部分植被化复式河道水流的二维解析解

运用涡粘模型理论对部分植被化复式河道的水流水深平均流速和边壁切应力分布进行了求解.通过对水流微元体进行纵向受力分析建立相应的控制微分方程,其中植被对水流的影响归结为拖曳力项.同时将复式渠道划分为3个子区域,通过联立求解各区域微分方程中的定解系数,最终得到均匀流的条件下各区水深平均流速的横向分布的解析解.在获得水深平均流速的横向分布后,可进一步给出对泥沙输移有重要影响的河床切应力的横向分布.通过与试验测得的资料比较,表明给出的解析解能够为工程设计提供足够精度的水力特性的预报.     

梯形渠道水力最优断面算图

一、基本原理 本文介绍利用算图来求解梯形渠道水力最优断面的一种方法.已知流量Q,比降i,边坡系数m及糙率n,采用巴甫洛夫斯基公式,求解最优断面的水深h,底宽b及流速v.参阅资料[1],问题归结为求解下列方程:     

河底和岸壁水流剪切应力“标准断面”法

零剪切应力分割线法和分割线表观剪切应力法是计算河底和河岸水流剪切应力的两种常用方法.为简化分割线表观剪切应力经验表达式,提出了“动量传输 平衡偏离”（momentum transfer equilibrium deviation,MTED）假设,认为表观剪切应力可由分割线一侧单位时间动量传输与其平衡值的差值来表示.为了确定平衡值,提出了标准断面的概念,所有矩形或者梯形断面都有对应的标准断面.基于MTED假设和标准断面的概念,建立了分割线表观剪切力以及河底和河岸剪切力占总剪切力比重的计算表达式.利用不同实验的200多个数据对不同的计算方法进行了对比分析,结果表明:该文的方法有效改善了计算精度,适用范围广,适用于矩形和梯形断面,以及河岸与河底糙率相同或不同的情况.     

考虑滩地植被的复式断面河道水流的二维解析解

针对滩地植被化的复式断面河道的水流特性,从沿水深积分的紊流时均运动微分方程出发,基于恒定均匀流的假设,给出了植被作用下河道水流水深平均流速沿横向分布的二维解析解.其中,将植被对水流的影响归结为拖曳力项,在模型中计及了二次流的影响,并对二次流强度系数K的取值进行了初步探讨.分别对顺直河道横断面和蜿蜒河道的顶点断面进行了计算,计算结果与试验资料符合良好,表明给出的二维解析解可用于植被作用下复式断面河道水流水深平均流速沿横向分布的数值预报.     

漫滩水流二次流项系数研究

基于SKM方法,引入二次流项系数,给出了漫滩水流水深平均流速沿横向分布的二维解析解．文中对SERC-FCF的系列试验进行了模拟,计算结果与实测资料吻合较好．在此基础上,进一步研究了复式河道断面形态对二次流项系数的影响,并分析了造成各种影响的原因．计算结果表明,二次流项系数的大小与断面形态有关,而其正负号与二次流的方向有关,这为二次流项系数的选取提供了参考依据．     

小角度斜向入流条件下复式断面明渠流速重分布线性理论

自然界中复式河道的来流方向常受径流量、滩槽形态影响,往往与主槽存在小幅度夹角,使得目前基于顺直河道假定的漫滩水流大量的理论成果难以适用.为研究斜向入流影响,采用平面二维浅水方程描述沿程均匀的复式断面明渠水流运动,选取斜向角度作为小参数,运用摄动法推导了小角度(θ20°)斜向入流条件下复式河道流速分布的线性解析解,并利用数值模拟结果进行验证,流速分布吻合较好.理论分析结果表明,斜向来流时由于出现垂直于河道方向流速分量,使得顺河道方向流速沿河宽分布偏离正向来流情况下的对称形态而重新分布,入流侧流速减小而对岸流速增大;在斜向角度θ=13°且滩槽水深比为3∶8的情况下,偏离幅度可达21.8%,该幅度随滩槽水深比的减小而增大.该文针对斜向来流对流速分布的修正将为进一步研究复式河道泥沙运动和河流演变提供更为准确的水动力条件.     

渠道水力计算表及其原理

大家知道,各种用途的各类渠道都要进行渠道设计.在渠道的横断面设计中,必须进行渠道的水力计算.渠道水力计算的主要任务是计算渠道的断面尺寸,以保证能通过所要求的流量,或校核已建成的渠道是否具有所要求的输水能力. 梯形渠道水力计算通常采用试算法,由于试算没有定量的规律可循,所以计算往往较繁.特别是在缺乏实践经验的情况下可能会作一些不必要的试算.     

渠道水力最优断面设计算图的一种编制方法

在进行渠道水力计算的过程中,当流量 Q、渠道底坡 i 及糙率 n 已知的情况下,都希望得到最小的过水断面面积,以减少土石方开挖量,或者说,在过水断面面积、渠底坡度和糙率一定的条件下,使渠道所通过的流量最大.水力学中把满足上述条件的断面形式叫做水力最优断面.     

部分植被化矩形河槽紊流时均流速分布分析解

研究了部分植被化矩形河槽紊流的水深平均流速分布．植被被视为不可移动的刚性多孔介质,植被对水流的阻力以多孔介质理论加以考虑,并综合考虑部分植被存在时矩形河槽紊动水流二次流的作用,建立了紊流动量方程．针对恒定均匀流的特点,对动量方程进行了简化,沿水深方向积分并引入参考量,形成无量纲形式的基于多孔介质理论紊动水流控制方程,进而对其求解给出了水深平均纵向时均流速分布的分析解．研究表明,在不同水流条件下的二次流强度系数具有相同的数量级．为验证分析解的正确性,在实验室采用MicoADV测量了部分植被化矩形河槽水流的流速分布．数值解与实验资料和日本学者的相关实验资料的对比表明,该方法可以准确预测部分植被化矩形河槽紊流水流的水深平均流速分布．     

复式断面明渠中水平热水浮力射流近区稀释特性研究

采用考虑浮力效应的RNG k-ε双方程模型,结合混合有限分析法对复式断面明渠流中水平圆孔热水浮力射流进行了数值模拟,对其近区稀释特性及二次流结构进行数值分析．同时,采用三维ADV测速仪及温度测量仪对其近区典型断面的:1) 横断面主流速等值线分布;2) 二次流;3) 横断面温度等值线分布进行了试验测量．数值模拟结果与试验资料的对比表明,考虑浮力效应的RNG k-ε模型能够较好模拟射流在环境来流、浮力效应、复式断面二次流等因素作用下,所表现出的分叉现象、附壁效应（Conada效应）和边滩二次流现象．     

A dynamic subgrid-scale model for the large eddy simulation of stratified flow

A new dynamic subgrid-scale (SGS) model, including subgrid turbulent stress and heat flux models for stratified shear flow is proposed by using Yoshizawa’s eddy viscosity model as a base model. Based on our calculated results, the dynamic subgrid-scale model developed here is effective for the large eddy simulation (LES) of stratified turbulent channel flows. The new SGS model is then applied to the large eddy simulation of stratified turbulent channel flow under gravity to investigate the coupled shear and buoyancy effects on the near-wall turbulent statistics and the turbulent heat transfer at different Richardson numbers. The critical Richardson number predicted by the present calculation is in good agreement with the value of theoretical analysis     

Two-layer flows of generalized immiscible second grade fluids in a rectangular channel

Unsteady one-dimensional flows of two incompressible and immiscible generalized second grade fluids in a rectangular channel are studied. A constant pressure gradient acts in the flow direction, while the channel walls have oscillating translational motions in their planes. The generalization considered in this paper consists into a mathematical model based on constitutive equations of second grade fluid with Caputo time-fractional derivative in which the history of the shear stress influences the velocity gradient. The velocity and shear stress fields in the Laplace transform domain are obtained. Numerical solutions for the real velocity and shear stress have been found by employing the Stehfest numerical algorithm for the inverse Laplace transform. The influence of the fractional parameters on the velocity and shear stress has been studied by numerical simulations and graphical illustrations. It is found that the memory effects are significant only for small values of the time t.     

Numerical modeling of hydrodynamic and hydrothermal characteristics in subtropical alpine lake

A three-dimensional, time-dependent hydrodynamic and hydrothermal model was performed and applied to the subtropical alpine Yuan-Yang Lake (YYL) in northeastern region of Taiwan. The model was driven with discharge inflow, heat, and wind stress to simulate the hydrodynamic and hydrothermal in the lake. The model was validated with measured water surface elevation, current, and temperature in 2008. The overall model simulation results are in quantitative agreement with the available field data. The validated model was then used to investigate wind-driven current, mean circulation, and residence time in the YYL. The modeling results reveal that the velocity field along the wind axis present the variations over depth with return current where the velocity at the surface layer is along the wind direction while it is opposite near 1 m below water surface. The simulated mean current indicates that the surface currents flow towards the southwest direction and form a clock-wise rotation. The calculated residence time is strongly dependent on the inflows and wind effects. Regression analysis of model results reveals that an exponential regression equation can be employed to correlate the residence time to change of discharge input. The residence time without wind stress is higher than that with wind effect, indicating that wind plays an important role in lake mixing. The calculated residence time is approximately 2-2.5 days under low inflow with wind effect.     

浅水亚临界航速舰船水压场计算方法研究

基于浅水波动势流理论和薄船假定,建立了浅水亚临界航速舰船水压场理论数学模型．采用有限差分方法,对浅水亚临界航速舰船水压场分布特征进行了数值计算．分析了航道岸壁、Froude数、色散效应对舰船水压场的影响,利用虚拟长度法改善了计算结果．通过与源汇分布法、Fourier积分变换法以及实验结果进行比对,验证了所建立的舰船水压场数学模型和计算方法的正确性．     

Nonlinear mathematical analysis for blood flow in a constricted artery under periodic body acceleration

This study analyses the pulsatile flow of blood through mild stenosed narrow arteries, treating the blood in the core region as a Casson fluid and the plasma in the peripheral layer as a Newtonian fluid. Perturbation method is employed to solve the resulting coupled implicit system of non-linear partial differential equations. The expressions for shear stress, velocity, wall shear stress, plug core radius, flow rate and longitudinal impedance to flow are obtained. The effects of pulsatility, stenosis depth, peripheral layer thickness, body acceleration and non-Newtonian behavior of blood on these flow quantities are discussed. It is noted that the plug core radius, wall shear stress and longitudinal impedance to flow increase as the yield stress and stenosis depth increase and they decrease with the increase of the body acceleration, pressure gradient, width of the peripheral layer thickness. It is observed that the plug flow velocity and flow rate increase with the increase of the pulsatile Reynolds number, body acceleration, pressure gradient and the width of the peripheral layer thickness and the reverse behavior is found when the yield stress, stenosis depth and lead angle increase. It is also recorded that the wall shear stress and longitudinal impedance to flow are considerably lower for the two-fluid Casson model than that of the single-fluid Casson model. It is found that the presence of body acceleration and peripheral layer influences the mean flow rate and mean velocity by increasing their magnitude significantly in the arteries.     

Scalar Conservation Laws with Nonconstant Coefficients with Application to Particle Size Segregation in Granular Flow

Granular materials will segregate by particle size when subjected to shear, as occurs, for example, in avalanches. The evolution of a bidisperse mixture of particles can be modeled by a nonlinear first order partial differential equation, provided the shear (or velocity) is a known function of position. While avalanche-driven shear is approximately uniform in depth, boundary-driven shear typically creates a shear band with a nonlinear velocity profile. In this paper, we measure a velocity profile from experimental data and solve initial value problems that mimic the segregation observed in the experiment, thereby verifying the value of the continuum model. To simplify the analysis, we consider only one-dimensional configurations, in which a layer of small particles is placed above a layer of large particles within an annular shear cell and is sheared for arbitrarily long times. We fit the measured velocity profile to both an exponential function of depth and a piecewise linear function which separates the shear band from the rest of the material. Each solution of the initial value problem is nonstandard, involving curved characteristics in the exponential case, and a material interface with a jump in characteristic speed in the piecewise linear case.     

基于系统识别方法的围岩弹性模量及水平地应力反演分析

利用有限元软件建立隧道开挖正演模型,基于新奥法隧道施工现场实测围岩收敛数据,采用灵敏度分析建立了参数调整算法,利用系统识别方法对隧道围岩弹性模量及水平地应力进行了反演分析,并讨论了初始值的影响.结果表明,系统识别反演分析法具有自适应能力强、反演分析过程收敛计算稳定性好和计算速度快等优点,在隧道及地下工程领域具有广阔的应用前景.     

An improved dynamic subgrid-scale model and its application to large eddy simulation of stratified channel flows

In the present paper, a new dynamic subgrid-scale (SGS) model of turbulent stress and heat flux for stratified shear flow is proposed. Based on our calculated results of stratified channel flow, the dynamic subgrid-scale model developed in this paper is shown to be effective for large eddy simulation (LES) of stratified turbulent shear flows. The new SGS model is then applied to the LES of the stratified turbulent channel flow to investigate the coupled shear and buoyancy effects on the behavior of turbulent statistics, turbulent heat transfer and flow structures at different Richardson numbers.     

Numerical modeling of surge overtopping of a levee

Levee protection/armoring is critical in flood fighting, and understanding the flow characteristics involved requires the evaluation of the overtopping processes with a variety of tools. The Adaptive Hydraulics Model (AdH) is used to calculate velocity and depth during an overtopping event. Using these values, the work explores the application of AdH for the estimation of associated shear stresses, so that appropriate measures are applied for protection to ultimately reduce the probability of levee failure during an overtopping event. Four different depths and three Manning’s n values were used for a total of twelve different test cases. Results show mean shear stress increases with increased surge depth and roughness. Additionally, the area of greatest shear stress is shown to be at the slope transitions from levee to berm. Values calculated in this effort should be considered in the design and implementation of levee protection.