Numerical Simulation Of Three-dimensional Flow Research Articles

Influences of channel bed morphology on flow structures in continuous curved channels

Introduction: The formation of bars and pools, characterized by concave and convex bed morphology, is a typical feature of curved rivers. The channel bed morphology has a significant influence on the flow structures in curved channels.Methods: Based on data from physical model experiments, this study employs the RNG k-ε model and the VOF (Volume of Fluid) method to perform three-dimensional numerical simulations of flow in continuous curved channels.Results: By comparing the variations in flow structures between channels with a flat bed and channels with bars and pools, the results show that the presence of bars and pools leads to an increase in longitudinal flow velocity on the convex bank side near the entrance of the upstream bend, while in the downstream bend it is opposite. The high-velocity region shifts slower towards the concave bank along the bend. The presence of point bars weakens the circulation near the convex bank in the upstream bend, resulting in a smaller circulation intensity. The decrease in circulation intensity is the largest (−23.91%) at the apex of the bend. In the downstream bend, the remaining circulation from the upstream bend attenuates slower in the pool and has a greater impact distance, increasing the circulation intensity in the downstream bend. The section near the bend entrance shows the largest increase in circulation intensity, with a rate of 128.18%. The unevenness of the bed topography increases the unevenness of the bed shear stress in the downstream bend.Discussion: The findings of this study contribute to a deeper understanding of the complex flow structures and evolution trends in natural curved rivers, providing scientific basis for the management of curved river channels.

Chaotic Transport of Solutes in Unsaturated Porous Media.

Unsaturated porous media, characterized by the combined presence of several immiscible fluid phases in the pore space, are highly relevant systems in nature, because they control the fate of contaminants and the availability of nutrients in the subsoil. However, a full understanding of the mechanisms controlling solute mixing in such systems is still missing. In particular, the role of saturation in the development of chaotic solute mixing has remained unexplored. Using three-dimensional numerical simulations of flow and transport at the pore scale, built upon X-ray tomograms of a porous medium at different degrees of liquid (wetting)-phase saturation, we show the occurrence of chaotic dynamics in both the deformation of the solute plume, as characterized by computed chaos metrics (Lyapunov exponents), and the mixing of the injected solute. Our results show an enhancement of these chaotic dynamics at lower saturation and their occurrence even under diffusion-relevant conditions over the medium's length, also being strengthened by larger flow velocities. These findings highlight the dominant role of the pore-scale spatial heterogeneity of the system, enhanced by the presence of an immiscible phase (e.g., air), on the mixing efficiency. This represents a stepping stone for the assessment of mixing and reactions in unsaturated porous media.

Minimal model of quasi-cyclic behaviour in turbulence driven by Taylor–Green forcing

We attempt to formulate the simplest possible model mimicking turbulent dynamics, such as quasi-cyclic behaviour (QCB), using only three variables. To this end, we first conduct direct numerical simulations of three-dimensional flow driven by the steady Taylor–Green forcing to find a similarity between a stable periodic orbit (SPO) at a small Reynolds number (Re) and turbulent QCB at higher Re. A close examination of the SPO allows the heuristic formulation of a three-equation model, representing the evolution of Fourier modes in three distinct scales. The model reproduces the continuous bifurcation from SPO to turbulence with QCB when Re is varied. We also demonstrate that, by changing model parameters, the proposed model exhibits a discontinuous transition from steady to chaotic solutions without going through an SPO.

Three-dimensional numerical simulation of flow past a rotating step cylinder

Flow past a rotating step cylinder is investigated through three-dimensional numerical simulations for a diameter ratio of 0.5 and a Reynolds number of 150. The step cylinder comprises two cylinders with different diameters arranged coaxially with a step between them. The rotation rate α is defined as the ratio of the rotation speed of the larger cylinder surface to the free-stream velocity. Vortex shedding happens for both cylinders at α = 0, 0.5 and 1, and is suppressed only for the larger cylinder at α = 2 and 3 and fully suppressed for both cylinders at α = 4. The vortex shedding suppression for the larger cylinder or for both cylinders has significant effects on the wake. The S-, N- and L-cells at α = 0 are in good agreement with those reported in previous studies and still exist at α = 1. The N-cell disappears at α = 0.5, and as a result, the L- and S-cells interact with each other directly at the step position. An additional cellular zone is found at α = 0.5 and 1 and this zone has multiple cells with vortex dislocation between them. At α = 2 and 3, there is a strong hub vortex in the streamwise direction behind the step and the vortices in the wake of the smaller cylinder form helical vortices after they roll around this hub vortex. The hub vortex is generated by the difference between the Magnus effect between the smaller and larger cylinders. At α = 4, the hub vortex still exists but the helical vortices disappear because vortex shedding is suppressed for both cylinders. At this rotation rate, the pressure on the cylinder surface oscillates with a frequency much higher than the vortex shedding frequency. The oscillation of the pressure is caused by the combination of periodic generation of ring vortices and their motion along the span of the larger cylinder.

On the origin of mode B instability of the wake of a square cylinder

Three-dimensional numerical simulations of flow passing a square cylinder are performed using a spectral element method. Reynolds numbers of 200 and 300, corresponding to modes A and B, respectively, are used to study the two- to three-dimensional transition of the wake flow. It is shown that the wake transition occurs in the early time prior to the onset of the vortex shedding event. For mode A, the spanwise instability grows exponentially downstream of the cylinder with constant wavelength in the early time dynamics and continues after the vortex separation. However, in the case of mode B, the exponential amplification reaches a saturation state before the vortex separation occurs. A splitting of the initially obtained wavelength of the spanwise instability occurs and is related to the splitting of spanwise vortical structures, which develops at the cores of the separation bubbles prior to the vortex shedding.

Numerical Simulation of Three-Dimensional Flow of Sudden Dam-Break over the Porous Bed

Dam break is a very important problem due to its effects on the economy, security, human casualties and environmental consequences. In this study, 3D flow due to dam break over the porous substrate is numerically simulated and the effect of porosity, permeability and thickness of the porous bed and the water depth in the porous substrate are investigated. Classic models of dam break over a rigid bed and water infiltration through porous media were studied and results of the numerical simulations are compared with existing laboratory data. Validation of the results is performed by comparing the water surface profiles and wave front position with dam break on the rigid and porous bed. Results showed that, due to the effect of dynamic wave in the initial stage of dam break, a local peak occurs in the flood hydrograph. The presence of porous bed reduces the acceleration of the flood wave relative to the flow over the solid bed and it decreases with the increase of the permeability of the bed. By increasing the permeability of the bed, the slope of the ascending limb of the flood hydrograph and the peak discharge drops. Furthermore, if the depth and permeability of the bed are such that the intrusive flow reaches the rigid substrate under the porous bed, saturation of the porous bed, results in a sharp increase in the slope of the flood hydrograph. The maximum values of the peak discharge at the end of the channel with porous bed occurred in saturated porous bed conditions.

Three-dimensional numerical simulation of flow and splash behavior in an oxygen coal combustion melting and separating furnace

Three-dimensional numerical simulation of flow and splash behavior in an oxygen coal combustion melting and separating furnace

Three-dimensional numerical simulation of flow in continuous bend under the influence of piers

With the development of social economy, more and more cities plan to build various kinds of Bridges in curved rivers to construct urban traffic network. The Realizable k-ε turbulence model and VOF method were used to capture the free water surface, Simple algorithm was used for coupling calculation to simulate the flow characteristics of continuous bend with piers placed at 45° forward bend section. Simulation results show that: (1) under the condition of continuous curve, set in the horizontal surface slope will change the channel distribution of piers, bridge piers and curve superposition, the piers downstream transverse surface slope significantly improved, but the bridge pier near area, on the other hand, because of the water resistance of piers, bridge pier influence and affect the part of the balance of the bend, transverse surface slope value is only under the condition of no pier 1/2;(2) The high longitudinal flow velocity area is more concentrated, the high velocity center moves down to the bottom of the channel, and the central position of the circulation structure moves to the convex bank, which can aggravate the scouring of the convex bank.

Numerical simulation of three-dimensional flow and heat transfer characteristics of liquid lead–bismuth

Liquid lead-bismuth cooled fast reactor is one of the most promising reactor types among the fourth-generation nuclear energy systems. The flow and heat transfer characteristics of lead-bismuth eutectic (LBE) are completely different from ordinary fluids due to its special thermal properties, causing that the traditional Reynolds analogy is no longer recommended and appropriate. More accurate turbulence flow and heat transfer model for the liquid metal lead-bismuth should be developed and applied in CFD simulation.In this paper, a specific CFD solver for simulating the flow and heat transfer of liquid lead-bismuth based on the k - ε - kθ- εθ model was developed based on the open source platform OpenFOAM. Then the advantage of proposed model was demonstrated and validated against a set of experimental data. Finally, the simulation of LBE turbulent flow and heat transfer in a 7-pin wire-wrapped rod bundle with the k - ε - kθ- εθ model was carried out. The influence of wire on the flow and heat transfer characteristics and the three-dimensional distribution of key thermal hydraulic parameters such as temperature, cross-flow velocity and Nusselt number were studied and presented.Compared with the traditional SED model with a constant Prt = 1.5 or 2.0, the k - ε - kθ- εθ model is more accurate on predicting the turbulence flow and heat transfer of liquid lead-bismuth. The average relative error of the k - ε - kθ - εθ model is reduced by 11.1% at most under the simulation conditions in this paper. This work is meaningful for the thermal hydraulic analysis and structure design of fuel assembly in the liquid lead-bismuth cooled fast reactor.

On three-dimensional boundary layer stability analysis on curved aerodynamic surfaces

Numerical simulation of three-dimensional flow past a prolate spheroid located at an angle of attack of 10° accompanied by the laminar-turbulent transition was performed for subsonic (M∞ = 0.136) and transonic (M∞ = 0.75) velocities. The general-purpose gas-dynamic package ANSYS Fluent in integration with a laminar-turbulent transition module based on software bundle LOTRAN 3 was used. The principles of numerical simulation of the main flow are discussed: requirements for the near-wall region allocation and independence of the results from various methods of presetting the position of forced turbulence. It was shown that two instability mechanisms take place in the boundary layer of the spheroid: the Tollmien-Schlichting instability and the cross-flow instability with a predominance of the last on side surfaces. A comparison of predicted position of the transition for different flow regimes was performed.

Effects of Junction Flow on the Free Convection Heat Transfer of a Heated Vertical Plate

The present study covers results of non-stationary numerical simulation of three-dimensional flow in the junction region of a low aspect ratio circular cylinder entirely immerged in laminar free convection boundary layer developing over a heated vertical plate. To categorize the effects of the cylinder surface temperature on the dynamics behavior of the forming vortical structure, its size and position both upstream and downstream of the cylinder the results, which prove the presence of a complex coherent vortical structure, are analyzed. In particular, the influence of horseshoe vortex system formed upstream of the cylinder on the free convection heat transfer of the heated vertical plate in the junction area is evaluated. Finally, the importance of the forming vortical structure on the heat transfer computation for the junction region is demonstrated.

Experimental and numerical investigation of the effect of number of parallel microchannels on flow boiling heat transfer

A combined numerical and experimental investigation to elucidate the two-phase flow behaviour and heat transfer during subcooled boiling of water in 1 × 1 cm2 footprint area heat sinks with six, ten, and fourteen parallel microchannels is performed. A three-dimensional, laminar, multi-phase, transient numerical model is developed to simulate flow boiling in microchannels. This study is one of the first studies which reports a three-dimensional numerical simulation of flow boiling in a large area multiple parallel microchannels heat sink including the effect of inlet and outlet plenums. We show an excellent agreement in heat transfer and pressure drop data with experimental results on a heat sink with fourteen parallel microchannels over a wide range of applied heat flux spanning various boiling regimes. The results show that the vapour blocking in the channel near the outlet is primarily responsible for instabilities and oscillations in the pressure drop, the surface temperature, and the mass flux. Intensified confinement due to the decrease in the number of the channels for a constant fin width results in increased surface temperatures. Similarly, increase in the number of the channels from six to fourteen improved heat transfer significantly wherein a significant drop of 45.5 °C in surface temperature with little increase of 37% in pressure drop is observed for a mass flux of 500 kg/m2s and a heat flux of 220 W/cm2. Microchannel heat sink with fourteen channels demonstrates on an average nearly 240% higher heat transfer coefficient in comparison to the heat sink with six channels. The numerical modelling framework used in this study can be used to provide design guidelines for microchannel heat sinks.

Numerical simulation for three-dimensional flow in a vortex tube with different turbulence models

Three-dimensional flow in a vortex tube with straight nozzle was investigated numerically. Four different turbulence models, Spalart-Allmaras, standard SST and realizable were proposed to replicate the flow in the vortex tube. The simulation results of pressure, temperature and velocity field were compared with the experimental data based on dimensionless analysis. It was found that the sudden gas expansion in the vortex chamber causes a pressure drop. The secondary circulations phenomena were observed and identified. Furthermore, the friction between the gas and wall surface and the energy transfer between different laminar flows are the main factors of energy separation in the vortex tube. The realizable model predicts energy separation better than other models studied in this research.

An investigation of kinematic parameters and stroke function on stroke reversal for three-dimensional vortex structures around a flapping insect wing

Abstract Complex flows or vortical structures are observed around flapping wings. In particular, as observed in various experiments and numerical simulations, these flows are affected when kinematic parameters and translational velocity functions are changed. In the present study, numerical simulations of three-dimensional flows around a flapping wing are conducted to investigate parameters and translational velocity functions on the stroke reversal stage using immersed boundary lattice Boltzmann method. We consider a flapping Drosophila wing without pitching motion, and the effects of the stroke amplitude and stroke reversal duration are investigated. First, at high stroke amplitude, we found that a hairpin-like vortex loop is obviously observed. Second, the stroke reversal duration affects remnant vortex structures in the wake. In addition, this parameter affects a time instant which the wake capture occurs. Third, it is shown that the translational velocity function has a significant effect on drag force. If the translational velocity function is modeled with trigonometric function, the drag force has a discrepancy with well-known experimental data. In order to solve the problem, we suggest a higher order polynomial. As a result, our function shows a better agreement with the experimental data.

Mathematical modeling of the generation of acoustic waves in a two-channel system

The purpose of this study is to determine the influence of the diameter of a subsonic conical nozzle on the generation of acoustic waves in a two-channel system. Three-dimensional numerical simulation of flow in the duct of an actual device was performed. A complete picture of the flow generated in the acoustic-convective drying system of the Institute of Theoretical and Applied Mechanics, SB RAS, was obtained. The results of the study show that to maintain the amplitude-frequency characteristics of the workflow with increasing diameter of the subsonic conical nozzle, it is necessary to reduce the settling chamber pressure. The numerically simulated amplitude-frequency characteristics of the acoustic flow generated in the working section of the drying system are in satisfactory agreement with the results of physical experiments.

Flame propagation in weightlessness above the burning surface of material

Numerical simulations of three-dimensional flow of a gas mixture with chemical reactions over a flat thermally destructing material surface are presented. To create an effective numerical model, two ways of determining the heat removal are considered, and a comparison with the analytical solution obtained within the frame of the boundary layer approximation is represented. As an example, the numerical simulations of three-dimensional flow in combustion chamber of hybrid rocket are performed. The temperature maps and molar fraction of fuel within the chamber for different times are shown.

Three-dimensional numerical simulation of flow in vertical slot fishways: validation of the model and characterization of the flow

ABSTRACT Vertical slot fishways allow energy dissipation as a function of the pool, longitudinal slope, baffle and vertical slot design. The mean and turbulent flow patterns in these structures must be compatible with the fish target. The design of these structures is commonly based on previous successful fishways as well as simplified theoretical equations and empirical relationships. To aid in the design of these structures, a three-dimensional hydrodynamic model was used to simulate the flow, and experimental studies were used to validate the model. The mean velocities, pressures and parameters indicative of turbulence were analyzed. The maximum flow velocities were up to 32% higher than the values obtained using a simplified theoretical equation. The evaluation of the volumetric dissipated power indicated that the mean value for the pool was lower than 150 W/m3; however, analysis of the spatial distribution showed that in some areas, the values can exceed 1000 W/m3. The results indicate that the numerical simulation was able to adequately represent the flow considering the computational cost involved. Accordingly, it can be used as a complementary tool for the design of new fishways and for the analysis of modifications in existing ones.

Numerical simulation for three-dimensional flow of Carreau nanofluid over a nonlinear stretching surface with convective heat and mass conditions

This article addresses (3D) flow of Carreau liquid in the presence of nanomaterials induced by a nonlinearly extendable surface. A nonlinear extendable surface generates the flow. Heat and mass transport via convective process is considered. The novel characteristics in regard to Brownian dispersion and thermophoresis are retained. The variation in partial differential framework to nonlinear ordinary differential framework is done through reasonable transformations. The graphical representation of transformed nonlinear ordinary differential framework is developed for both situations (n 1). An efficient numerical solver namely NDSolve is used to tackle the governing nonlinear framework. The contributions of various interesting variables are studied graphically. Physical amounts like surface drag coefficients, transfer of heat and mass rates are portrayed by numeric esteems.

Дослідження впливу інтерференції лопаткових вінців на втрати в ступені осьового компресора

Мета: дослідження втрат повного тиску у вхідному напрямному апараті, в робочому колесі й напрямному апараті ступеня осьового компресора з урахуванням їх газодинамічної інтерференції. Метод дослідження: дослідження виконано методом чисельного моделювання тривимірної течії в ступені осьового компресора. Побудовано неструктуровану адаптивну розрахункову сітку. Газодинамічний розрахунок течії в ступені осьового компресора виконано з використанням системи рівнянь Нав’є – Стокса, яка замикалася моделлю турбулентної в’язкості SST. Результати: було проведено серію газодинамічних розрахунків течії при різних значеннях осьової швидкості на вході в ступінь компресора. Колова швидкість на периферійному радіусі на розрахунковому режимі складала u=238.64 м/с. Коефіцієнт швидкості на вході в ступінь змінювався в діапазоні =0.35…0.7. За результатами розрахунку побудовано залежності коефіцієнтів втрат повного тиску від коефіцієнта швидкості на вході для вхідного напрямного апарату, робочого колеса і напрямного апарату. Аналіз результатів дослідження показує, що найбільший внесок у загальний баланс втрат повного тиску вносять втрати в напрямному апараті. Обговорення: унаслідок газодинамічної інтерференції робочого колеса і напрямного апарату відбувається збільшення втрат в напрямному апараті. Взаємний вплив лопаткових вінців робочого колеса і напрямного апарату призводить до суттєвої трансформації швидкостей і тисків в міжлопаткових каналах. Як наслідок, має місце перерозподілення втрат, обумовлених коловою і радіальною нерівномірністю потоку, кінцевими перетіканнями і дією відцентрових сил. Можна очікувати, що зменшення рівня втрат в ступені осьового компресора можна забезпечити шляхом впливу на пограничний шар в кінцевому зазорі робочого колеса.

Three-dimensional numerical simulation of flow in trapezoidal cutthroat flumes based on FLOW-3D

Three-dimensional numerical simulation of flow in trapezoidal cutthroat flumes based on FLOW-3D