用PDF方程法分析悬沙垂线浓度分布

从颗粒运动的PDF(概率密度分布函数)输运方程出发。建立颗粒相的质量、动量和脉动速度二阶矩方程．对于明渠二维恒定均匀流。利用垂向动量方程导出了新的泥沙扩散方程。表明颗粒脉动强度梯度、升力、重力沉降和紊动扩散都影响悬沙运动。说明了传统扩散方程的不足．理论分析了水沙两相物理属性和水流条件对泥沙扩散系数和浓度分布特征的影响。并通过细颗粒试验资料的分析进行了定量研究．     

泥沙输运模拟综述——现状及其发展趋势

随着流体力学中数值方法的飞速发展,计算模型已经成为研究流体运动,泥沙输运和不同环境(诸如河流,湖泊及沿海地区)中相应污染物归趋过程等方面非常有吸引力的工具,在过去的30多年里,发展了许多计算水动力学/泥沙输运模型.文章追溯当前具有代表性的(一维、二维、三维)模型的发展历程,描述他们各自的特点,优势及局限,力图作为对模型方面感兴趣读者的第一指南,同时也为大家讨论模型的局限性,未来的发展趋势和研究需求方面搭建一个平台.给出了模型的表达,时空特性,水动力学和沉积物的耦合方式,处理非恒定流,推移质和悬移质,泥沙交换过程,泥沙类型(粘性或非粘性)及非均匀泥沙输运的能力.总结了不同模型的应用实例,读者可以运用这些例子作为研究模型设置,模型率定及模型验证的参考.给出了选择泥沙输运模型应遵循的原则,模型输入及率定方面存在的问题及改进的途径.探讨了现有水动力学/泥沙输运模型在处理复杂湍流,泥沙携带,流动与输沙耦合,非均匀泥沙,离散和扩散系数,河岸来沙处理等方面的局限性及改进的方向.最后,对基于多相流思想的泥沙输运模型及其它一些交叉性问题作了评述与展望.     

明渠非定常水流的垂向结构实验研究

本文报告了明渠非定常水流垂向结构的实验研究,通过 LDV 激光测速,我们给出了非定常水流的速度削面,湍流度,Eulerian质量输运速度的垂向分布。实验结果表明:在非定常周期很长时非定常流动的速度剖面,湍流强度及其粗糙度的影响基本上与定常流动相一致,当非定常流动为纯往复流动时,存在一个逆时针方向的环流,这种环流有利于底部的质量输运。     

明渠中悬移质的弥散-对流方程及悬浮机理

悬移质泥沙通常构成冲积河流总输沙量的主体, 研究悬移质的悬浮机理具有重要的意义. 以双流体模型为基础, 通过引入弥散速度的概念, 建立了悬移质泥沙的输沙方程以及泥沙扩散系数的本构关系. 应用该方程分析了二维明渠均匀流中悬移质泥沙浓度垂向分布规律, 并与Einstein 和Chien 的泥沙浓度实验资料及经典扩散理论进行了对比. 以此为基础, 分析了紊动扩散、颗粒自身的紊动、颗粒碰撞应力对泥沙悬浮的影响在垂向上的变化, 以及浓度、粒径等对这些因素的影响. 结果表明, 泥沙颗粒在明渠紊流中的扩散是浑水的紊动扩散、颗粒自身的紊动、颗粒碰撞应力3 部分不同机制共同作用的结果, 把泥沙颗粒的悬浮简单归因于水流的紊动是不全面的.     

长江口拦门沙地区泥沙输运

本文建立了长江口拦门沙地区泥沙运动的数学模型，所得水流和含沙量与现场观测资料相符合，分析了潮波和径流作用下泥沙输运规律，预测了整治工程实施后主航道地貌变化．     

定床弯道内水沙两相运动的数值模拟

在适体同位网格中采用非正交曲线坐标系下的三维k-ε-kp固液两相双流体湍流模型研究弯道内水流和悬浮泥沙运动,主要计算了试验室S型水槽内清水流动的三维流场、120°弯道内水沙两相流动中底沙与底流的运动轨迹以及S型水槽内水沙两相流动的两相流场和泥沙浓度场. 对于S型水槽内清水流动,数值结果与试验结果吻合良好. 120°弯道内水沙两相流动中固液两相的运动轨迹在弯道直线段基本重合,在弯道内泥沙轨迹逐步偏离水体轨迹,其偏离程度随泥沙粒径增大而增大. 从S型水槽内水沙两相流动计算结果中发现泥沙纵向流速在壁面附近比水流纵向速度大,在远离壁面区域比水流纵向速度小;弯道内泥沙横向流速比水流横向流速小;垂向流速在直线段和泥沙沉速相当,在弯道内受螺旋水流影响而变化;两相流速差别随泥沙粒径增大而变大;泥沙浓度呈现下浓上稀的分布,在弯道内横向断面上呈现凸岸大凹岸小的分布,泥沙浓度随泥沙粒径增大而减小.      

河型发育过程的二维数值模拟研究

改进已有二维平面水沙模型,进一步考虑河床粗化、弯道二次流及河床结构对泥沙输运的影响;并针对河湾形态对崩岸的影响提出新的非粘性土崩岸模拟方法,使其能模拟边滩的形成。利用改进后模型分别对Yen弯道水槽泥沙冲淤试验及Friendkin室内弯曲小河塑造试验进行模拟;模拟结果与试验观测资料吻合较好,表明考虑弯道对泥沙输运及河岸崩塌影响的模型能够更准确反映天然河流演化过程,为研究河型演化机理及各控制因素变化规律提供新手段。     

低浓度固液两相流的颗粒相动理学模型

用广义Fokker-Planck扩散模型描述液相湍动对颗粒的挟带作用，用修正的BGK模型描述粒间碰撞效应，建立了封闭的颗粒相PDF输运方程．运用Chapman-Enskog迭代法求得方程的二阶近似解，获得颗粒相脉动速度二阶矩和三阶矩闭合关系．模型与颗粒流模型相容，与液相湍流闭合模型是否相容依赖于扩散模型的具体形式，并据此比较了不同的涡一颗粒作用模型．模型与二维明渠流轻质沙和天然沙试验资料符合很好．表明细小粒径颗粒能够充分跟随水流运动；大粒径颗粒的相间平均速度差和壁面滑移速度明显，近壁区内的颗粒沿流向和垂向脉动强度都可能大于水流，并存在一定程度的颗粒碰撞效应．     

细颗粒泥沙净冲刷和输移的大涡模拟研究

在传统水沙输移数值模拟研究中一般采用雷诺时均模拟技术(Reynolds-averaged simulation,RANS).与RANS相比,大涡模拟技术(large eddy simulation,LES)能够更加精确反映细部流动结构,计算机的发展使得采用LES探讨水流和泥沙运动规律成为可能.本文尝试给出净冲刷条件下悬沙计算的边界条件,采用动态亚格子模式对循环槽道和长槽道中的水流运动和泥沙输移进行了三维大涡模拟研究.利用直接数值模拟(direct numerical simulation,DNS)结果对LES模型进行了率定,计算结果符合良好,在此基础上初步探讨了泥沙浓度、湍动强度和湍动通量等的分布特征.结果表明,净冲刷条件下输沙平衡时泥沙浓度符合Rouse公式分布,单向流动中泥沙浓度沿着流向逐渐增大.泥沙浓度湍动强度和湍动通量都在近底部达到最大值,沿着垂向迅速减小.湍动黏性系数和扩散系数基本上在水深中间处达到最大.湍动Schmidt数沿着水深方向不是常数,在近底部和自由水面附近较大,在水深中间处较小.     

基于有限体积法的三维局部冲刷数值模型研究及应用

局部冲刷的三维数值模拟可预测水力冲刷的破坏程度和破坏机制,进而提供更加合理的工程措施以减轻或避免局部冲刷造成的工程破坏。基于有限体积法和非结构化的计算网格构建了以水动力学模型、泥沙冲淤和河床变形方程为基础的三维局部冲刷数值模型。水动力学模型中的湍流模型为剪切应力SST k-ω输运模型,泥沙冲淤以底床切应力大小和分布为基础,水沙模型的耦合采用单向弱耦合方式。首先,通过ANSYS-FLUENT软件数值计算水动力学模型后,将水力特性数据单向传递至泥沙模块,并应用UDF函数二次开发实现泥沙模型的数值计算。利用动网格技术重构因河床地形更新引起的变形网格。与动床圆柱冲刷和丁坝局部冲刷的试验结果进行比较,验证了局部冲刷数值模型的可靠性。从平衡冲深时的冲刷深度和冲坑内水流特性等结果的对比可以看出,该数值模型成功地模拟出最终冲刷地形和形态,并能捕捉不同时刻的三维地形变化。根据数值模型的建立及应用结果分析主要得到以下结论,以切应力观点为基础开发该模型时,具有简易性和较强的可靠性;单元体泥沙通量的重构和床面坡度等因素均影响模型的精度;FLUENT软件提供的动网格技术能较好重构小变形网格,但是重构因地形变化引起的大变形网格时略显不足。     

Erosion and sedimentation of a bump in fluvial flow

The 2D laminar quasistationary interacting boundary layer flow with mass transport of a suspended sediment is solved over an erodable bump (or dune) in the case of a fluvial régime. It is assumed that if the skin friction goes over a threshold value, the bump is eroded, then, the concentration of sediment in suspension is convected but falls at a constant settling velocity. This changes the shape of the dune, exemples of displacement toward final equilibrium states are presented.     

振荡潜流带沉积层?水界面污染物输运的研究

潜流带中污染物质交换与输运特性是影响水资源环境的重要问题之一. 本文对底部为高渗透沉积层的三维槽道振荡流高Schmidt数传质问题进行了大涡模拟研究. 采用动力学亚格子模型来封闭滤波后的三维不可压缩Navier-Stokes方程以及污染物输运方程, 同时采用修正的Darcy-Brinkman-Forcheimer模型来描述沉淀有锌离子污染溶质的可渗透沉积层. 通过对沉积层内外流场和浓度场的统计特性以及瞬态结构的分析, 探究了上覆水体中振荡流驱动作用对污染物输运的动力学影响以及扩散率随振荡周期和振荡角的变化规律. 研究结果表明, 浓度通量中的湍流浓度分量在垂向物质交换中起主导作用, 流向、展向速度, 湍流强度和污染物浓度的波动跟随振荡驱动力呈现准周期变化, 同时发现沉积层?水交界面处的湍流浓度通量与法向湍流强度两者之间的变化具有明确的相关性. 并且在较大振荡角和低频振荡的情况下, 沉积层?水交界面处的有效扩散率增大, 这主要是来自于沉积层?水交界面处流体的猝发行为有效促进了湍流混合和物质交换, 进而增强了污染物的垂向输运.      

用LDA测量U形水槽内的振荡流

本文介绍LDA用于测量U形水槽内的振荡流。水槽直管段为正方形,弯管段内侧弯面及外侧弯面是专门计算设计的。对于水平直管段、竖管段及弯管段共七个截面,测量了边界层外振荡流速的振幅分布。对于水平直管段上底面及下底面各三个截面,测量了边界层內振荡流速的振幅分布及相位分布。测量结果和无限长槽道的理论计算结果作了比较,二者基本符合。     

Mobile bed thickness in skewed asymmetric oscillatory sheet flows

A new instantaneous mobile bed thickness model is presented for sediment transport in skewed asymmetric oscillatory sheet flows. The proposed model includes a basic bed load part and a suspended load part related to the Shields parameter, and takes into account the effects of mass conservation, phase-lag, and asymmetric boundary layer development, which are important in skewed asymmetric flows but usually absent in classical models. The proposed model is validated by erosion depth and sheet flow layer thickness data in both steady and unsteady flows, and applied to a new instantaneous sediment transport rate formula. With higher accuracy than classical empirical models in steady flows, the new formula can also be used for instantaneous sediment transport rate prediction in skewed asymmetric oscillatory sheet flows.     

Large eddy simulation of boundary layer flow under cnoidal waves

Water waves in coastal areas are generally nonlinear, exhibiting asymmetric velocity profiles with different amplitudes of crest and trough. The behaviors of the boundary layer under asymmetric waves are of great significance for sediment transport in natural circumstances. While previous studies have mainly focused on linear or symmetric waves, asymmetric wave-induced flows remain unclear, particularly in the flow regime with high Reynolds numbers.Taking cnoidal wave as a typical example of asymmetric waves, we propose to use an infinite immersed plate oscillating cnoidally in its own plane in quiescent water to simulate asymmetric wave boundary layer. A large eddy simulation approach with Smagorinsky subgrid model is adopted to investigate the flow characteristics of the boundary layer. It is verified that the model well reproduces experimental and theoretical results. Then a series of numerical experiments are carried out to study the boundary layer beneath cnoidal waves from laminar to fully developed turbulent regimes at high Reynolds numbers, larger than ever studied before.Results of velocity profile, wall shear stress, friction coefficient, phase lead between velocity and wall shear stress, and the boundary layer thickness are obtained. The dependencies of these boundary layer properties on the asymmetric degree and Reynolds number are discussed in detail.     

Towards the numerical simulation of the horizontal slug front

Steps towards the numerical simulation of the flow behind the slug front in horizontal slug flow performed with a streamfunction-vorticity representation of the mean flow and an energy dissipation model for the turbulence are discussed. The flow field consists of two vortices, one saddle point and four stagnation regions. Attention is focused on the following boundary conditions: moving wall jet, moving wall, free jet velocity discontinuity and vertical liquid-gas open surface. A dissipation flux boundary condition is suggested to simulate the interaction of the turbulent eddies with the open surface. A method to assess the necessity to use a transport model equation for the dissipation rather than a geometric specification of a length is suggested. Three different ways to characterize the mixing zone length are proposed.     

Unsteady MHD free convection flow past a vertical permeable flat plate in a rotating frame of reference with constant heat source in a nanofluid

The unsteady magnetohydrodynamic flow of a nanofluid past an oscillatory moving vertical permeable semi-infinite flat plate with constant heat source in a rotating frame of reference is theoretically investigated. The velocity along the plate (slip velocity) is assumed to oscillate on time with a constant frequency. The analytical solutions of the boundary layer equations are assumed of oscillatory type and they are obtained by using the small perturbation approximations. The influence of various relevant physical characteristics are presented and discussed.     

Two approaches to modeling unstable flow and mixing of variable density fluids in porous media

Density variation of aqueous phase fluids flowing in a porous medium, resulting from spatial and temporal variation of solute concentration, often gives rise to unstable flow, and therefore has a significant effect on solute transport. Studies on simulating unstable flow and mixing of variable density fluids in seemingly homogeneous porous media are rare. In this study, a three-dimensional (3-D) and a one-dimensional (1-D) model were developed to simulate unstable flow and mixing in a vertical, nominally 1-D system. In the 3-D model, the fluid flow and solute transport equations were solved numerically with a very fine spatial discretization. The 1-D numerical model was derived from a theoretical model to simulate the flow and mixing of fluids with variable density and viscosity at the field scale. To evaluate the models, simulated results were compared with experimental data from displacement experiments in a vertical sand column. The results show that the 1-D model provides fairly good prediction of breakthrough curves and that the 3-D model is able to qualitatively simulate breakthrough curves for highly unstable flow and mixing.Contribution from the Alabama Agric. Exp. Sta. as AAES Journal No. 3-955037.     

振荡边界层研究及其应用

综述了振荡边界展的研究现状,包括:层流边界层,转捩和湍流边界层的理论、数值和实验研究的最新进展．在自然界,最典型的例子是波浪、潮汐作用下的水底边界层．同时讨论了水底边界层的特点及其中物质输送的动力学过程,还给出了实际应用的例子．      

New interpretation of specific sign of Reynolds stress in the boundary layer on a flat plate

The specific sign of Reynolds stress in the boundary layer on a flat plate at zero incidence is newly interpreted in present paper based on the theory of vortex-induced vortex. It avoids some problems appeared in a traditional explanation, on the basis of relationship between mean and fluctuating flows due to the transport of momentum. Through the analysis of local flow field in the immediate neighborhood of wall, the characteristics of Reynolds stress are identified through introducing turbulence-induced small-scale streamwise eddies above the flat plate. The positive Reynolds stress is theoretically verified. And such new interpretation illustrates that the generation of Reynolds stress, as well as fluctuating velocity, is intrinsically independent of the mean flow. But its specific sign would be determined by the mean flow due to the inertial forces. Other features,such as the intensity relationship among three components of fluctuating velocity, are also presented.