Large eddy simulation of free-surface flows

This paper introduces and discusses numerical methods for free-surface flow simulations and applies a large eddy simulation(LES) based free-surface-resolved CFD method to a couple of flows of hydraulic engineering interest. The advantages, disadvantages and limitations of the various methods are discussed. The review prioritises interface capturing methods over interface tracking methods, as these have shown themselves to be more generally applicable to viscous flows of practical engineering interest, particularly when complex and rapidly changing surface topologies are encountered. Then, a LES solver that employs the level set method to capture free-surface deformation in 3-D flows is presented, as are results from two example calculations that concern complex low submergence turbulent flows over idealised roughness elements and bluff bodies. The results show that the method is capable of predicting very complex flows that are characterised by strong interactions between the bulk flow and the free-surface, and permits the identification of turbulent events and structures that would be very difficult to measure experimentally.     

模拟波浪运动的三维自由表面流动数值模型

基于Navier-Stokes方程建立了三维自由表面流动数值模型来模拟波浪运动.模型中Navier-Stokes方程的求解采用两步投影法,计算域的空间离散采用交错网格.采用满足自由表面运动学边界条件的水位控制方程来计算自由表面.通过数值结果与解析解的比较,验证了本文建立的三维自由表面流动数学模型可以精确的模拟线性色散波以及孤立波沿平底坡和非平底坡的传播.     

自由表面流动数值模拟方法的探讨

本文建立和开发了一个用于求解具有自由表面的二维不可压缩粘性流体流动的Navier-Stokes方程的数值模型，这个模型采用ALE方法描述流体的运动，基本方程组中的流速和压力利用Projection 2方法来解耦，空间离散采用有限单元法，而时间离散则采用semi-implicit方法，自由表面位置则通过求解相应的偏微分方程来确定，而计算过程中网格的调整则采用了新的方法，算例结果令人满意，表明方法可行。     

Wavy properties and analytical modeling of free-surface flows in the development of the multi-domain method

The wavy(oscillatory both in space and in time) properties of free-surface flows due to presence of floating bodies are analyzed within the framework of the potential-flow theory by assuming that the fluid is perfect and flow irrotational. A so-called new multi-domain method has been developed based on the fluid domain division by an analytical control surface surrounding bodies and the application of different methods adapted in the external and internal domains. In the analytical domain external to the control surface, the fundamental solution satisfying the linear boundary condition on the free surface associated with a point singularity(often called Green function and referred here as point solution) is applied to capture all wavy features of free-surface flows extending horizontally to infinity. Unlike classical studies in which the control surface is discretized, the unknown velocity potential and its normal derivatives are expressed by expansions of orthogonal elementary functions. The velocity potential associated with each elementary distribution(elementary solutions) on the control surface can be obtained by performing multi-fold integrals in an analytical way. In the domain internal to the control surface containing the bodies, we could apply different methods like the Rankine source method based on the boundary integral equations for which the elementary solutions obtained in the external domain playing the role of Dirichlet-to-Neumann operator close the problem.     

悬沙冲淤问题的湍流两相模型

在双流体模型框架内统一二阶矩两相湍流模型的基础上，建立了改进的悬沙冲淤问题的湍流两相模型-湍流代数应力模型，这一模型不仅能反映两相湍流的各向异性特征，并且更深刻揭示了悬沙颗粒与流体的湍流相互作用的机理，以及浮力、回流及旋流等的影响。适用于河口悬沙冲淤及水库泥沙异重流等工程问题的模拟计算。     

A hybrid DEM/CFD approach for solid-liquid flows简

A hybrid scheme coupling the discrete element method(DEM)with the computational fluid dynamics(CFD)is developed to model solid-liquid flows.Instead of solving the pressure Poisson equation,we use the compressible volume-averaged continuity and momentum equations with an isothermal stiff equation of state for the liquid phase in our CFD scheme.The motion of the solid phase is obtained by using the DEM,in which the particle-particle and particle-wall interactions are modelled by using the theoretical contact mechanics.The two phases are coupled through the Newton’s third law of motion.To verify the proposed method,the sedimentation of a single spherical particle is simulated in water,and the results are compared with experimental results reported in the literature.In addition,the drafting,kissing,and tumbling(DKT)phenomenon between two particles in a liquid is modelled and reasonable results are obtained.Finally,the numerical simulation of the density-driven segregation of a binary particulate suspension involving 10 000 particles in a closed container is conducted to show that the presented method is potentially powerful to simulate real particulate flows with large number of moving particles.     

A hybrid RANS-LES model for combining flows in open-channel T-junctions

In the present study, a hybrid RANS-LES (Reynolds Averaged Navier-Stokes equation — Large Eddy Simulation) model is developed for the simulation of open-channel T-junction flows. The hybrid approach can save computational effort comparing to the LES approach. The comparison between the computational results and the detailed experimental data shows that this relatively new modeling approach is more accurate than the RANS approach in the prediction of the combining flows in T-junctions.

     

3-D hybrid LES-RANS model for simulation of open-channel T-diversion flows

The study of flow diversions in open channels plays an important practical role in the design and management of open-channel networks for irrigation or drainage.To accurately predict the mean flow and turbulence characteristics of open-channel dividing flows,a hybrid LES-RANS model,which combines the large eddy simulation (LES) model with the Reynolds-averaged Navier-Stokes (RANS) model,is proposed in the present study.The unsteady RANS model was used to simulate the upstream and downstream regions of a main channel,as well as the downstream region of a branch channel.The LES model was used to simulate the channel diversion region,where turbulent flow characteristics ate complicated.Isotropic velocity fluctuations were added at the inflow interface of the LES region to trigger the generation of resolved turbulence.A method based on the virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of the LES region in order to take downstream flow effects computed by the RANS model into account and dissipate the excessive turbulent fluctuations.This hybrid approach saves computational effort and makes it easier to properly specify inlet and outlet boundary conditions.Comparison between computational results and experimental data indicates that this relatively new modeling approach can accurately predict open-channel T-diversion flows.     

高浓度水流流速仪的研制与应用

作为一种复杂的固液两相流动，高浓度水流由于固体颗粒浓度高、颗粒级配宽及流动的可视性差等特点，致使其测流一直成为制约高浓度水流运动机理研究的瓶颈性问题。本文依据毕托管工作原理，通过液压传感方式感应测量高浓度水流动水压力和计算机数据采集处理等方法，成功地研制了新型感压式高浓度水流流速仪，并在室内天然沙与粉煤灰高含度水流试验中得到了应用。该流速仪研制成功为进一步研究高浓度水流流速分布规律、揭示其水流结构与输沙机理创造了有利条件。     

A hybrid model for simulating velocity field of a river with complex geometry plunged by multiple jets

1. INTRODUCTION Currently, when complicated movements of flow and sediment are simulated, physical models are usually used. Physical models must have a sound theoretical foundation, and much experience is needed on the use of rigid bed river models in riv…     

Comparative study of MPS method and level-set method for sloshing flows

This paper presents a comparative study of a meshless level-set method in the simulation of sloshing flows. The numerical moving particle semi-implicit （MPS） method and a grid based schemes of the MPS and level-set methods are outlined and two violent sloshing cases are considered. The computed results are compared with the corresponding experimental data for validation. The impact pressure and the deformations of free surface induced by sloshing are comparatively analyzed, and are in good agreement with experimental ones. Results show that both the MPS and level-set methods are good tools for simulation of violent sloshing flows. However, the second pressure peaks as well as breaking and splashing of free surface by the MPS method are captured better than by the level-set method.     

虚拟水库大坝的三维建模方法研究

结合潘家口和大黑汀水库大坝三维模型建设工作流程和关键技术的介绍,得出虚拟水库大坝三维建模的一般方法。     

河道堤防模拟决口水利计算简化法

本文分析了堤防决口泛区水流机理,提出了决口水利计算简化算法的公式及列表试算求解的方法,说明了决口泛区图绘制应注意的问题。     

明渠水流边壁影响修正的计算方法

针对窄深明渠及岸壁附近的明渠水流泥沙运动进行研究时,常常需要除掉岸壁的作用以便了解床面剪切应力的影响.以往对边壁影响修正的方法是基于一些没有完全被验证的假设,床面剪切力的计算数值可能受所采取的边壁修正的方法影响.本研究在分析可靠资料的基础上,发现爱因斯坦对数阻力公式不但可以用于计算不同断面形式的平均的水流情况,而且可以分别用于边壁和床面的阻力计算.将分别对应于边壁和床面的爱因斯坦阻力公式应用阻力计算中,可以解决方程组不闭合的问题.试验资料分析表明,本文提出的方法有较高的精度.     

Assessment of the predictive capability of RANS models in simulating meandering open channel flows

The predictive capability of Reynolds-averaged numerical simulation(RANS) models is investigated by simulating the flow in meandering open channel flumes and comparing the obtained results with the measured data. The flow structures of the two experiments are much different in order to get better insights. Two eddy viscosity turbulence models and different wall treatment methods are tested. Comparisons show that no essential difference exists among the predictions. The difference of turbulence models has a limited effect, and the near wall refinement improves the predictions slightly. Results show that, while the longitudinal velocities are generally well predicted, the predictive capability of the secondary flow is largely determined by the complexity of the flow structure. In Case 1 of a simple flow structure, the secondary flow velocity is reasonably predicted. In Case 2, consisting of sharp curved consecutive reverse bends, the flow structure becomes complex after the first bend, and the complex flow structure leads to the poor prediction of the secondary flow. The analysis shows that the high level of turbulence anisotropy is related with the boundary layer separation, but not with the flow structure complexity in the central area which definitely causes the poor prediction of RANS models. The turbulence model modifications and the wall treatment methods barely improve the predictive capability of RANS models in simulating complex flow structures.     

复杂黏弹性流体运动的数值计算方法

本文提出了一个能够描述具有复杂黏弹性的流体在各种条件下发生变形和流动的数学模型,并建立了相应的数值求解方法.数值方法是在计算流体动力学中常用的SMAC方法的基础上改进时间步进格式,依据问题的性质合理选用显式或隐式,从而达到提高计算效率的目的.数值模型被应用于Oldroyd-B黏弹性流体平板剪切流起动问题的计算,得到的数值解与理论解吻合良好.论文还对针对平板间平行流动进行数值实验,对算法的稳定性和收敛性进行了分析.     

氧化镁微膨胀混凝土在变温条件下膨胀规律数值模拟的当量龄期法

通过对氧化镁混凝土在变温条件下膨胀特性的分析，指出了传统计算方法的不足，提出了计算变温过程中膨胀变化的当量龄期法，即采用变温后曲线相同膨胀量处的切线斜率计算膨胀量。并以温度突升突降过程、温度突变的典型过程及典型的温度过程的3个算例与试验结果进行验证，证明该计算方法可行。     

Membrane finite element method for simulating fluid flow in porous medium

A new membrane finite element method for modeling fluid flow in a porous medium is presented in order to quickly and accurately simulate the geo-membrane fabric used in civil engineering. It is based on discontinuous finite element theory, and can be easily coupled with the normal Galerkin finite element method. Based on the saturated seepage equation, the element coefficient matrix of the membrane element method is derived, and a geometric transform relation for the membrane element between a global coordinate system and a local coordinate system is obtained. A method for the determination of the fluid flux conductivity of the membrane element is presented. This method provides a basis for determining discontinuous parameters in discontinuous finite element theory. An anti-seepage problem regarding the foundation of a building is analyzed by coupling the membrane finite element method with the normal Galerkin finite element method. The analysis results demonstrate the utility and superiority of the membrane finite element method in fluid flow analysis of a porous medium.     

用蒙特卡罗法解有压隧洞出口段水流问题

利用不规则游动网格的蒙特卡罗法对有压隧洞出口段水流进行了数值模拟。根据数值计算与模型试验结果的比较,得出具有凸形曲面边界（包括折面底坡）负压区的存在不仅是边界层分离所致,边壁弯曲使过流产生的离心力也是产生负压的重要原因的结论。计算结果表明：蒙特卡罗法灵活简单、结果稳定,计算结果与实测结果吻合较好。     

自由面区域变换法解三体船兴波问题

为求解三体船兴波问题,基于线性自由面条件的势流兴波理论Rankine源方法,利用物理平面与计算平面的转换关系计算自由面上物理量的导数,开发了三体船兴波阻力预报程序.采用该方法和程序,对不同侧体布局的Wigley三体船兴波阻力及波形进行计算,并将计算结果与模型试验结果、其他作者数值计算结果进行比较,验证了该方法对三体船的适用性,表明采用贴水线网格及自由面区域变换法解三体船兴波问题可提高三体船兴波问题求解效率.