LARGE-EDDY AND DETACHED-EDDY SIMULATIONS OF THE SEPARATED FLOW AROUND A CIRCULAR CYLINDER

The separated turbulent flow around a circular cylinder is investigated using Large-Eddy Simulation (LES), Detached-Eddy Simulation (DES, or hybrid RANS/LES methods), and Unsteady Reynolds-Averaged Navier-Stokes (URANS). The purpose of this study is to examine some typical simulation approaches for the prediction of complex separated turbulent flow and to clarify the capability of applying these approaches to a typical case of the separated turbulent flow around a circular cylinder. Several turbulence models, i.e. dynamic Sub-grid Scale (SGS) model in LES, the DES-based Spalart-Allmaras (S-A) and k ? ω Shear-Stress-Transport (SST) models in DES, and the S-A and SST models in URANS, are used in the calculations. Some typical results, e.g., the mean pressure and drag coefficients, velocity profiles, Strouhal number, and Reynolds stresses, are obtained and compared with previous computational and experimental data. Based on our extensive calculations, we assess the capability and performance of these simulation approaches coupled with the relevant turbulence models to predict the separated turbulent flow.     

THE TURBULENT MODEL OF LIQUID-PARTICLE FLOWS AND ITS APPLICATION TO NUMERICAL PREDICTION OF OIL POLLUTION

Simulation of particle turbulent transport in present mathematical models of two-phase turbulent flows is algebraic model based on local following theory. The predicted results bythese models are not consistent with that of experiments available. The motion of two-phase flowis described by a unified model of multi-fluid in this paper. The two-fluid turbulent model oftwo-phase flow based on k-e equations for particles and fluid, and corresponding numerical thch-nique are both developed. The proposed model and the numerical technique are successfully em-ployed to numerical modelling for oil-water two-phase turbulent recirculating flow.     

ON THE ORIGIN OF TURBULENT SECONDARY FLOW IN A STRAIGHT DUCT OF NON-CIRCULAR CROSS-SECTION

The origin of turbulent secondary flow in a straight duct of non-circular cross-section is discussed from a theoretical standpoint and examined from a numerical simulation ofthe turbulent flow in a square duct,It is reached that the driving force of such secondary flowcomes from the second-order terms of the difference in gradients of normal and transverseReynolds stresses in the axial vorticity equation.The present results obtained compare well withavailable experimental results for both primary and secondary flow quantities.     

NUMERICAL CALCULATION OF TURBULENT FLOW WITH FREE SURFACE

The problem of free surface is one of main difficulties in the numerical calculation of turbulent flow. This pa-per presents the concept of "elastic-lid" for the first time, which is used to calculate the steady turbulent flow do-mains with complex free surfaces. This method can be combined with the existing steady turbulence models. Com-pared with MAC and other unsteady methods, it is simpler and can give us the parameters related to turbulentflow. The method is supported by the techniques of "block-off" and "elastic-grid". which can be selected for dif-ferent problems. Finally, three examples are given to show the application of the "elastic-lid" method. The calcu-lated results are in good agreement with the measured.     

ADJOINT METHOD OF PARAMETER IDENTIFICATION FOR SOME NONLINEAR REACTION-DIFFUSION SYSTEMS

This paper deals with the problem of determining two unknown parameters of some nonlinear reaction-diffusion models. These reaction-diffusion models are derived from applications in the groundwater flow transport, environ mental sciences, gas dynamics, heat and mass transfer, industrial automatization and some other engineering technological fields. The adjoint method based on the variational principle is a relatively new optimal control method. It is used in the identification of the unknown diffusion coefficient, and some coefficients of the nonlinear sink or source terms in these systems.At first, the problem is transferred into an optimization problem of minimizing a functional, and the adjoint equations of the governing equations are derived from the adjoint method.Then, the formulas are given to calculate the gradient of the objective function with respect to the couple of unknown parameters. At last, an iterative gradient-based optimization algorithm is presented for solving the optimization problem. A numerical example is offered in the end. It shows the effectiveness of the proposed approach.     

Numerical simulation of turbulent flow around a forced moving circular cylinder on cut cells

Fixed and forced moving circular cylinders in turbulent flows are studied by using the Large Eddy Simulation （LES） and two-equation based Detached Eddy Simulation （DES） turbulence models. The Cartesian cut cell approach is adopted to track the body surface across a stationary background grid covering the whole computational domain. A cell-centered finite volume method of second-order accuracy in both time and space is developed to solve the flow field in fluid cells, which is also modified accordingly in cut cells and merged cells. In order to compare different turbulence models, the current flow past a fixed circular cylinder at a moderate Reynolds number, Re = 3 900, is tested first. The model is also applied to the simulation of a forced oscillating circular cylinder in the turbulent flow, and the influences of different oscillation amplitudes, frequencies and free stream velocities are discussed. The numerical results indicate that the present numerical model based on the Cartesian cut cell approach is capable of solving the turbulent flow around a body undergoing motions, which is a foundation for the possible future study on wake induced oscillation and vortex induced vibration.     

NUMERICAL STUDY OF TURBULENT FLOW AROUND TWIN PLATE AT LARGE ATTACK ANGLE

The present work is mainly aimed to study the fluid dynamic force on twin plate in a turbulent flow by solving the governing equations numerically with the well known approach SIMPLE, in which the multiple time scale turbulent k-ε model has been employed to determine the eddy viscosity of turbulent flow field. The turbulence model is used with a restriction for the kinetic energy of smaller turbulent scale to achieve numerical stability. The lengths of recirculation zone for the turbulent flow around twin plate with the definite angle of attack AOA = 50° and constant flow blockage ratio 10% are in good agreement with experimental data, which indicates the wall functions used in the numerical investigation are acceptable as well. It is found that the drag force and distribution of wall pressure are intimately influenced by the arrangement of twin plate at constant flow blockage. The results of multiple time scale k-εmodel are compared with single time scale k-εmodel, indicating that the drag force of     

PREDICTION OF CAVITATION INCEPTION IN PIPELINES

Based on the analysis of the averaged axisymmetric turbulent flow and the motionand growth of gas nuclei in the flow,this paper presents a method for predicting cavitation in-ception in pipelines.Mathematic model is proposed and the fluctuating pressure in the flow andother random factors with respect to gas nuclei are treated using the Monte-Carlo method.Sothat the bridge between macroscopic aspect of cavitation in the flow and the microscopic event ofindividual nucleus cavitation is set up.Numerical investigation and experimnental test are carriedout for the flow past an orifice in a uniform pipe with circular section.     

《南水北调与水利科技》优先数字出版声明

The turbulent flows through the channels with abrupt cross-sectional changes are common and importantphysical process in nature.For a better prediction of the mean flow and turbulent characteristics for this problem,atwo-dimensional depth-averaged numerical model is developed.The model is robust and accurate in reproducing therecirculation flow behind a groyne and turbulent flows in channels with abrupt cross-sectional changes,when com-pared to the available experimental data of mean velocities and turbulence kinetic energy.Our results reveal that theabrupt cross-sectional change of a channel can affect the flow pattern significantly and introduces the complex turbu-lence characteristics.In particular,when the channel has an abrupt expansion,the mean flow pattern is mainly in lon-gitudinal direction with rather small transverse component.Meanwhile,a recirculating region forms behind the expan-sion position and the turbulence has very strong intensity within this region.For the flow in the channel with an ab-rupt contraction,the longitudinal component of the flow is decreased by the obstruction on one side and accelerated onthe other side,whereas the transverse velocity is small.The turbulence is extraordinarily strong in the regions adja-cent to the contraction wall in the narrow channel.In both cases of abrupt cross-sectional changes,the TKE is genera-ted dominantly by the shear of the longitudinal velocities.     

INTRINSIC FEATURES OF TURBULENT FLOW IN STRONGLY 3-D SKEW BLADE PASSAGE OF A FRANCIS TURBINE

The turbulent flow, with the Reynolds number of 5.9× 105, in the strongly 3-D skew blade passage of a true Francis hydro turbine was simulated by the Large Eddy Simulation (LES) approach to investigate the spatial and temporal distributions of the fully developed turbulence in the passage with strongly 3-D complex geometry. The simulations show that the strong three-dimensionality of the passage has a great amplification effect on the turbulence in the passage, and the distributions of the turbulence are diversely nonuniform, for instance, the rise of turbulent kinetic energy in the lower 1/3 region of the passage is more than 45%, whereas its rise in the upper 1/3 region is less than 1%. With the LES approach, the details of the flow structures at the near-wall surfaces of the blades could be obtained. Several turbulent spots were captured.     

底流消能的跌坎型消力池再附长度的试验研究

再附长度是衡量坎后底滚回流区水流特征的一个重要参数,再附长度随水流入射角度、跌坎深度的变化而变化。根据平面紊动射流理论,建立淹没紊动射流的扩散方程,推导出再附长度计算的半经验公式,并结合水力学试验,对公式进行验证。结果表明,通过试验观测所得观测值与通过半经验公式计算所得理论值吻合较好。     

DIRECT NUMERICAL SIMULATION OF TURBULENT FLOW IN A STRAIGHT SQUARE DUCT AT REYNOLDS NUMBER 600

This article presents the direct numerical simulation results of the turbulent flow in a straight square duct at a Reynolds number of 600, based on the duct width and the mean wall-shear velocity. The turbulence statistics along the wall bisector is examined with the turbulent flow field properties given by streamwise velocity and vorticity fields in the duct cross section. It was found that the solutions of the turbulent duct flow obtained in a spatial resolution with 1.2×10⁶ grid points are satisfactory as compared to the existing numerical and experimental results. The results indicate that it is reasonable to neglect the sub-grid scale models in this spatial resolution level for the duct flow at the particular friction Reynolds number.     

LARGE EDDY SIMULATION OF FREE SURFACE TURBULENT CHANNEL FLOW WITH HEAT TRANSFER

1. INTRODUCTIONFreesurface turbulent flow exists widely in engineering. Due to difficulties in measurement and simulation, very little has been understood about the structures and transport mechanism of turbulence near the freesurface. Komori and Ueda…     

REVIEW OF SOME RESEARCHES ON NANO-AND SUBMICRON BROWNIAN PARTICLE-LADEN TURBULENT FLOW

The study of nano-and submicron Brownian particle-laden turbulent flow has wide industrial applicability and hence has received much attention.The purpose of the present paper is to provide and review some researches in this field.The topics are related to the universality,particularity,complexity and importance of nano-and submicron Brownian particle-laden turbulent flow,the models of particle general dynamical equation,the collision behavior of particles.Finally,several open research issues are identified.     

LARGE EDDY SIMULATION OF THERMALLY-STRATIFIED TURBULENT CHANNEL FLOW WITH TEMPERATURE OSCILLATION ON THE BOTTOM WALL

Thermally-stratified shear turbulent channel flow with temperature oscillation on the bottom wall of the channel was investigated with the Large Eddy Simulation (LES) approach coupled with dynamic Sub-Grid-Scale (SGS) models. The effect of temperature oscillation on the turbulent channel flow behavior was examined. The phase-averaged velocities and temperature, and flow structures at different Richardson numbers and periods of the oscillation was analyzed.     

TWO DIMENSIONAL SIMULATION OF WIND-DRIVEN CIRCULATION IN RESERVOIR

1 .　INTRODUCTIONThethree physicalcausesaffectingreservoirflowandsedimentationarewind ,solarradiationandriverinflow .Themostimportantcauseformixinganddispersionofmassandenergyiswind ,whichatmoderatespeedcanbreakupthermalstratificationanddensityflow .Thusnumericalsimulationofwinddrivencirculationisessentialinprovidinginformationonmasstransportconditioninreservoirandindeterminingthelocation ,wherethewatercanbesafelyabstracted .Italsohaspracticalapplicationinmonitoringreservoir .Thisisveryim…     

NUMERICAL SIMULATION OF THE ENERGY DISSIPATION CHARACTERISTICS IN STILLING BASIN OF MULTI-HORIZONTAL SUBMERGED JETS

3-D numerical simulation was carried out for the water flow in a stilling basin with multi-horizontal submerged jets by using two different turbulence models, namely, the VOF RNG k - ? and Mixture RNG k - ? turbulence models. The calculated water depth, velocity profile and pressure distribution are in good agreement with the data obtained in experiments. It indicates that the numerical simulation can effectively be used to study the water flow movement and the energy dissipation mechanism. The numerical simulation results show that the turbulent kinetic energy distribution obtained by using the Mixture turbulence model covers a region about 18% larger than that calculated by using the VOF turbulence model, and is in better agreement with the actual situation. Furthermore, the Mixture turbulence model is better than the VOF turbulence model in calculating the air entrainment.