后向台阶流动研究综述

综述近年来后向台阶流动研究的进展及现状。对研究台阶后再附着点位置、时均流动的漩涡结构、起动涡、雷诺应力和剪切层中大涡结构等重要成果进行了归纳；对大涡模拟在后向台阶流动中的研究状况进行介绍；对现有的研究方法及存在问题进行分析，并指出了进一步的研究方向。     

带自由表面的后向台阶起动流的流场模拟

对雷诺数Re=1350带自由表面的后向台阶起动流进行数值模拟,得到其自由表面、再附着长度以及台阶后分离涡涡心位置随时间变化的规律,并进一步讨论了自由表面的波动和台阶后大尺度涡结构的产生、发展及溃灭的过程对再附着长度的影响。     

应用PIV进行二维后向台阶流流动特性的研究(2)二维后向台阶流起动流的研究

利用PIV技术，对二维后向台阶起动流的瞬态速度场进行了观测。得到层流至湍流情况下起动流的一些运动规律。本文分析了流动中起动涡等旋涡的运动和发展特征。分析了起动流过程中再附着点的位置、起动涡等旋涡涡心的位置等的变化规律。     

方型人工鱼礁周围水流运动的数值模拟研究

采用计算流体动力学(CFD)技术,模拟得到了5种不同来流速度(0.1、0.2、0.4、0.6和0.8 m/s)下方型人工鱼礁(边长为3 m)周围的水流场。铅垂平面上的计算结果表明:当水流贴近礁体迎流面时,水流抬升而形成上升流;不同来流速度工况下,上升流最大速度均约为来流速度的0.64倍,上升流平均速度均为来流速度的0.12倍,而上升流的最大高度为礁体高度的2.62～2.65倍;由于流动分离,在礁体顶面形成一小旋涡区,而在礁体背水面后端形成背涡区;5种来流速度下的背涡区长度均为礁体高度的3.0～3.5倍,而背涡区高度为礁体高度的1.1～1.2倍。水平面上的计算结果表明:紧贴礁体四周为水流减速区,在礁体两侧形成小旋涡区(缓流区),而在礁体背部形成一大旋涡区(背涡区);背涡区内的水流旋涡结构随来流速度大小而变,但背涡区的最大宽度基本不随来流速度的改变而改变,约为礁体宽度的2倍。     

明渠突扩分离流底部回流长度统计特性研究

利用大涡模拟技术,对明渠突扩分离流三维流动进行了数值模拟,得到明渠突扩分离流流场精细速度分布特征,并与试验结果进行了比较,指出:在高雷诺数下,明渠突扩分离流的底部回流长度随时间不断震荡,但整体统计特征值趋于稳定,统计平均长度与试验结果相符,并且对雷诺数的依赖很小。     

串列梯形柱绕流水动力特性数值分析

该文采用数值模拟分析了雷诺数为150和间距比为3的串列双梯形柱绕流水动力特性。当梯形柱背流面与迎流面长度比为d/D=0、0.3和0.5时,双柱的上侧旋涡融合,在尾部形成了不对称的P+S泄涡模式,导致下游柱体承受的平均升力偏离零值。d/D=0.7的双梯形柱泄涡转变为2S模式,表现为上游柱体旋涡与下游柱体旋涡融合后交替脱落。双方柱(d/D=1)间隙以及下游方柱尾部均形成了准静态涡,剪切层有小幅摆动,但未卷曲形成旋涡。d/D=0和0.3时,上游柱体后方由于边界层再附着形成了次生涡,增大了局部压力。随着d/D的增大,上游柱体旋涡形成长度增长,双柱间隙流动趋于稳定,下游柱体前缘的横向速度脉动逐渐减弱,其后方的尾迹也趋于对称。上游柱体的遮蔽效应使其平均阻力大于下游柱体,而间隙流的不稳定性与边界层的交替再附着使得下游柱体升力偏大。双方柱绕流场处于拟稳定状态,其水动力系数远小于三棱柱和梯形柱。相较于双圆柱,三棱柱和梯形柱的边界层分离点靠前,增大了绕流的升阻力。     

海山后尾迹涡形成因素的数值研究

该文采用regional ocean modeling system(ROMS)海洋模式,对理想channel中的海山模型,研究均匀分层流作用下,不同的伯格数和来流条件对海山诱导的尾迹涡及垂向流动的影响。结果表明,伯格数越大,越有利于海山后尾迹涡的产生,涡的强度不断增大,且涡脱落频率加快;随流速的增加,海山后尾迹涡的强度增大,涡脱落频率加快,且海山周围的垂向流动增强;当来流速度较低(0.05 m/s)及较高(0.8 m/s)时,均不利于在海山后形成稳定的垂向流动。     

人工鱼礁开口比对流场效应影响的三维数值模拟研究

采用CFD技术,模拟得到了开口立方体人工鱼礁绕流的三维水流场。通过速度场模拟结果分析,揭示出了流场效应随开口比变化的规律:1上升流最大速度与来流速度的比、上升流平均速度与来流速度的比、以及上升流最大高度与礁体高度的比均随开口比的增大而降低;2背涡区的尺度随开口比的增大而显著降低,当开口比为0.1和0.2时,在礁体背流面后端形成尺度较大而速度很小的尾涡区,而当开口比为0.5和0.6时背流面后端已无明显旋涡区。此外,模拟结果也揭示出上升流特性参数和速度场分布图案几乎不受来流速度大小的影响。     

陷落腔剪切层自持振荡的数值计算分析

该文主要研究陷落腔剪切层自持振荡的水动力特性,以不可压缩Navier-Stokes方程为控制方程,利用计算流体力学软件CFX中的高级湍流模型Larger eddy simulation(LES),使用simple算法进行数值仿真计算.数值计算结果表明:随着腔中的大涡沿着随边向上爬升并趋于稳定,在腔口导边处不断有附着涡脱出,而后又很快的破碎;得出不同雷诺数下腔口剪切层处的压力系数沿周向的分布规律及剪切层自持振荡的频率特性,总结出剪切层处的各阶振荡频率随着雷诺数的不同趋于常数;将数值计算得出的结果与物理实验的数据进行对比分析,从而验证了数值计算的可靠性.     

明渠柱体上游马蹄涡的运动学特征研究

马蹄涡是水下柱体结构周围床面冲刷的主要动力。为掌握马蹄涡的运动学特征,引入了基于旋转强度的马蹄涡识别方法,提出了基于椭圆涡拟合的马蹄涡特征提取方法,其中,椭圆涡由Oseen涡和倾斜单向剪切流叠加而成。基于上述方法,通过开展明渠圆柱绕流PIV实验,对柱体上游对称面时均流场中马蹄涡的运动学特征进行了研究。结果表明,在柱体雷诺数大于104的紊流柱体绕流中,马蹄涡上游的流动分离点随柱体雷诺数的增大而缓慢向下游移动,但马蹄涡的位置保持不变,涡中心距柱体中轴约0.67D(D为柱体直径或宽度),距床面约0.06D;随着柱体雷诺数的增大,马蹄涡的半径不变而强度增加,使得马蹄涡作用于床面的剪切应力增大。研究结果建立了客观提取马蹄涡运动学特征的数学方法,得到了流动分离点、马蹄涡位置等运动学特征随水流条件的变化规律,揭示了明渠柱体绕流与其它流动中柱体绕流的差异,初步构建了马蹄涡的动力作用概念模型。     

SIMULTANEOUS FLOW VISUALIZATION AND WALL-PRESSURE MEASUREMENT OF THE TURBULENT SEPARATED AND REATTACHING FLOW OVER A BACKWARD-FACING STEP

Unsteady behaviors of the large-scale vortical structure superimposed in the turbulent separated and reattaching flow over a backward-facing step were convincingly delineated by performing simultaneous measurements of the wall pressure fluctuations and visualizations of the flow. Toward this end,a synchronized instrumentation system integrated with the microphone array and the high-speed camera was established. The smoke-wire technique was employed to visualize the unsteady events. A thorough analysis based on the wall pressure fluctuations disclosed that the large-scale vortical structure shedding at the frequency of fH /U 0= 0.064 gave a primary contribution to the wall pressure fluctuations,and consequently dominated unsteady behaviors of the turbulent shear layer. The convection velocity of the large-scale vortical structure was determined as U C=0.55 U 0. The instantaneous flow visualizations and wall pressure were compared in a straightforward manner. Below the separation bubble and the reattachment zone,the negative peak of the time-varying wall pressure was in phase with passage of the local large-scale vortical structure. In the redeveloping turbulent boundary layer,the decaying large-scale vortical structure was clearly revealed.     

跌坎下游流场的时均流速分布和脉动特性

本文采用颗粒示踪的方法试验研究了跌坎下游主流重新接触床面流区的时均流速分布，流速的脉动强度及各点脉动流速的概率密度分布。试验结果表明，跌坎下游水流分离，具有稳定的漩涡发展，漩涡产生的频率约为每２秒３次。重新接触点离跌坎的距离约在３￣６倍坎高之间。在主流重新接触床面的流区，紊动剪切应力最大，在量测的流区，脉动流速的概率密度接近正态分布。     

EXPERIMENT STUDY ON SEDIMENT INCIPIENCE IN BACKWARDFACING STEP FLOW

Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtained using the technique of Particle Tracking Velocimetry (PTV). The optical amplification technique was employed to measure the instantaneous flow velocities near the bed and the instantaneous bed shear stress was given. The experimental observations revealed a new insight into the oscillation of the large-scale structure and the three-dimensional characteristics of the flow. In particular,very high turbulence intensity,instantaneous horizontal velocity near the bed and the bed shear stress near the reattachment point were observed. The sediment incipient probability obtained from the sequent images of sediment particles near the bed indicates that the critical instantaneous shear stress of the sediment incipience is independent of flow conditions.     

风琴管喷嘴喉部结构对射流流场流动结构影响的数值研究

本文提出了一种采用间歇涡环模型述射流流场的大尺度流动结构特性，并依据边界元方法优化设计风琴管等自激振荡型喷管的内流结构曲面和其喉部结构的数值实验方法，计算结果表明内流过渡曲面为椭球面而出口形状为锥面的风琴管式喷嘴喷射出的圆形射流流场具有良好的大尺度涡环结构。     

基于粒子图像测速技术的明渠圆柱上游行近流特征研究

现有实验测量和数值模拟方法难以准确捕获近水面流动特征,为全面掌握墩柱上游行近来流的内部结构和水面特征,通过明渠圆柱绕流试验,采用粒子图像测速技术对柱体上游的对称垂面进行了测量,获得了行近来流的瞬时二维流场和水面线。在此基础上,对圆柱上游的时均流动和壅水特征进行了研究,得出上游行近来流从水面至床面依次由上升流、近水平流和下降流构成,形成典型的三层结构,其中,上升流厚度随柱体雷诺数增加而变厚,下降流无量纲厚度基本不随柱体雷诺数改变。在靠近柱体的床面附近,边界层与床面分离,分离区内为向上游流动的逆流以及一个马蹄涡和一个次生涡;在靠近柱体的水面附近存在表层逆流以及两个逆向涡。研究结果揭示出边界层分离、表层逆流及柱体本身依次促使行近来流减速,使得沿程水深雍高,壅水末端至圆柱迎水面的水面线符合一元四次函数分布,无量纲壅水长度随着柱体雷诺数的增加而延长。研究成果可作为基于流体体积法等具有自由水面计算能力的明渠墩柱绕流数学模型的验证依据。     

Turbulent flow structure at a 90-degree open channel confluence: Accounting for the distortion of the shear layer

City channels often have a smaller width-to-depth ratio in comparison to natural rivers due to the limited land availability. The penetration of the tributary into the main channel can cause the distortion of the shear layer. The main purpose of the present study is to investigate the mean and turbulent flow structure in the distorted shear layer in a discharge-adjustable plexiglass circulating flume. Three-dimensional velocities were collected and hydrodynamics and turbulence characteristics such as mean velocity field, turbulent kinetic energy, Reynolds shear stress, turbulence spectrum, and occurrence probabilities of quadrant events were analyzed. The results showed that a stronger helical cell was formed and extended for a longer distance downstream when the tributary channel had a higher flow rate than the main channel. The maximum Reynolds shear stress and the ejection and sweep events were mainly distributed at the middle zone of the water depth, rather than near the water surface, which were coincident with the shear layer as indicated by the turbulence kinetic energy. No obvious energy concentration was observed, and the power law relations for individual velocity components all had an exponent slightly larger than −5/3 in the flow frequency. The distortion of the shear layer resulted in an increase in occurrence probabilities of ejection and sweep events within the shear layer, which were related to the turbulence presenting vortices induced by wall. If the discharge ratio remained unchanged, an increase in the discharge of both channels resulted in an increase in some parameters, such as velocity, turbulent kinetic energy, and the absolute values of Reynolds shear stress, while the shear layer was distorted to a larger extent as the discharge of each channel decreased. All these results suggested that sediment transport, bed morphology and contaminant transport in the distorted shear layer at city channel confluences may differ significantly from that at natural river confluences.     

曲率缓变型反弧段水流的时均特性

本文提出一种曲率缓变型反弧曲线设计方法，用激光测速系统研究了该反弧水流的时均特性，得出反弧段水流时均流速分布，壁面压强，壁面切应力及紊流边界层的沿程变化，与单圆弧反弧体型试验成果比较表明，的反弧曲率的连续缓变，使壁面压强分布和空化数变化缓和，消除了曲率突变存在的逆、顺压力梯度；过渡层紊流强度减小２０－３０％；切应力系数有所下降，反弧末端切应力系数Ｃ＇ｆ减小１０％，本文还对反弧水流紊流边界层厚度的变     

非淹没圆柱群局部水流场特征分析

本文基于带自由表面水动力学数值模型,采用双方程k-ω的剪切应力输运(SST)湍流模式,对非淹没刚性圆柱群水流场进行了三维数值模拟。通过与实验数据的对比分析,验证了模型的有效性及模拟精度。通过圆柱群绕流场特征的模拟分析,探究了不同圆柱雷诺数和固体体积分数对柱群内剪切层渗入长度(δ)、柱群后尾流区剪切层长度以及床面应力空间分布特征的影响。     

ANALYSIS OF HYDRODYNAMICS FOR TWO-DIMENSIONAL FLOW AROUND WAVING PLATES

Hydrodynamic characteristics for two-dimensional flow around a waving plate are investigated. Under large Reynolds number approximation, the flow is assumed to be a combination of the outer potential flow and a thin vortex layer, which consists of a boundary layer and a shed free shear layer. A nonlinear mathematical formulation for describing the outer unsteady potential flow coupled with an unsteady boundary layer equation for the inner viscous flow adjacent to the waving plate is developed. A semi-analytical method with a nonlinear Kutta condition imposed at the trailing edge is used to solve the velocity field of the outer flow and the evolution of wake vortex induced by a large-amplitude waving plate. The unsteady boundary layer equation is solved by extending Pohlhausen’s method to its unsteady counterpart. The thrust and viscous drag coefficients, propulsive efficiency, and the pattern of wake vortex sheet are discussed.     

单边突扩浅水回流水流结构研究

试验资料表明,回流段流速及底部切应力横向分布沿流向呈自相似性。根据相似原理,水深平均方程可化为常微分方程,经求解获得了无量纲速度剖面公式。验证表阴,速度剖面公式不仅适用于垂线平均流速剖面,而且也适用于沿水深各流层速度剖面。利用速度剖面公式计算的垂线平均流速场与实测值符合较好。拟合了无量纲切应力剖面公式,探讨了主回流区阻力分布规律。 此外,还分析了影响回流长度的因素,建立了回流长度经验公式。