添加网格对增强表面活性剂减阻流体传热性能的试验研究

本文应用热电偶测温系统和PIV（Particle Image Velocimetry）,激光测量系统对二维流道内添加网格后CTAC（Cetyltrimethyl Ammonium Chloride）表面活性剂减阻流体的传热性能和流动结构进行试验研究。研究表明：在完全减阻区内,减阻流体的传热性能大幅度降低;当雷诺数超过临界雷诺数后,传热性能呈指数级快速增长;网格的添加破坏了表面活性剂的胶束网状结构,在一定程度上恢复了流场的湍流强度,从而使传热效果显著增强。     

表面活性剂减阻流体热边界层温度脉动及传热特性实验研究

该文针对在水中添加阳离子表面活性剂产生的减阻作用同时降低传热性能的现象,应用相位多普勒激光测量系统和热电偶温度测量系统对二维流道内表面活性剂减阻流体热边界层内的湍流温度脉动以及传热特性进行了实验研究.研究表明,表面活性剂减阻流体平均温度梯度在流动缓冲层很大,热扩散率较低,而水的热阻主要发生在近壁区域的高扩散层.表面活性剂减阻流体温度脉动强度与速度脉动强度有相似的分布趋势,其峰值都向外层移动.在近壁处的高扩散层温度脉动频率很小,而在平均温度梯度很大的低扩散层,减阻流体的温度脉动频率有所增强.而水的温度脉动频率比表面活性剂溶液高且在整个断面上几乎没有变化.对于表面活性剂热流体,湍流温度脉动对轴向热流量的贡献更大,使得轴向热流量的最大值出现在靠近温度脉动强度最大值所在的位置.表面活性剂减阻流体法向湍流热流量分布与雷诺应力分布有相似的趋势.     

河漫滩湿地淹没柔性植物水流涡量场研究

含植物水流的涡量场构成了河漫滩湿地水生物生境的物理基础,形成水、营养物质的循环以及能量的分配过程。本文通过玻璃水槽试验用二维标准PIV测量研究了含淹没柔性植物二维垂直涡量场的分布。研究表明:流速梯度是形成含植物水流涡量的关键因素,水流过植物冠层附近受到的干扰大,因而流速梯度大;涡量最大值出现在淹没柔性植物冠层以下约50%处;植物冠层处,涡量绝对值大,拟熵大,能量耗散率也高。植物冠层是造成水流紊动耗散的主要原因。     

河道含淹没刚性植物二维涡量场特性

含植物水流的涡量场构成了水生动物生境的物理基础,形成水、物质的循环以及能量的分配过程。本文通过玻璃水槽试验用二维标准PIV测量研究了含淹没刚性植物二维垂直涡量场的分布,得出流速梯度是形成含植物水流涡量的关键因素。植物顶层附近对水流的干扰大,因而流速梯度大;涡量最大值出现在淹没刚性植物顶层以下约30%处;植物顶层处,涡量绝对值大,拟熵大,能量耗散率也高。植物顶层是造成水流紊动耗散的主要原因。     

水垫塘内淹没冲击射流中的大尺度涡结构及其特征

高坝下游水垫塘消能与防护问题目前尚未得到妥善解决。本文应用现代紊流理论的观点，对水垫塘内的复杂流动结构进行了系统的研究，对主射流区的大尺度涡体和壁射流区的紊动涡体的拟序结构进行了分析，从紊流力学的角度对脉动压力的起源及水垫塘内水体能量的转移与耗散做了机理性解释，为高坝下游水垫塘的消能与防护的进一步研究提供了理论依据。     

高分辨率粒子示踪测速技术在光滑明渠紊流黏性底层测量中的应用

在光滑明渠紊动水流的研究中,黏性底层具有重要的作用,壁面剪切应力与涡量均源于此流区。由于充分发展明渠紊流的黏性底层的物理尺度在0.1 mm量级,常规方法难以实际测量该区内的速度分布。本文构建了测量黏性底层的高分辨率HR-PTV与测量全流区的普通PIV的耦合测量系统,对光滑明渠紊流黏性底层的流动特性进行了研究。HR-PTV系统采用阈值递增法、灰度截断、椭圆度与充实度指标等方法进行图像处理得到清晰的粒子图像;使用匹配几率法对粒子坐标进行匹配,计算粒子速度。得到了明渠紊流黏性底层(0.3 mm以内)的平均流速、紊动强度、偏态系数和峰度系数的垂线分布。从拟合的平均流速分布得出摩阻流速u*,结果与牛顿流体本构方程一致;黏性底层与全流区的时均流速、紊动强度和雷诺应力均能用拟合所得的u*合理无量纲化,并能光滑连接,表明由此得出的u*适用于全流区。将明渠紊流的各种分布与封闭槽道流DNS数据进行对比,验证了HR-PTV在黏性底层所得各阶速度矩的准确性。     

丁坝绕流的k-ε紊流模型数值解

本文采用雷诺方程组描述丁坝绕流运动,并用紊流k-ε双方程模型闭合雷诺方程组。根据控制体积法导出沿水深平均的二维偏微分方程的离散格式,然后用SIMPLER程式进行求解。通过计算,获得丁坝绕流流场内流速、水化、紊动能和紊动粘性系数的断面分布规律,其中流速和水位的计算值与实测资料较为一致。     

近壁紊流随机理论及其应用

本文概述了作者近年来在研究近壁紊流方面取得的成果。基于紊流具有多层次的涡旋结构,提出了近壁紊流的随机理论;通过对糙率尾部水流分离的研究,阐明了出现光滑区、过渡区和粗糙区的原因;通过统计分析,明确了处于从层流向紊流过渡状态下的紊动出现机率;基于对粘弹性流体方程式的分析,导出了含高分子聚合物紊流的减阻参教。应用上述理论,全面阐述了明渠流、管道流、边界层流和减阻流的时均结构、脉动结构和阻力规律,并得到了与现有试验资料一致的结果。     

复式河槽漫滩水流的三维数值计算

采用紊流模型中的雷诺应力模型(RSM)对非对称复式河槽水流的三维紊流进行了数值模拟。数值模拟计算结果与试验结果的比较表明RSM能够很好地模拟滩槽交互区内的二次流、主流速分布和床面剪切力,此模型模拟复式河槽是可靠的。最后用RSM模拟了对称复式河槽在不同水深下的流速场及床面剪切力分布,得出当水深增加时,滩槽交互区二次流动纵向涡增强,而浅滩主流速及床面剪切力迅速增大的结论。     

河道漫滩及湿地上淹没柔性植物水流的紊流特性

植物在河流生态的修复中起重要作用,因此对含植物明渠水流的紊流机理的研究显得非常重要。将河漫滩及湿地中含淹没柔性植物的水流视为充分发展了的紊流,在实验室中用特定材料模拟柔性植物,通过水槽试验用三维ADV对淹没柔性植物段紊流特性进行了测量研究,对试验结果计算分析表明含淹没柔性植物的垂向流速分布不再符合指数分布;其紊流具有明显的各向异性特性;植物冠层交界处,水流紊动交换强烈,该处紊流强度及雷诺应力出现最大值。     

开孔促淤板周围水流紊动特性试验研究

为深入了解开孔促淤板周围的水流特性,通过室内概化水槽试验分析了开孔促淤板前后的水流紊动特性,采用声学多普勒流速仪(ADV)测量了开孔促淤板前后典型断面上的三维瞬时流速,分析了典型断面上的时均流速、紊动强度、雷诺切应力和紊动能的分布规律。试验结果表明:板后近板区出现回流区,且回流区与开孔促淤板格栅位置相对应;板后水流相对紊动强度、雷诺切应力及相对紊动能明显大于板前,开孔促淤板对板后近板区水流影响较大;随着与开孔促淤板距离的增大,回流现象消失,流速变化趋于稳定,水流相对紊动强度减小且最终趋于稳定,相对紊动能出现折减且折减率逐渐降低;靠近近板区xOy面雷诺切应力大于yOz面雷诺切应力,靠近下游xOy面雷诺切应力与yOz面雷诺切应力相差较小。     

水流与壁面间切应力估计方法的比较研究

精确估计水流与壁面间切应力在泥沙输移和水利工程安全方面具有重要意义。通过选取并整理文献数据,对现今七种主要的水流与壁面间切应力的估计方法在不同雷诺数的条件下进行评价和比较,发现总体上湍流动能外推法和雷诺应力外推法估计壁面切应力的可靠性和精确度最高,平均相对误差分别为3.2%和5.2%。湍流能量法、湍流动能法和雷诺应力法次之,以壁定理的精确度最低,但其所需的数据较容易获取。二阶应力法的精确度取决于阻力系数的取值。其中,壁定理和二阶应力法的估计值偏高而更加安全,湍流动能外推法的估计值较为准确,而其余方法给出的估计值偏低。所得结果对不同条件下水流与壁面间切应力估计方法的选择以及直接测量仪器的校准有一定的参考价值。     

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.     

Comparison of Direct and Indirect Boundary Shear Stress Measurements along Vegetated Streambanks

Estimates of boundary shear stress along vegetated streambanks are needed to predict streambank fluvial erosion. Because fluvial shear stress cannot be directly measured in the field, reliable estimation techniques using field instrumentation are needed. This study evaluated local bank shear stress estimation methods applicable to sloping, vegetated streambanks. Two reaches of a second order stream were modelled in a flume using a fixed‐bed Froude‐scale modelling technique. One reach was dominated by dense shrubs while the other reach was located in a mature forest. Direct measurements of local bank shear stress using a hot‐film anemometer were compared to estimates based on velocity measurements (logarithmic method, Reynolds stresses, and turbulent kinetic energy). For channels with no or widely spaced vegetation, the velocity‐based estimates underestimated the bank shear stress due to secondary flow contributions. For banks with dense vegetation, Reynolds stresses and turbulent kinetic energy estimates were statistically similar to direct measurements on average, but substantial error occurred when making point comparisons. Velocity‐based estimates generally over predicted bank stress in areas of high shear at the vegetation edge and underpredicted stress within dense vegetation. Ultimately, results suggest that none of tested techniques can be broadly applied to streambanks, and flow structure is critical in selecting the appropriate estimation technique. Copyright © 2016 John Wiley & Sons, Ltd.     

明渠岸边横向取水口的三维数值计算

本文是在明渠岸边横向取水口的试验研究基础上的配套数值模拟。采用雷诺平均的N—S方程，以各向异性的雷诺应力模型(RSM)封闭方程组，用SIMPLEC算法对压力—速度场求解。计算结果与试验实测结果吻合较好，反映了此项模拟的可信性，可籍以扩大、分析一些运动规律。数值计算结果启示了横向取水口水流的复杂三维流动特性，据之对取水口附近水流各区的划分、水流的分离和反向流动、螺旋流以及床面剪应力的分布等复杂水力和泥沙输移物理现象有了进一步认识。     

各向异性对无切变边界层与轴对称应变湍流特性的影响

本文运用湍流快速畸变理论对各向异性条件对无切变边界层及轴对称应变湍流特性的影响进行了探讨，对无切变边界层，所进行扮析与实验资料表明，此时各向异性的影响在于增加纵向紊动强度及减小横向紊动强度，但其对垂向紊动影响甚小，对轴对称应变湍流，本文也对紊动强度与雷诺应力随向各异性条件的变化进行了分析。     

EFFECT OF STREAMLINE CURVATURE AND SOME OTHER CONDITIONS ON THE TURBULENT STRUCTURES IN THE OTHER REGION OF AN OBSTACLE

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推移质运动对清水和宾汉体泥浆湍流结构的影响

通过试验研究了清水湍流和泥浆湍流中加入推移质后时均流速分布、脉动强度、概率密度分布、相关系数、频谱密度分布等的变化.试验结果证明,清水和泥浆湍流中加入推移质后时均流速减小,时均流速梯度增大,脉动加强.泥浆过渡区湍流上部为层流,有流核,加入推移质后,上部层流转变为湍流,流核消失.分析证明,这是由于推移质运动增大了雷诺应力和时均流速梯度,因而增大了时均流动向脉动传递的能量的缘故,清水湍流中加入推移质后脉速概率密度分布偏离正态,偏态系数加大.泥浆湍流中加入推移质后相关系数降低,脉动的随机性增大.泥浆湍流的脉动能量集中于低频脉动.由于推移质也可以吸收低频脉动能来支持其运动,所以加入推移质后泥浆湍流的低频脉动能量降低,高频脉动能量增大.     

Numerical investigation of flow through vegetated multi-stage compound channel

This paper addresses the problem of the renormalization group k turbulence modeling of a vegetated multi-stage compound channel. Results from Micro acoustic Doppler velocimeter（ADV） tests are used with time and spatial averaging（doubleaveraging method） in the analysis of the flow field and the characterization. Comparisons of the mean velocity, the Reynolds stress, and the turbulent energy distribution show the validity of the computational method. The mean velocity profile sees an obvious deceleration in the terraces because of vegetation. Secondary flow exists mainly at the junction of the main channel and the vegetation region on the first terrace. The bed shear stress in the main channel is much greater than that in the terraces. The difference of the bed shear stress between two terraces is insignificant, and the presence of vegetation can effectively reduce the bed shear stress.     

Fully rough submerged plane wall-jets

This study presents the turbulent flow field in submerged plane wall-jets on horizontal fully rough walls detected by a Vectrino velocimeter. For the comparison between the fully rough and smooth submerged wall-jets, the smooth submerged wall-jet case was also revisited. The two-dimensional Reynolds averaged boundary layer equations of a steady turbulent flow are analyzed to determine the velocity and Reynolds shear stress profiles in the fully developed zone of smooth and fully rough submerged wall-jets. The response of the turbulent flow characteristics in submerged wall-jets to wall roughness is examined from the point of view of similarity characteristics, growth of the length scale, and decay of the velocity and turbulence characteristics scales; and compared with the response of those to smooth and transitionally rough walls. The significant observation is that with an appropriate scaling, the velocity, Reynolds shear stress and turbulence intensities in the fully developed zone of fully rough submerged wall-jets are reasonably similar. The rate of decay of jet-velocity on fully rough walls is greater than that on smooth wall, but it is less than that on transitionally rough walls due to the presence of roughness sub-layer on the fully rough walls. Analysis of the third-order moments of velocity fluctuations reveals that the jet-layer is associated with the arrival of low-speed fluid parcel causing an effect of retardation; while the inner-layer of circulatory flow is associated with the arrival of high-speed fluid parcel causing an effect of acceleration. Wall roughness influences the peak values (both positive and negative) of the third-order moments increasing them significantly. Thus, on fully rough walls, the arrival processes of low- and high-speed fluid parcels become stronger in the jet-layer and the inner-layer of circulatory flow, respectively. Also, the peak values of streamwise and vertical flux of turbulent kinetic energy increase significantly in the presence of fully rough walls.