Numerical simulation and analysis of confined turbulent buoyant jet with variable source

In this work, experimental and numerical investigations are undertaken for confined buoyant turbulent jet with varying inlet temperatures. Results of the experimental work and numerical simulations for the problem under consideration are presented. Four cases of different variable inlet temperatures and different flow rates are considered. The realizable k-ε turbulence model is used to model the turbulent flow. Comparisons show good agreements between simulated and measured results. The average deviation of the simulated temperature by realizable k-ε turbulent model and the measured temperature is within 2%. The results indicate that temperatures along the vertical axis vary, generally, in nonlinear fashion as opposed to the approximately linear variation that was observed for the constant inlet temperature that was done in a previous work. Furthermore, thermal stratification exits, particularly closer to the entrance region. Further away from the entrance region the variation in temperatures becomes relatively smaller. The stratification is observed since the start of the experiment and continues during the whole course. Numerical experiments for constant, monotone increasing and monotone decreasing of inlet temperature are done to show its effect on the buoyancy force in terms of Richardson number.     

Large-eddy simulation of suspended sediment transport in turbulent channel flow

The numerical simulation of the non-cohesive sediment transport in a turbulent channel flow with a high concentration is a challenging but practical task. A modified coherent dynamic eddy model of the Large Eddy Simulation (LES) with a pick-up function is used in the present study to simulate the sediment erosion and the deposition in a turbulent channel flow. The rough wall model is used instead of the LES with the near-wall resolution to obtain the reasonable turbulent flow characteristics while avoiding the high costs in the computation. Good results are obtained, and are used to analyze the sediment transport properties. The results show that the streamwise vortices play an important role in the riverbed erosion and the sediment pick-up, which may serve as guidelines for the sediment management and the water environment protection engineering.     

Flow and heat transfer characteristics of falling water film on horizontal circular and non-circular cylinders

This paper presents a two-dimensional CFD study of the falling film evaporation of horizontal tubes with different shapes applied in the seawater desalination. The flow and heat transfer characteristics of the falling water film on one circular tube and two non-circular shaped tubes, a drop-shaped tube and an oval-shaped tube, are analyzed, respectively. The Volume Of Fluid (VOF) method is employed to investigate the influence of the mass flow rate and the feeder height on the distribution of the film thickness and the heat transfer performance. The numerical results show that the minimum value of the film thickness appears approximately at the angular positions of 125 o , 160 o and 170 o for the smooth circular, oval-and drop-shaped tubes, respectively. The film thickness grows with the increase of the mass flow rate and the decrease of the feeder height, while the variation pattern varies for different tubes. Moreover, compared with the circular tube, the drop- and oval-shaped tubes have a lower dimensionless temperature and a thinner thermal boundary layer, which means a better heat transfer performance. Finally, the numerical results correlate well with the experimental and predicted data in literature.     

Numerical simulation of the internal wave propagation in continuously densitystratified ocean

A numerical model is proposed to simulate the internal wave propagation in a continuously density-stratified ocean, and in the model, the momentum equations are derived from the Euler equations on the basis of the Boussinesq approximation. The governing equations, including the continuity equation and the momentum equations, are discretized with the finite volume method. The advection terms are treated with the total variation diminishing （TVD） scheme, and the SIMPLE algorithm is employed to solve the discretized governing equations. After the modeling test, the suitable TVD scheme is selected. The SIMPLE algorithm is modified to simplify the calculation process, and it is easily made to adapt to the TVD scheme. The Sommerfeld＇s radiation condition combined with a sponge layer is adopted at the outflow boundary. In the water flume with a constant water depth, the numerical results are compared to the analytical solutions with a good agreement. The numerical simulations are carried out for a wave flume with a submerged dike, and the model results are analyzed in detail. The results show that the present numerical model can effectively simulate the propagation of the internal wave.     

Numerical prediction of hydrodynamic forces on a ship passing through a lock with different configurations简

A CFD method is used to numerically predict the hydrodynamic forces and moments acting on a ship passing through a lock with a constant speed. By solving the RANS equations in combination with the RNG k-e turbulence model, the unsteady viscous flow around the ship is simulated and the hydrodynamic forces and moments acting on the ship are calculated. UDF is com-piled to define the ship motion. Meanwhile, grid regeneration is dealt with by using the dynamic mesh method and sliding interface technique. Under the assumption of low ship speed, the effects of free surface elevation are neglected in the numerical simulation. A bulk carrier ship model is taken as an example for the numerical study. The numerical results are presented and compared with the available experimental results. By analyzing the numerical results obtained for locks with different configurations, the influences of approach wall configuration, lock configuration symmetry and lock chamber breadth on the hydrodynamic forces and moments are demonstrated. The numerical method applied in this paper can qualitatively predict the ship-lock hydrodynamic interaction and pro-vide certain guidance on lock design.     

NUMERICAL STUDY OF THE PITCHING MOTIONS OF SUPERCAVITA-TING VEHICLES

The pitching motions of supercavitating vehicles could not be avoided due to the lost water buoyancy.In order to have some insight for the design of the supercavitating vehicles,the fixed frequency and free pitching motions are investigated.A numerical predicting method based on the relative motion principle and the non-inertia coordinate system is proposed to simulate the free pitching motions of supercavitating vehicles in the longitudinal plane.Homogeneous and two fluid multiphase models are used to predict the natural and the ventilated supercavitating flows.In the fixed frequency pitching motions,a variety of working conditions are considered,including the pitching angular velocities and the supercavity scales and the results are found to be consistent with the available experimental results in literature.The mesh deformation technology controlled by the moment of momentum equation is adopted to study the free pitching motions and finally to obtain the planing states proposed by Savchenko.The numerical method is validated for predicting the pitching motions of supercavitating vehicles and is found to enjoy better calculation efficiency as comparing with the mesh regeneration technology.     

Modeling of thermodynamics of ice and water in seasonal ice-covered reservoir

A width-averaged 2-D numerical model for simulating vertical distributions of flow and water temperature in reservoirs with an ice cover is developed. In this model, the 2-D flow and water temperature distributions are solved by the finite volume method with the k-? turbulent model. The heat conduction in the ice cover is modeled by the vertical heat transfer and the heat exchanges through the air-ice and ice-water interfaces. The model is applied to a 153 km long reservoir in Songhua River and the simulated results are in a good agreement with the field data of both the vertical water temperature and the ice thickness. The simulated results show that the ice cover thickness in the reservoir is not uniform, the maximum thickness appears in the middle reach, the outflow temperature has an obvious variation as compared with the natural temperature, and a buoyant flow occurs in the reservoir surface at the freeze-up and break-up periods. The model can effectively simulate the water temperature and the ice conditions of large reservoirs in cold regions.     

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     

EXPERIMENTAL AND NUMERICAL INVESTIGATIONS ON SECONDARY FLOWS AND SEDIMENTATIONS BEHIND A SPUR DIKE

For a physical model of the approach navigation channel of Three Gorges Project(TGP), flow patterns around a non-submerged spur dike placed as a mountain in a long rectangular glass flume were experimentally investigated with and without “sucking-spouting” water respectively. Based on rigid lid assumption and Dynamic Smagorinsky Model, all these vortex flows around the spur dike were numerically simulated and analyzed, which probably affect the whole flow field and then probably lead to sedimentations for silt-laden two-phase flows. Meanwhile, silt-laden flows were also investigated experimentally. Both the secondary flow region and the silt sedimentations downstream of the spur dike decrease greatly with “sucking-spouting” water compared to those ones without “sucking-spouting” water. The Finite Volume Method (FVM) was used to discretize the governing equations together with a staggered grid system, where the second order difference is employed for the diffusion terms and the source terms while the upwind difference QUICK is used for the convection terms. The computational results are all in fairly good agreement with the experimental data.     

RESEARCH OF INNER FLOW IN A DOUBLE BLADES PUMP BASED ON OPENFOAM

The inner flow analysis of centrifugal pumps has gradually become an important issue for the hydraulic design and performance improvement.Nowadays,CFD simulation toolbox of pump inner flow mainly contains commercial tools and open source tools.There are some defects for commercial CFD software for the numerical simulation of 3-D turbulent internal flow in pump,especially in capturing the flow characteristics under the off-design operating conditions.Additionally,it is difficult for researchers to do further investigation because of the undeclared source.Therefore,an open source software like Open Field Operation and Manipulation (OpenFOAM) is increasingly popular with researchers from all over the world.In this paper,a new computational study was implemented based on the original solver and was used to directly simulate the steady-state inner flow in a double blades pump,with the specific speed is 111.In order to disclose the characteristics deeply,three research schemes were conducted.The ratios () of the flow rate are 0.8,1.0 and 1.2,respectively.The simulation results were verified with the Particle Imaging Velocimetry (PIV) experimental results,and the numerical calculation results agree well with the experimental data.Meanwhile,the phenomena of flow separation under the off-design operating conditions are well captured by OpenFOAM.The results indicate that OpenFOAM possesses obvious strong predominance in computing the internal flow field of pump.The analysis results can also be used as the basis for the further research and the improvement of centrifugal pump.     

Numerical and experimental study of Boussinesq wall horizontal turbulent jet of fresh water in a static homogeneous environment of salt water

This paper investigates a numerical and experimental study about buoyant wall turbulent jet in a static homogeneous environment. A light fluid of fresh water is injected horizontally and tangentially to a plane wall into homogenous salt water ambient. This later is given with different values of salinity and the initial fractional density is small, so the applicability of the Boussinesq approximation is valid. Since the domain temperature is assumed to be constant, the density of the mixture is a function of the salt concentration only. Mathematical model is based on the finite volume method and reports on an application of standard k- ? turbulence model for steady flow with densimetric Froude numbers of 1-75 and Reynolds numbers of 2 000-6 000. The basic features of the model are the conservation of mass, momentum and concentration. The boundaries of jet body, the radius and cling length are determined. It is found that the jet spreading and behavior depend on the ratio between initial buoyancy flux and momentum, i.e., initial Froude number, and on the influence of wall boundary which corresponds to Coanda effect. Laboratory experiments were conducted with photographic observations of jet trajectories and numerical results are described and compared with the experiments. A good agreement with numerical and experimental results has been achieved.     

Modeling of fully nonlinear wave evolution over a submerged bar

A three-dimensional fully nonlinear numerical wave tank (NWT) is developed by using a time-domain higher-order boundary element method (HOBEM). The source generation of waves is adopted to generate input waves. The mixed Eulerian-Lagrangian (MEL) method is utilized to track and refresh the free surface. The model is applied to three examples of wave evolution and harmonic decomposition during propogation over a submerged bar and numerical results are compared with the experimental data and the solutions of Boussinesq model. It shows that the present model agrees better with the measurements than the Boussinesq model and are more adaptable for stronger nonlinear waves.

     

Towards full predictions of temperature dynamics in McNary Dam forebay using OpenFOAM

Hydroelectric facilities impact water temperature; low velocities in a reservoir increase residence time and enhance heat exchange in surface layers. In this study, an unsteady three-dimensional model was developed to predict the temperature dynamics in the McNary Dam forebay. The model is based on the open-source code OpenFOAM. RANS equations with the Boussinesq approximation were used to solve the flow field. A realizable κ-ε model that accounts for the production of wind turbulence was developed. Solar radiation and convective heat transfer at the free surface were included. The result of the model was compared with the field data collected on August 18, 2004. Changes in diurnal stratification were adequately predicted by the model. Observed vertical and lateral temperature distributions were accurately captured. Results indicate that the model can be used as a numerical tool to assess structural and operational alternatives to reduce the forebay temperature.     

水轮发电机组推力轴承油槽内流动及传热特性研究

为解决推力轴承瓦温过高的问题,采用数值模拟方法分析了某水轮发电机组推力轴承内循环冷却系统油槽内润滑油的流动及传热特性。结果表明,靠近镜板处油流速度大,远离镜板位置油流速度小;镜板内缘压力最低,沿着推力轴瓦径向向外压力逐渐增大;初始瓦温、初始油温一定时,随着水温降低,整体油温下降幅度并不大。根据研究结果,对推力轴承内循环冷却系统提出适当增大冷却器U型管直线段的长度、增大冷却器管径、提高冷却水的压力和流量、加大油冷却器铜管间距的改进建议。     

三角闸门浮箱设置方案对比分析与研究

针对中高水头工作门浮箱设置研究甚少的现状,以引江济淮工程庐江船闸下闸首三角闸门为研究对象,利用有限元软件 ANSYS,建立浮箱式三角闸门和非浮箱式三角闸门两种不同数值模型,详细对比分析两种模型的门体、桁架等主要构件结构强度及刚度变化,以及浮箱设置对顶、底枢等运转件设计的影响;对中高水头三角闸门推荐选择非浮箱式的设计,并为同类型三角闸门设计提供参考。     

三峡水库近坝区三维流场及温度场的数值模拟

基于三维不可压缩流动的N-S方程建立水流水温模型,模型在动量方程中考虑浮力作用并取消了静水压力假定,采用大涡模拟紊流模型计算紊动扩散,并考虑水面散热及其太阳辐射的影响因素,适合于对复杂流态及其有密度分层流动的数值模拟。应用所建立的数学模型对三峡近坝区从庙河至坝前水域的三维流场及温度场进行了数值模拟,计算出的水温分布与坝前观测的水温分布二者符合良好。模拟结果表明,三峡水库在蓄水至139 m水位时,垂向没有明显的水温分层,与实测结果相符。     

热分层湍流直接模拟的高精度算法和数值分析

该文发展了高阶精度有限差分格式的直接数值模拟方法,并将其成功地应用于带有自由面的热分层槽道湍流问题的计算,与低阶精度格式相比,高精度格式有着更宽的低耗散波段,对中高波数分量有较好的模拟能力。该文直接在非均匀网格上采用Lagrange多项式插值构造迎风偏置格式,克服了传统二阶格式在网格变化剧烈时精度损失的缺点,提高了在近壁区的计算精度。对三维槽道湍流以及带有自由面的热分层湍流的流场和温度场的统计特征、脉动特征以及能谱进行了分析,进一步验证了高精度差分方法对热分层剪切湍流问题直接模拟的有效性。总体上讲,该格式精度高,对高频非物理振荡有抑制作用,可以适应变化的空间网格,有效地模拟复杂湍流流动结构,并正确地揭示其流动机理.     

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.     

Influence of artificial ecological floating beds on river hydraulic characteristics

The artificial ecological floating bed is widely used in rivers and lakes to repair and purify polluted water. However, the water flow pattern and the water level distribution are significantly changed by the floating beds, and the influence on the water flow is different from that of aquatic plants. In this paper, based on the continuous porous media model, a moveable two-layer combination model is built to describe the floating bed. The influences of the floating beds on the water flow characteristics are studied by numerical simulations and experiments using an experimental water channel. The variations of the water level distribution are discussed under conditions of different flow velocities（ v= 0.1 m/s, 0.2 m/s, 0.30 m/s, 0.4 m/s）, floating bed coverage rates（20%, 40%, 60%） and arrangement positions away from the channel wall（ D= 0 m, 0.1 m, 0.2 m）. The results indicate that the flow velocity increases under the floating beds, and the water level rises significantly under high flow velocity conditions in the upstream region and the floating bed region. In addition, the average rising water level value（ARWLV） increases significantly with the increase of the floating bed coverage rate, and the arrangement position of floating beds in the river can also greatly influence the water level distribution under a high-flow velocity condition（v ？0.2 m /s）.