明渠恒定均匀流机械能损失构成分析

明渠水流机械能损失的确定是水力学乃至工程流体力学中的重要内容。从明渠均质不可压缩液体恒定总流出发,以流体力学中黏性流体运动数学模型为基础,本质上揭示了机械能损失的构成和相互转化关系。成果表明矩形明渠均匀流紊流的机械能损失由时均流速梯度引起的损失与紊动能耗散引起的损失组成,当雷诺数较小时,时均流速梯度引起的损失是机械能损失的主要来源,而雷诺数较大时,紊动能耗散引起的损失是机械能损失的主要来源。在宽浅明渠中,雷诺数约为9.5×10³时,均流速梯度引起的损失和紊动能耗散引起的损失相等。     

管壁均匀密集加糙圆管流的新总流机械能方程及流动数值模拟

能量(机械能)方程是水力学的理论基础;能量损失(机械能损失)的计算既是水力学乃至工程流体力学的主要研究内容之一,也是工程实际中颇为关注的问题。以往水力学中的能量方程系以重力场中理想不可压缩流体的伯努利方程为基础而得到,其既不能直接反映紊动对流动的作用,能量损失也无明确的表达式。本文在我们现有研究工作基础上,通过理论分析与数值模拟,得到如下成果:(1)构建了管壁均匀密集加糙圆管流的新总流机械能方程;(2)通过数值模拟,对管壁均匀密集加糙圆管流的机械能损失及其构成进行了研究。构建的方程包含了紊动的作用;研究成果表明,均匀密集加糙圆管流总流机械能损失与雷诺数及管壁光滑度有关,且可分解为与时均流速梯度相关的损失及与紊动耗散相关的损失两部分。     

管道进口过渡段的水力学特性

边界层沿流程发展是否达到管道中心是判别管道流态已否形成均匀流的基本条件,本文把管道中边界层正在发展的流段称为进口过渡段。根据管流的特性及卡门——勃兰特粗糙管流速方程推导出:管流中紊流边界层厚度的沿程发展,形成均匀流的临界位置,管道进口过渡段的阻力系数,能量损失及损失系数(不包括进口处的局部损失),过渡段中边界层外区势流速度与压力沿程变化的计算公式。并发现在有压管道中紊流边界层的发展比在宽阔的明渠中快。     

论矩形明渠流边界切应力分布规律

本文提出了流体内部任一微小水体所含机械能总是沿到边壁最短距离的方向向边界传递，并通过粘性作用而予耗散的假定。依此假定本文导出了能适合各种宽深比的矩形明渠边界切应力分布公式，同时还给出了矩形明渠流边壁、底壁糙率不同时的边界切应力分布公式，所得公式与实验资料较为相符。     

南水北调中线工程总干渠水力学 仿真模型研究

以节制闸为边界将南水北调中线总干渠分为若干子段,采用闸门过闸流量公式、能量方程、圣维南（Saint-Venant）方程组和低压管流方程分别计算闸门水位流量关系、明渠恒定流、明渠非恒定流和有压建筑物非恒定流,以此建立总干渠水力学计算模型。通过双重迭代方法对水流方程、过闸流量方程进行联合求解,以模拟总干渠运行调度动态过渡过程,以京石段2008年输水实测数据对水力学模型进行验证,结果表明模型具有良好的收敛性和较高的精度。     

细颗粒原状土起动流速试验研究

为了得到更为客观科学的泥沙基本活动规律,利用有压矩形管进行了细颗粒泥沙起动试验。试验结果表明,管内水压力对细颗粒泥沙的起动有重要影响,水压力越大,管中泥沙起动断面平均流速和起动切应力越大。根据有压矩形管的水流分布特性和水压特点,将矩形管中水流转换为普通明渠水流,转换得到的明渠中泥沙起动垂线平均流速与试验矩形管中泥沙起动断面平均流速基本相同,验证了利用有压矩形管进行细颗粒泥沙起动流速试验的可行性和代表性。在试验矩形管与实际明渠的泥沙起动流速转换过程中,并未考虑不同水柱高度下曼宁糙率系数的变化,这方面还有待进一步研究。     

明渠恒定渐变流水面变化规律探讨

针对明渠恒定渐变流的水面变化问题,根据水力学能量方程,建立了明渠恒定渐变流的能量微分方程,并推导出水位及水深的沿程变化规律,通过数学推导,得出明渠恒定渐变流在缓流和急流状态下的水面变化规律。该规律不需对河渠水面线进行分区分情况讨论,适用于各种河渠断面,适用性强,在河渠设计中,也可以通过该规律反算河渠参数,使河渠满足水位和水深的工程要求,实用性强。     

多孔介质强化传热的理论与实验研究

本文从理论上分析了绕花丝多孔体(孔隙率ε＞95％)的管内换热以及流动特性，建立贴附多孔层圆管内流体的流动模型。通过用Brinkman—extended Darcy方程分析多孔层内流体的流动，以及在能量方程中加入弥散系数，得到模型内速度场和温度场的理论解。实验结果表明：在层流范围内其实验数据与理论解吻合较好，说明理论模型能够分析多孔体管内的层流流动和换热。通过改进这一模型还可以为各种多孔介质强化管的研究提供理论基础。     

矩形明渠消力池水跃的水头损失研究

为了研究沿程水头损失与局部水头损失的变化规律,根据沿程水头损失的基本定义,推求矩形明渠消力池水跃区沿程水头损失与床面阻力系数、水跃共轭水深比、跃前断面宽高比及跃前断面水深的理论关系,提出了矩形明渠水跃区沿程水头损失及其系数和局部水头损失系数的理论公式,给出了沿程水头损失系数、局部水头损失系数和总水头损失系数的简单拟合公式。研究表明:沿程水头损失随着跃前断面水深和床面阻力系数的增大而增大,随着水跃共轭水深比和跃前断面宽高比的增大而减小;局部水头损失系数随着跃前断面弗劳德数的增大而增大;水跃区局部水头损失占比随着弗劳德数的增加而增加,弗劳德数为3时的局部水头损失占比达到90%,弗劳德数为6时的局部水头损失占比已达到95%以上。研究成果可进一步完善并丰富水跃理论体系。     

明渠冰盖下流动的综合糙率

明渠冰盖下流动的综合糙率nc是确定冰期水位和流量关系的关键参数,本文根据冰盖区和床面区的边界条件和流量的连续性条件提出了通用的综合糙率公式,研究了冰盖下一般矩形明渠和宽浅明渠流速分布及流速比a对综合糙率nc的影响,在此基础上,计算分析比较了通用综合糙率公式、Einstein公式、Belokon-Sabaneev公式和Larsen公式的差异和使用条件,结果表明:对于宽深比b/H较小的矩形明渠,采用Belokon-Sabaneev公式将产生很大的计算偏差,Larsen公式只适合宽浅明渠,而Einstein公式是通用综合糙率公式的最好近似,推荐在工程设计中采用。     

The mechanical energy equation for total flow in open channels

The mechanical energy equation is a fundamental equation of a 1-D mathematical model in Hydraulics and Engineering Fluid Mechanics. This equation for the total flow used to be deduced by extending the Bernoulli＇s equation for the ideal fluid in the streamline to a stream tube, and then revised by considering the viscous effect and integrated on the cross section. This derivation is not rigorous and the effect of turbulence is not considered. In this paper, the energy equation for the total flow is derived by using the Navier-Stokes equations in Fluid Mechanics, the results are as follows：（1） A new energy equation for steady channel flows of incompressible homogeneous liquid is obtained, which includes the variation of the turbulent kinetic energy along the channel, the formula for the mechanical energy loss of the total flow can be determined directly in the deduction process.（2） The theoretical solution of the velocity field for laminar flows in a rectangular open channel is obtained and the mechanical energy loss in the energy equation is calculated. The variations of the coefficient of the mechanical energy loss against the Reynolds number and the width-depth ratio are obtained.（3） The turbulent flow in a rectangular open channel is simulated using 3-D Reynolds averaged equations closed by the Reynolds stress model（RSM）, and the variations of the coefficient of the mechanical energy loss against the Reynolds number and the width-depth ratio are discussed.     

The calculation of mechanical energy loss for incompressible steady pipe flow of homogeneous fluid

The calculation of the mechanical energy loss is one of the fundamental problems in the field of Hydraulics and Engineering Fluid Mechanics. However, for a non-uniform flow the relation between the mechanical energy loss in a volume of fluid and the kinematical and dynamical characteristics of the flow field is not clearly established. In this paper a new mechanical energy equation for the incompressible steady non-uniform pipe flow of homogeneous fluid is derived, which includes the variation of the mean turbulent kinetic energy, and the formula for the calculation of the mechanical energy transformation loss for the non-uniform flow between two cross sections is obtained based on this equation. This formula can be simplified to the Darcy-Weisbach formula for the uniform flow as widely used in Hydraulics. Furthermore, the contributions of the mechanical energy loss relative to the time averaged velocity gradient and the dissipation of the turbulent kinetic energy in the turbulent uniform pipe flow are discussed, and the contributions of the mechanical energy loss in the viscous sublayer, the buffer layer and the region above the buffer layer for the turbulent uniform flow are also analyzed.     

有限长圆管中Stokes流动的半数值半分析解法

本文研究不可压缩蠕流穿过有限长圆管的轴对称流动的半数值(配置法)半分析解法。对不同长度圆管进行了数值计算,结果表明无论是压差或摩阻系数,都与管长之间存在着近似的线性律。     

Theoretical analysis and numerical simulation of mechanical energy loss and wall resistance of steady open channel flow

The mechanical energy loss and the wall resistance are very important in practical engineering. These problems are investigated through theoretical analysis and numerical simulation in this paper. The results are as follows.(1) A new mechanical energy equation for the total flow is obtained, and a general formula for the calculation of the mechanical energy loss is proposed.(2) The general relationship between the wall resistance and the mechanical energy loss for the steady channel flow is obtained, the simplified form of which for the steady uniform channel flow is in consistent with the formula used in Hydraulics deduced by ? theorem and dimensional analysis.(3) The steady channel flow over a backward facing step with a small expansion ratio is numerically simulated, and the mechanical energy loss, the wall resistance as well as the relationship between the wall resistance and the mechanical energy loss are calculated and analyzed.     

结点局部能量损失对管网水力计算的影响

对于管径较大、管段长度较短的管网,结点处的动量交换和局部能量损失显著影响管网中管段通过流量的计算结果。进行管网水力计算时,忽略结点处的局部能量损失或采用不变的局部能量损失系数均是不合适的。文中分别应用三种计算方法,即忽略结点处局部能量损失、采用恒定不变的结点局部能量损失系数,以及根据试验成果所得的与水流条件有关的半经验公式,针对短管段构成的管网系统进行了计算比较,管段流量的计算结果相差很大。研究结果还表明,与同一结点相接的各管段间存在显著的能量交换,因此,忽略或采用恒定不变的能量损失系数均不能正确地反映这些水流现象。     

EXPERIMENTAL AND NUMERICAL ANALYSIS OF LAMINAR AND LOW TURBULENT FLOW DISTRIBUTIONS IN INLET DIVIDING HEADER OF SHELL AND TUBE HEAT EXCHANGER

Flow distribution headers play a major role in heat exchangers. The selection of header diameter, branch pipe diameter, branch pipe spacing etc. is based on the designer's experience and general guide lines. The proper selection of the header dimensions will yield uniform flow distribution in heat exchangers, which in turn will enhance the heat exchanger efficiency. In this work, the flow distribution in branch pipes and the pressure variation across the branch pipes in laminar and low turbulence region is studied with two models of the inlet dividing headers. When the numerical analysis has been applied, its inability to predict the no flow condition through the branch pipes is revealed. The results are presented in the form of flow rate ratio through branch pipes and nondimensional coefficients across branch pipes which are useful to apply the existing mathematical models for the present experimental setup.     

圆管壁粗糙度对圆管内流态影响研究

基于尼古拉兹圆管实验思想,利用颗粒流程序中的流体计算模块,实现了对圆管壁粗糙度的模拟,进而分析了不同粗糙度对圆管断面流态分布的影响以及在不同压力差作用下平均流速与圆管壁粗糙度之间的关系。研究结果表明:圆管内流体流速受管壁的扰动影响,在横截面上呈U型分布,而非抛物线型分布;圆管壁对管内流体流态的影响范围随管壁粗糙度变化而变化,圆管管壁相对粗糙度越大,管内受扰动流体范围越大,即管内层流范围越小;在管壁粗糙度一定的情况下,圆管断面流量随上下端面压力差的增大而增大;在相同压力差作用下,圆管断面流量随管壁相对粗糙度增大而逐渐减小,颗粒的阻流作用明显;随着管壁粗糙度的增大,圆管上下端面受压力差作用的影响范围也随之增大,使管内流体流态变得更加复杂。     

基于能量比系数的圆柱栅绕流LES方法研究

圆柱阵列流场不但是工程上常见的流动换热结构,而且绕流问题与湍流涡发展研究密切相关。该文采用数值模拟算法研究了不同雷诺数下的单列圆柱栅绕流的不可压缩流动。比较了非定常RANS方程计算得到的湍动能和耗散率,引入湍流能量比系数对初始计算网格进行重新划分和局部加密,进而建立适宜于大涡模拟网格尺度,其结果与PIV和RANS结果进行对比。结果表明,在湍流流场区域能量比系数达到30%-40%的情况下,就可得到工程实际需要的精确计算结果,这大大减少LES算法所需要的计算网格总数和时间。     

急流弯道双曲底板的体形设计

本文提出采用双曲底板的方法消除急流渠道弯段的冲击波。在柱面座标系中建立不可压缩粘性流体的运动方程，导出无量纲形式的自由水面方程并建立了相应连续性方。通过迭代法求解自由水面方程并计算出自由水面高程，由连续性方程求出水流深度。从水面高程中减去水深得出底板高程。文中给出了按此方法设计的马河水库溢洪道弯道段水流计算结果。结果证明渠道内水面平稳，没有发生冲击波。底板设计由编程计算完成。