTHOR]

The rational design of flow fields is of vital importance for the internal distribution uniformity and overall power output of solid oxide fuel cells (SOFCs). This study reports the design of discrete cylindrical landing flow fields for planar SOFCs to tackle the in-plane unevenness problems. The effect of key geometric parameters on the internal mass and current distribution and the overall power output of SOFCs has been investigated using three-dimensional (3D) multi-physics numerical simulations. It is found that the cylindrical landing flow fields significantly reduce the local variation of mass distribution and improve the uniformity of current distribution compared with the parallel landing flow fields. The overall output performance of the cell is improved by the cylindrical landing flow field (D2.0-S1.0) due to the synergetic effect of increased pressure drop, enhanced gas transport and reduced ohmic loss. The results of this study demonstrate the effective of the cylindrical landing flow fields for improving the distribution uniformity of planar SOFCs and provide theoretical insights for further development and optimization of the cylindrical landing flow fields for related applications. • The cylindrical landing flow field is designed and studied by 3D numerical modeling for planar SOFCs. • The effectiveness of this design for improving the distribution uniformity is demonstrated. • Relationships of key structural parameters with in-plane distribution and power output are identified. • Effect of contact resistance on distribution properties is decoupled with structural variation. • Theoretical insights for further development of cylindrical landing flow fields are provided. [ABSTRACT FROM AUTHOR]

In this paper, we study the convergence analysis for a robust stochastic structure-preserving Lagrangian numerical scheme in computing effective diffusivity of time-dependent chaotic flows, which are modeled by stochastic differential equations (SDEs). Our numerical scheme is based on a splitting method to solve the corresponding SDEs in which the deterministic subproblem is discretized using a structure-preserving scheme while the random subproblem is discretized using the Euler-Maruyama scheme. We obtain a sharp and uniform-in-time convergence analysis for the proposed numerical scheme that allows us to accurately compute long-time solutions of the SDEs. As such, we can compute the effective diffusivity for time-dependent chaotic flows. Finally, we present numerical results to demonstrate the accuracy and efficiency of the proposed method in computing effective diffusivity for the time-dependent Arnold-Beltrami-Childress (ABC) flow and Kolmogorov flow in three-dimensional space. [ABSTRACT FROM AUTHOR]

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In this paper, we study the propagation speeds of reaction-diffusion-advection fronts in time-periodic cellular and chaotic flows with Kolmogorov-Petrovsky-Piskunov (KPP) nonlinearity. We first apply the variational principle to reduce the computation of KPP front speeds to a principal eigenvalue problem of a linear advection-diffusion operator with space-time periodic coefficient on a periodic domain. To this end, we develop efficient Lagrangian particle methods to compute the principal eigenvalue through the Feynman-Kac formula. By estimating the convergence rate of Feynman-Kac semigroups and the operator splitting method for approximating the linear advection-diffusion solution operators, we obtain convergence analysis for the proposed numerical method. Finally, we present numerical results to demonstrate the accuracy and efficiency of the proposed method in computing KPP front speeds in time-periodic cellular and chaotic flows, especially the time-dependent Arnold-Beltrami-Childress flow and time-dependent Kolmogorov flow in three-dimensional space. [ABSTRACT FROM AUTHOR]

This study investigates heat transfer characteristics and three-dimensional flow of non-Newtonian Casson nanofluid over a linearly stretching flat surface in the rotating frame of a reference. The current model includes the Buongiorno nanofluid model comprises nanoparticles' haphazard motion and thermo-migration. It also considered mechanisms for viscous heating and constant heat flux at the boundary. The nonlinear partial differential system modeling includes the non-Newtonian Casson fluid model and the boundary layer approximation. The system governing equations were nondimensionalized and numerically solved. A parametric study was conducted to analyze the significance of dimensionless parameters on velocities, the concentration, temperatures, Nusselt number, friction factors, and Sherwood number. The study reveals that the Casson nanoliquid temperature enhanced significantly due to the mechanisms of haphazard motion and thermo-migration. The momentum layer thickness of nano Casson fluid reduced due to the rotation phenomenon while the thermal layer structure amended notably. In the absence of rotation, there is no transverse velocity. The thermal layer structure is enhanced owing to the viscous heating process. The intense haphazard motion and thermo-migration mechanisms lead to maximum heat transfer rate at the plate. In addition, results show that the Coriolis force strength elevation shows similar axial and transverse velocities behavior. In addition, the nanoparticle concentration is observed higher due to the rotation aspect and Casson fluid parameter. Furthermore, the Casson fluid factor decreases with velocities, but the trend is the opposite for the high Casson fluid factor. The thermal and solute layer thickness growth is due to the nanoparticles' thermo-diffusion. In conclusion, the larger rotation factor increases the friction factors. The maximum plate heat transfer rate is when higher Nb and Nt are higher. [ABSTRACT FROM AUTHOR]

In this paper, we study the problem of computing the effective diffusivity for particles moving in chaotic flows. Instead of solving a convection-diffusion type cell problem in the Eulerian formulation (arising from homogenization theory for parabolic equations), we compute the motion of particles in the Lagrangian formulation, which is modeled by stochastic differential equations (SDEs). A robust numerical integrator based on a splitting method was proposed to solve the SDEs and rigorous error analysis for the numerical integrator was provided using the backward error analysis technique in our previous work. However, the upper bound on the error estimate is not sharp. To improve our result, we propose a new and uniform in time error analysis for the numerical integrator that allows us to get rid of the exponential growth factor in our previous error estimate. Our new error analysis is based on a probabilistic approach, which interprets the solution process generated by our numerical integrator as a Markov process. By exploring the ergodicity of the solution process, we prove the convergence analysis of our method in computing effective diffusivity over infinite time. We present numerical results to verify the accuracy and efficiency of the proposed method in computing effective diffusivity for several chaotic flows, especially the Arnold--Beltrami--Childress flow and Kolmogorov flow in three-dimensional space. [ABSTRACT FROM AUTHOR]

A radial turbine is one of the vital components of a helium liquefaction plant. The design of a turbine becomes critical due to its compact size and high‐speed configuration. In this study, numerical optimization has been performed for the three‐dimensional steady flow of helium gas in the radial inflow turbine of a helium liquefaction plant at a nominal condition. The mean line design is an appropriate method to obtain the approximate results. The computational fluid dynamics simulation algorithm is adopted in this study to reach the final results and Ansys CFX is used for the simulation. From the analysis, it has been reported that the number of rotor blades was overestimated in the mean line design. Performance parameters like total‐ to‐static efficiency and velocity ratio were also found to be optimum numerically under a preliminary design condition. Finally, power of 1.7 kW was achieved at total‐to‐static efficiency of 71.4%. The deviation in analytical and numerical results is within ±10% for performance as well as geometric parameters. [ABSTRACT FROM AUTHOR]

The pulsating airflow through human bronchial tree is of great significance for understanding its function and morphology. Fractal theory and numerical simulation are applied in this paper to study the global and local flow characteristics in the bronchial tree under unstable conditions. First, the pulsating flow impedance of fractal bronchial tree is derived, and the structure of bronchial tree is optimized by minimizing flow impedance. It has been shown that the optimal structure depends on the physical law governing the airflow. The optimized diameter ratio between parent and daughter branches for pulsating flow is different from Murray's law, and the fractal dimension for branch diameter lies in 2 and 3. Afterwards, the local pulsating flow field by three-dimensional (3D) numerical simulation on a symmetrical bronchial model is compared with the global flow characteristics by fractal analysis. The numerical results show that asymmetrical airflow characteristics can be found at high Reynolds number, and the velocity distribution of the main bronchus is more irregular and the turbulence phenomenon is more evident. This work can help to understand the association between function and structure of the bronchial tree, and it may shed light on the physical mechanisms and drugs targeting of pulmonary disease. [ABSTRACT FROM AUTHOR]

The self-excited instabilities acting on laminar separation bubbles in the absence of external forcing are studied by means of linear stability analysis and direct numerical simulation. Previous studies demonstrated the existence of a three-dimensional modal instability, that becomes active for bubbles with peak reversed flow of approximately $7\,\%$ of the free-stream velocity, well below the ${\approx } 16\,\%$ required for the absolute instability of Kelvin–Helmholtz waves. Direct numerical simulations are used to describe the nonlinear evolution of the primary instability, which is found to correspond to a supercritical pitchfork bifurcation and results in fully three-dimensional flows with spanwise inhomogeneity of finite amplitude. An extension of the classic weakly non-parallel analysis is then applied to the bifurcated flows, that have a strong dependence on the cross-stream planes and a mild dependence on the streamwise direction. The spanwise distortion of the separated flow induced by the primary instability is found to strongly destabilize the Kelvin–Helmholtz waves, leading to their absolute instability and the appearance of a global oscillator-type instability. This sequence of instabilities triggers the laminar–turbulent transition without requiring external disturbances or actuation. The characteristic frequency and streamwise and spanwise wavelengths of the self-excited instability are in good agreement with those reported for low-turbulence wind-tunnel experiments without explicit forcing. This indicates that the inherent dynamics described by the self-excited instability can also be relevant when external disturbances are present. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of floating offshore wind turbines is extremely complex due to the motions of the floating platform. The surge and pitch motions are the most influential motions among the six degrees-of-freedom motions. In view of this, the aim of this study is to investigate the unsteady aerodynamic characteristics of a floating offshore wind turbine under single (surge or pitch) and combined motions using computational fluid dynamics simulations, In addition, the coupling technique of dynamic mesh and sliding mesh is employed, as well as the unsteady Reynolds averaged Navier-Stokes method. The numerical simulation method in this paper is first validated by comparison to the results of the blade element momentum method and the vortex method. Then, the aerodynamic characteristics of the floating offshore wind turbine under harmonic platform motions with different periods and amplitudes are investigated. The results show that the increase of amplitude and frequency aggravate the fluctuation of the overall aerodynamic performance of the wind turbine. In addition, the combined surge-pitch motion reduces the average power generation indicating that complex platform motions adversely affect the power generation of floating offshore wind turbines. • The effects of wind load were analyzed by comparing the platform motions under different amplitudes and frequencies. • Two representative platform motions and the combined motion were considered for analysis. • Analyze the aerodynamic characteristics of different airfoil sections to reveal the variations of overall performance. • The detailed analysis of the wake effects showed the three-dimensional flow under the platform motions. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) analysis coupled with pitching motion of a reentry capsule is performed, and a model equation for the aerodynamic force coincident with the CFD result is proposed. The self-excitation of pitching oscillation and the subsequent limit-cycle oscillation are reproduced in a fine-grid CFD simulation. The axis of the vortex ring in the wake extracted by the phase average is displaced to the lower side of the capsule base when the pitch angle α = 0 and α > 0. Such a displacement induced the dynamic component of pitching moment around α = 0. Subsequently, the pitching moment coefficient is decomposed into a Fourier series, where the amplitude of the third harmonics is larger than the dynamic component of the fundamental frequency. The proposed model equation for the pitching moment, which fully includes the third harmonics, reproduces the same amplitude and the same frequency of the CFD result in the case of limit-cycle oscillation. Compared to conventional models, the present model was found to give a better approximation of the dynamic component αCMdy of the unsteady aerodynamic work per unit time. [ABSTRACT FROM AUTHOR]

With the increasing number of long tunnelling and urban subway constructions, mixed-face ground conditions are frequently encountered. Rock fragmentation mechanism under disc cutter cutting in TBM tunneling through the mixed-face ground is complex and can lead to engineering difficulties. During TBM tunneling in mixed-face ground with soft rock in upper layer and hard rock in the lower layer, reduction of the advance rate and reduced rotational speed of cutter head occur compared with homogeneous ground. As a result, the muck in the working chamber cannot be replaced timely, leading to the formation of mud cake. Additionally, the disc cutters cannot rotate normally and are worn eccentrically and severely. Finally, the cutters collide with hard rock periodically at the interface between soft and hard rock, thus being subject to a huge impact load, even overload on some cutters, resulting in chipping of the cutter ring and damage to the cutter holder. This paper presents numerical analysis of the disc cutter cutting process considering the difference of rock-cutting behaviors of disc cutters in the mixed-face ground with the aid of PFC3D code. Based on the forces imposed on the disc cutter and rock crack propagation, TBM tunneling in the mixed-face ground is investigated. The decrease of the mean rolling force of the disc cutter causes rotation hindering in the disc cutter in soft rock stratum leading to flat cutter wear. The gap of the normal force between the soft rock and hard rock generates the overturning moment of the cutter head, which causes the eccentricity and vibration of the cutter head. [ABSTRACT FROM AUTHOR]

The Honji instability of in-line forced cylinder near a plane wall is investigated using Floquet analysis and direct numerical simulation using the open-source code Nektar++. The onset of the Honji instability is analyzed over a range of the gap-to-diameter ratios (e / D) from 2.0 to 0.03125, with fixed Keulegan–Carpenter number K C = 2, Stokes number β = 150, and R e = 300. As e / D decreases from 2 to 0.375, the Honji instability is primarily on the gap side of the cylinder. For 0. 25 < e / D < 0. 375 , the location of the onset of the Honji instability switches from the gap side to the top side of the cylinder. This is due to the blockage effect, which results in the two different trends in the onset of the Honji instability with decreasing e / D. For 0. 125 < e / D < 0. 25 , the favorable pressure gradient suppresses the onset of instability at the gap side. The Honji instability is observed again at the top side of the cylinder at smaller gap ratio (e / D = 0. 0625), which is due to increased flow strength at the top side. At even small gap ratio, i.e. e / D = 0. 03125 , the secondary vortexes and Honji instability together enhance the three-dimensional flow. • The secondary vortices and Honji instability are mutually enhanced at small gap ratios. • For gap ratios e/D¿0.375, the Honji instability is influenced by the blockage effect. • For gap ratios 0.375¿e/D¿0.125, the Honji instability is affected by the favorable pressure gradient. • For gap ratios e/D = 0.03125, a reversed oscillating flow is encountered. • Mode switch is observed for small gap ratio e/D = 0.03125. [ABSTRACT FROM AUTHOR]

Commonly, the base stabilities of sheeted excavation pits against seepage failure by heave are evaluated by using Terzaghi's rectangular-shaped failure plane. However, seepage failures usually occur in the corner areas of polygon-shaped excavation pits, where relatively high hydraulic gradients develop as a result of three-dimensional seepage flows. In this study, it is shown by means of numerical analyses and model tests that quicksand condition that requires no three-dimensional failure body can be used to assess the base stabilities of sheeted excavation pits against seepage failure in cohesionless soils. Thereby, uncertainties with respect to the shapes of three-dimensional failure bodies are eliminated. [ABSTRACT FROM AUTHOR]

In this paper, a deterministic stress decomposition is applied over the numerical three-dimensional flow solution available for a single volute centrifugal pump. The numerical model has proven in previous publications its robustness to obtain the impeller to volute-tongue flow interaction, and it is now used as starting point for the current research. The main objective has been oriented toward a detailed analysis of the lack of uniformity in the flow that the volute tongue promotes on the blade-to-blade axisymmetric pattern. Through this analysis, the fluctuation field may be retrieved and main interaction sources have been pinpointed. The results obtained with the deterministic analysis become of paramount interest to understand the different flow features found in a typical centrifugal pump as a function of the flow rate. Moreover, this postprocessing tool provides an economic and easy procedure for designers to compare the different deterministic terms, also giving relevant information on the unresolved turbulence intensity scales. Complementarily, a way to model the turbulent effects in a systematic way is also presented, comparing their impact on the performance with respect to deterministic sources in a useful framework, that may be applied for similar kinds of pumps. [ABSTRACT FROM AUTHOR]

In the present paper, two-phase three-dimensional turbulent flow simulations are carried out by applying computational fluid dynamics (CFD) technique to the internal flow of a horizontal separator that is used in petroleum industry. Two different geometries are considered; the separator with a straight plate at the top and the separator with the straight plate on the side of the separator. Effects of the location, distance between the inlet of the separator and the diverter plate and inlet velocity on the separation efficiency are investigated by employing the standard k- ε turbulence model. For these purposes, three different distances between the straight diverter plate and the inlet to the separator (0.1 m, 0.15 m and 0.2 m) and four different velocities (0.25 m/s, 0.5 m/s, 0.75 m/s, and 1 m/s) are taken into account by means of Euler mixture model. It is revealed that the maximum separation efficiency is 99.772% when the mixture enters the separator from the top with the inlet velocity of 0.25 m/s and the plate is located 0.2 m away from the inlet section of the separator. An inverse correlation is detected between the inlet velocity and the efficiency of the separation since increasing the inlet velocity decreases the efficiency of the separation. [ABSTRACT FROM AUTHOR]

We analyze the effects of inlet periodic pulsatile flows on the flow inside a slot-die manifold for different locations of the inlet feed port. This type of flow rate disturbance can be caused by a coating liquid pumping mechanism that may lead to coating thickness nonuniformity. In this study, we analyze the transient behavior of the three-dimensional flows inside the slot-die manifold using parallel finite element computations. For a small magnitude of inlet flow rate disturbance with a given frequency, the local flow rate per unit width and the velocity profile at the feed slot exit become periodic. Therefore, the amplification factor can be used to quantify the effect of the inlet pulsatile flows. We found that the flow inside the feed slot, which is the confined channel with a high aspect ratio, dominates the dynamics of the outlet flow. However, the overall trend of the amplification factor along the cross-web direction resembles the steady-state pressure profile inside the cavity, and hence the steady-state profile of the outlet flow rate per unit width, which is strongly affected by the choice of the inlet port location. [ABSTRACT FROM AUTHOR]

Hybrid nanofluid has been streamlined as a new class of nanofluid, marked by its thermal properties and potential utilities which serve the purpose to enhance the rate of heat transfer. The main aim of the current analysis is to present a comparison between the behavior of traditional nanofluid and emerging hybrid nanofluid in the presence of micropolar fluid theory, rotation and porous medium over an exponentially stretched surface. The constructed mathematical differential system is solved numerically by means of the BVP-4C technique. The comparison between behavior of pure water, nanofluid, hybrid nanofluid over velocity, microrotation and temperature distribution has been visualized graphically. For better comprehension of flow characteristics and heat transfer rate, variation in skin friction coefficients in addition to the Nusselt number of nanofluid along with hybrid nanofluid is scrutinized. We perceive from the present study that the rate of heat transfer of nanofluid is lower than that of hybrid nanofluid even in the presence of micropolar effects, rotation and porosity. [ABSTRACT FROM AUTHOR]

• A single fiber motion in a wall-bounded shear flow is investigated. • Direct numerical simulations are performed using a novel Cartesian grid method. • Validity is ascertained by comparison with Jeffery's theory in a far-field condition. • Effects of the fiber-wall distance and aspect ratio on the fiber rotation are discussed. • The three-dimensional flow field and its relation to the hydrodynamic torque are closely examined. In order to investigate the effect of a solid wall on the interaction between the motion of a fiber and a suspending fluid flow, direct numerical simulations of single-fiber dynamics in a wall-bounded shear flow are conducted. A Cartesian grid method using a consistent direct discretization approach and its extension to a moving boundary problem is used for accurate prediction of a three-dimensional fluid flow around a rotating fiber. The fiber is modeled as an ellipsoid with an aspect ratio ranging from 2 to 8, while the suspending fluid is assumed to be an incompressible Newtonian fluid with a particle Reynolds number of less than 0.1. After the validity of the simulations is ascertained in a far-field condition through comparisons with theoretical results, the wall effect on the fiber motion is investigated in detail. The wall effect appears when the minimum distance between the fiber and the wall c is less than 5 times the length of the major radius of the fiber b. The rotation period increases with decreasing fiber-wall distance and increasing aspect ratio, and the ratio of the rotation period with respect to the theoretical period is well normalized by (c / b)/(b / d), where d is the equivalent radius obtained from the cross-sectional area of the fiber. Based on visualization of the flow field, the fiber motion is found to be strongly affected by the hydrodynamic torque caused by the shear stress and the pressure distribution on the fiber surface. The pressure distribution acts as a decelerating torque on the fiber when the fiber is parallel to the wall, while it acts as an accelerating torque when the fiber is perpendicular to the wall. These pressure variations are augmented as the fiber-wall distance decreases, resulting in an increase in the rotation period as well as time-averaged fiber orientation in the streamwise direction. [ABSTRACT FROM AUTHOR]

Abstract A three-dimensional numerical study has been carried out for thermo-hydraulic performance evaluation of a novel design recharging microchannel. In this proposed design of recharging microchannel fresh fluid flows multiple times over total cooling length. Thus, minimizing the maximum temperature within the cooling area. Conjugate effects are considered in the analysis to investigate the effect of axial wall conduction on the overall thermal performance of the recharging microchannel. The parametric variations considered in this study includes solid to fluid thickness ratio δ sf (0.5–2), width ratio ω sf (0.5–2), solid to fluid conductivity ratio k sf (21.957–657.075), channel aspect ratio ε (0.5–2) and channel length L (30–120 mm). Simulations are performed for different flow Re (50–200) and applied constant heat flux q″ (5–20 W/cm2) at the bottom face of the substrate. The study reveals that axial back/wall conduction increases with increasing channel aspect ratio, solid to fluid thickness, width, and conductivity ratio; whereas it decreases with increasing channel length and flow Reynolds number in recharging microchannel. It is also observed that applied heat flux does not affect axial wall conduction. Finally, it is found that the overall thermal performance of recharging microchannel is better than that of a simple microchannel. Next, axial wall/back conduction is very less in recharging microchannel compared to simple microchannel. This provides efficient cooling solution for better thermal management of high heat flux generating devices. Highlights • Fresh fluid enters at periodic intervals, thus carry more heat out of the system. • The proposed design minimizing the maximum temperature within the cooling area. • Average Nusselt number is higher in the proposed recharging microchannel. • Thermal resistance is less in recharging microchannel than simple microchannel. • Thermal performance of recharging microchannel is higher than simple microchannel. [ABSTRACT FROM AUTHOR]

The elementary task is to calculate the growth rates of disturbances when the eN method in transition prediction is performed. However, there is no unified knowledge to determine the growth rates of disturbances in three-dimensional (3D) flows. In this paper, we study the relation among the wave parameters of the disturbance in boundary layers in which the imaginary parts of wave parameters are far smaller than the real parts. The generalized growth rate (GGR) in the direction of group velocity is introduced, and the conservation relation of GGR is strictly deduced in theory. This conservation relation manifests that the GGR only depends on the real parts of wave parameters instead of the imaginary parts. Numerical validations for GGR conservation are also provided in the cases of first/second modes and crossflow modes. The application of GGR to the eN method in 3D flows is discussed, and the puzzle of determining growth rates in 3D flows is clarified. A convenient method is also proposed to calculate growth rates of disturbances in 3D flows. Good agreement between this convenient method and existing methods is found except the condition that the angle between the group velocity direction and the x-direction is close to 90° which can be easily avoided in practical application. [ABSTRACT FROM AUTHOR]

In this study, the numerical analysis of the three-dimensional flow fields of a series of tandem-arranged equal-strength hydrogen jets injected into a Mach 8 hypersonic crossflow is presented. The study comparatively analyzes the flow field similarities and differences between different configurations, namely, single, double, and quadruple jets. The results indicate that downstream jets do not have a significant effect on the first Mach cell and only slightly lift the bow shock at downstream compared with the single jet case. Additionally, high-temperature regions in all cases are in close proximity to the hydrogen stoichiometric equivalence ratio interface, making the fuel more prone to pre-ignition. Moreover, flow patterns in inter-gap regions and vortex structures are compared casewise. In addition, hydrogen injected from each orifice is traced, and the mass fraction distribution contributed by each jet on the symmetry plane and the inter-gap region is analyzed to derive the mixing characteristics. [ABSTRACT FROM AUTHOR]

Theoretical and numerical study of the convection processes, which are accompanied by evaporation/condensation, in the framework of new non-standard problem is largely motivated by new physical experiments. One of the principal questions is to understand the character and to evaluate the degree of influence of particular factors or their combined action on the structure of the joint flows of liquid and gas-vapor mixture. The flow topology is determined by four main mechanisms: natural and thermocapillary convection, tangential stresses and mass transfer due to evaporation at the interface. The mathematical modeling of the fluid flows in an infinite channel with a rectangular cross section is carried out on the basis of the solution of a special type of the convection equations. The effects of thermodiffusion and diffusive thermal conductivity in the gas phase and evaporation at the thermocapillary interface are taken into consideration. Numerical investigations are performed for the liquid – gas (ethanol – nitrogen) system under normal and low gravity. The fluid flows are characterized as translational and progressively rotational motions and can be realized in various forms. [ABSTRACT FROM AUTHOR]

We study the collapsible behaviour of a vessel conveying viscous flows subject to external pressure, a scenario that could occur in many physiological applications. The vessel is modelled as a three-dimensional cylindrical tube of nonlinear hyperelastic material. To solve the fully coupled fluid-structure interaction, we have developed a novel approach based on the Arbitrary Lagrangian-Eulerian (ALE) method and the frontal solver. The method of rotating spines is used to enable an automatic mesh adaptation. The numerical code is verified extensively with published results and those obtained using the commercial packages in simpler cases, e.g. ANSYS for the structure with the prescribed flow, and FLUENT for the fluid flow with prescribed structure deformation. We examine three different hyperelastic material models for the tube for the first time in this context and show that at the small strain, all three material models give similar results. However, for the large strain, results differ depending on the material model used. We further study the behaviour of the tube under a mode-3 buckling and reveal its complex flow patterns under various external pressures. To understand these flow patterns, we show how energy dissipation is associated with the boundary layers created at the narrowest collapsed section of the tube, and how the transverse flow forms a virtual sink to feed a strong axial jet. We found that the energy dissipation associated with the recirculation does not coincide with the flow separation zone itself, but overlaps with the streamlines that divide the three recirculation zones. Finally, we examine the bifurcation diagrams for both mode-3 and mode-2 collapses and reveal that multiple solutions exist for a range of the Reynolds number. Our work is a step towards modelling more realistic physiological flows in collapsible arteries and veins. [ABSTRACT FROM AUTHOR]

The verified Darcy-Brinkman model and boundary perturbation method are used to study the Brinkman flow in a tube with a bumpy surface, assuming the amplitude of the bumps is small compared to the mean tube radius. This study is important to understand the abnormal flow conditions caused by the boundary irregularities in diseased vessels. The mean rate flow is found, up to second-order correction, as a function of circumferential and longitudinal wave numbers and the permeability parameter of the porous medium. Numerical results displaying the velocity components and bumpiness functions are obtained for various values of the physical parameters of the problem. The results are tabulated and represented graphically for various physical parameters. It is found that, for every permeability parameter and for given bump area, there exists a circumferential wave number, for which the flow resistance is minimized. The limiting cases of Stokes and Darcy's flows of the bumpiness function are discussed and compared with the available results in the literature. [ABSTRACT FROM AUTHOR]

The flow and heat transfer of magnetohydrodynamic three-dimensional Maxwell nanofluid over a permeable stretching/shrinking surface with convective boundary conditions is numerically investigated. The partial differential equations governing the flow and heat transfer are transformed to a set of ordinary differential equations by using the suitable transformations for the velocity, temperature and concentration components. These equations have been solved numerically by employing the bvp4c function in Matlab. Numerical solutions are obtained for the skin friction coefficient and the local Nusselt number. Dual solutions are discovered and hence the stability analysis has been done to identify which solution is stable and physically realizable and which is not stable. Solutions are obtained for the skin friction coefficients and local Nusselt number for several values of the parameters, namely the suction parameter, Deborah number, Biot number and Prandtl number. The solutions are presented in some graphs and tables and are analyzed and discussed in detail. [ABSTRACT FROM AUTHOR]

In this work we present a FCT- like Maximum-Principle Preserving (MPP) method to solve the transport equation. We use high-order polynomial spaces; in particular, we consider up to 5th order spaces in two and three dimensions and 23rd order spaces in one dimension. The method combines the concepts of positive basis functions for discontinuous Galerkin finite element spatial discretization, locally defined solution bounds, element-based flux correction, and non-linear local mass redistribution. We consider a simple 1D problem with non-smooth initial data to explain and understand the behavior of different parts of the method. Convergence tests in space indicate that high-order accuracy is achieved. Numerical results from several benchmarks in two and three dimensions are also reported. [ABSTRACT FROM AUTHOR]

A variety of concepts of deep draft semi-submersibles (DDS) have been developed in order to improve the vertical motions. However, the DDS may experience critical vortex-induced motions (VIMs) stemming from the fluctuating forces on the columns. Aiming to investigate the excitation loads and the fluid mechanism of VIM, this paper presents experimental and numerical studies of the three-dimensional (3-D) flow around four circular section cylinders in a square configuration at subcritical Reynolds numbers (Re). The experimental measurements were carried out by using the piezoelectric load cells in a wind tunnel, while the numerical simulations were conducted by employing the Large Eddy Simulation (LES) method. The effects of three spacing ratios (L/D) and four array attack angles (α) ranging from 0° to 45° in 15° increments were investigated. The results show that the vortex structures are the key to understand the behaviours of the forces and the Strouhal number (St). Consequently, the downstream cylinders are normally subjected to smaller mean drags but more serious fluctuating forces than the upstream cylinders, due to the influence of unsteady wake vortices. Furthermore, at the present range ofL/Dratios, the effect of spacing ratio on the excitation forces is not significant. [ABSTRACT FROM AUTHOR]

In this study, the three-dimensional flow behaviors and aerodynamics characteristics of a NACA0003 wing with an aspect ratio (AR) of 1 at Reynolds numbers of 1.0×10 5 has been investigated both experimentally and numerically. The force measurement and flow visualization (oil flow visualization) through the wind tunnel experiment were applied to verify the reliability of the simulation. The investigation with different angles of attack reveals that flow structure contained three dimensional laminar separation bubble (LSB) and wing-tip vortex. As increasing angle of attack, the area occupied by LSB expands both streamwise and spanwise. The wing-tip vortex has strong impact on the formation of LSB. In addition, the spanwise load distribution has shown that both three dimensional LSB and wing-tip vortex provide additional vortex force. [ABSTRACT FROM AUTHOR]

A fluctuating inflow around the surface of rotor blade in the spanwise direction presents a more significant challenge in the performance of wind turbine. In this paper, three-dimensional (3D) experimental and computational investigations of a straight-bladed VAWT (Vertical Axis Wind Turbine) are proposed and analyzed with two straight blades. In wind tunnel experiments, LDV (Laser Doppler Velocimeter) system is presented to investigate the influence of spanwise direction on the straight-bladed of NACA0021 symmetric airfoil in unsteady wind condition. In numerical analysis, 3D transient CFD (Computational Fluid Dynamics) models have been performed to simulate the flow field characteristics of VAWT at the same experimental conditions as wind tunnel experiments. From comparing the results of wind tunnel experiments and numerical analysis, it is found that momentum amount is the largest at the blade center height and the smallest at the blade tip. Furthermore, it is well able to predict the experimental results using CFD model based on k– ε Shear Stress Transport turbulence model. [ABSTRACT FROM AUTHOR]

This article experimentally and numerically analyzes the effect of turbulators with different geometries (Type I, Type II, Type III, and Type IV) located at the inlet of the inner pipe in a concentric-type heat exchanger. Experiments were performed at parallel-flow conditions in the same and opposite directions to investigate the impact of manufactured turbulators on heat transfer and pressure drop. In the numerical study, ANSYS 12.0 Fluent code program was used, and basic protection equations were solved in the steady-state, three-dimensional, and turbulence-flow conditions. Results were obtained from numerical analysis conducted at different flow values of air (7, 8, 9, 10, 11, and 12 m3/h). The distribution of temperature, velocity, and pressure was demonstrated as a result of numerical analyses. Experimental and numerical results were compared, and it was observed that they were in conformity with each other. When the data obtained from the analyses were examined, the highest heat transfer, pressure drop, and friction factor increase were detected to be in the Type IV turbulator. [ABSTRACT FROM PUBLISHER]

A three-dimensional Computational Fluid Dynamics (CFD) model based on the VOF method is developed to investigate the hydrodynamic characteristics of falling film over horizontal tubes. The model not only considers gravity and surface tension, but also considers drag force when counter-current gas flow is imposed. For different liquid flow rates, laminar model, and k-ε turbulence model are employed to improve the model accuracy. Simulations are performed for the film Re varying from 100 to 1000 and gas flow rate ranging from 0 m/s to 0.5375 m/s. Based on the simulations, the gas flow effects on flow mode transitions agree well with the experimental data. In the circumferential direction, all bulk film thicknesses increase with the increased gas flow rate in the three basic flow modes — droplet, column, and sheet. In both the droplet and column flow mode, the position in which the thinnest film is located changes when gas flow is imposed. In the axial direction, the maximum film thickness is approximately twice that of the minimum in the droplet flow mode. Regular stable – crest – stable distribution can be observed in the column flow mode and the film distribution nearly has no variation in the sheet flow mode over time. [ABSTRACT FROM AUTHOR]

This paper is concerned with the three-dimensional rotating flow of nanofluid induced by a convectively heated deformable surface. The base fluid is treated as water while three different types of nanoparticles namely copper oxide-CuO, copper-Cu and silver-Ag are analyzed. Two different models for effective thermal conductivity of nanofluids are employed. A self-similar form of the governing differential system is formulated via adequate transformations. Shooting approach combined with fifth order Runge–Kutta method is used to determine the velocity and temperature distributions above the sheet. Our computations reveal that skin friction coefficient has direct relationship with the volume fraction of nanoparticles. Further the surface heat transfer rate is an increasing function of the solid volume fraction of nanoparticles. Sketch for three-dimensional streamlines is also obtained and discussed for a particular case. [ABSTRACT FROM AUTHOR]

Turbulent natural convection with and without radiation transfer in two-dimensional (2D) and three-dimensional (3D) air-filled differentially heated cavities is numerically investigated using various RANS (Reynolds Averaged Navier–Stokes) turbulence models and the Discrete Ordinates radiation model. Five different two-equation eddy-viscosity models including the standard k – ε model, the renormalization group (RNG) k – ε model, the realisable k – ε model, the standard k – ω model and the shear-stress transport (SST) k – ω model are selected for comparison. Qualitative and quantitative data are presented to demonstrate the effects of three-dimensionality, radiation transfer and the thermal boundary conditions on the horizontal surfaces on the numerical solution of the convective flow in the cavity. The present numerical results are compared against published experimental and direct numerical simulation data. It is found that the predicted thermal stratification in the interior of the cavity is improved when the simulation is extended from 2D to 3D and when the effect of radiation transfer is accounted for. The discrepancy with regard to the interior stratification between the experiment and numerical simulation is mainly caused by the negligence of radiation transfer. The thermal boundary conditions on the horizontal surfaces also have a significant impact on the numerical solution, especially when the radiation transfer is not accounted for. Further, the present results show that all the RANS models are capable of capturing the main features of the flow and the overall performance of these turbulence models in terms of predicting time-averaged quantities is acceptable. It is found that the variation of the numerical results obtained with the three k – ε models is very small, whereas the discrepancy between the two k – ω models is significant. The SST k – ω model has the best overall performance and the standard k – ω model has the worst overall performance. [ABSTRACT FROM AUTHOR]

The complex transport phenomena and their effects on the weld pool dynamics and surface rippling in moving gas tungsten arc welding (GTAW) under pulsed currents are studied by using a 3D transient numerical model. The distributions of the melt-flow velocity and temperature, and weld bead formation are simulated. The effects of welding conditions, including welding current waveform, pulse frequency and welding speed, on the weld penetration, formation and final appearance of ripples are discussed. It is found that surface ripples are formed under pulsed current due to the up-and-down weld pool motion, caused mainly by the periodically varied current and solidification rate of weld pool. The results show that for the cases with the same average current, the pulsed current leads to the deeper weld penetration than continuous current, and the higher peak current corresponds to the higher ripples and deeper penetration. The larger pulse frequency results in the more uniform thermal energy distributions on the workpiece and tends to decrease the solidification rate, leading to the more uniform penetration depths, the smaller pitch and height of the ripples. A slow travel speed is helpful to reduce pitch of the ripples but also at the risk of the reduced effective penetration. Finally, a GTAW experiment for the case of continuous current is conducted to validate the modeling predictions in terms of weld width, penetration depth and the formation of ripples. [ABSTRACT FROM AUTHOR]

Flow past two cylinders of different diameters in close proximity is simulated numerically for a constant diameter ratio of 0.45, a gap ratio of 0.0625, and a Reynolds number of 1000 (defined using the diameter of the main cylinder). The effect of the position angle α of the small cylinder relative to the large one on force coefficients and wake flow patterns are studied. Depending on the position angle α of the small cylinder, four wake flow modes are identified: the upstream interference mode for α = 0°, 22.5°, and 45°, the intermittent attached gap flow mode for α = 67.5° and 90°, the attached gap flow mode for α = 112.5° and 135°, and the wake interference mode for α = 157.5° and 180°. The RMS lift coefficients of both cylinders are reduced significantly compared with that of a single cylinder, regardless of the position angle of the small cylinder. Although the variation trends of the mean drag and lift coefficients with the position angle of the small cylinder obtained from the two-dimensional (2D) and three-dimensional (3D) simulations are similar, the 2D simulations overestimate the mean drag coefficient, the RMS drag and lift coefficients compared with those obtained from the 3D simulations. [ABSTRACT FROM AUTHOR]

We perform the systematic numerical study of high vorticity structures that develop in the 3D incompressible Euler equations from generic large-scale initial conditions. We observe that a multitude of high vorticity structures appear in the form of thin vorticity sheets (pancakes). Our analysis reveals the self-similarity of the pancakes evolution, which is governed by two different exponents e-t/Tl and et/Tω describing compression in the transverse direction and the vorticity growth, respectively, with the universal ratio Tl/Tω ≈ 2/3. We relate development of these structures to the gradual formation of the Kolmogorov energy spectrum Ek ∝ k-5/3, which we observe in a fully inviscid system. With the spectral analysis, we demonstrate that the energy transfer to small scales is performed through the pancake structures, which accumulate in the Kolmogorov interval of scales and evolve according to the scaling law ωmax ∝ l-2/3 for the local vorticity maximums ωmax and the transverse pancake scales l. [ABSTRACT FROM AUTHOR]