【作者】 何建波；

【导师】 王虹；

【作者基本信息】 清华大学 ， 水利工程， 2009， 硕士

【摘要】 在与人们生活息息相关的很多领域,流体力学都取得了巨大的成功,但是,人们对天然河流尤其是河流弯道中的水流结构、紊动特性、泥沙及污染物的输移扩散特性的认识却极其有限,这使得人们在预测河床演变、洪水、突发污染事故等与流体力学(水力学)相关的问题时遇到很大的困难。因此,对天然河流弯道中水流结构特性进行研究就显得尤为必要。但是,由于获取实测资料困难、昂贵、精度差,人们对天然河流的基础性研究都是从室内明渠弯道水槽入手的。本文即在此方面进行了探索。本试验设计了一个由4个等尺度90o弯段组合连接而成的连续弯道概化模型,采用三维电磁流速仪对该模型中的水流进行了精细测量,得到了每个测点三个方向的瞬时速度时间序列,并对其进行了时均值分析和紊动特性分析。本文分析了水面线、纵向水深平均速度、水动力轴线,三个方向流速的垂线分布特点沿流程和横向的不同之处,以及环流沿流程的形成、发展、变化过程,得到了单一弯道水流中的典型结果,如水面线的扭曲、时均流速沿横向、垂向的分布在不同断面上变化、环流在弯道中的沿程演变过程等。同时,也得到了与简单弯道水流不同的结果,如某些断面上表面水流的流向、垂线上纵向流速的最大位置、环流形式的多样性等,这些不同点对研究天然河流连续弯道中的流动有参考价值。本文还对试验数据进行了紊动特性分析,包括紊动能谱、紊动强度、紊动切应力、紊动能、紊动剪切效率、紊动时间比尺等,对弯道水流的紊动特性有了大致的认识,如紊动主要集中在10Hz以下,紊动的欧拉积分时间比尺为0,弯道紊动的随机性大等。分析了不同环流形式与各种紊动量的互相影响的关系,得出了环流区域以外的水流紊动强,而其内部的紊动弱;紊动引起的剪切效率在两岸附近和底部附近几乎为0,在水面附近最大,并且其最大位置的分布区域与环流形式有关,在固体边界和水面附近以外的区域,有局部极值出现,这些极值位于环流的边界附近;边壁附近紊动的自相关时间增长,紊动结构的时间尺度增大等一系列结论。更多还原

【Abstract】 Despite the many achievements fluid mechanics has made in many areas closely related to people’s lives, our understanding of the flow structure, characteristics of turbulent flow, contaminant dispersion and sediment transport in river bends are still very limited, which results in difficulties in predicting the dispersion of contaminants and the fluvial processes in the bends of meandering river channels. Therefore research works on the dynamic characteristics of turbulent flow in river bends are urgently needed. Because of the difficulties, high cost and poor accuracy of field measurements, laboratory flume experiment of flow in a bend is a first option for researchers to learn more about this subject. This thesis presents such a flume experiment to study turbulence structures in consecutive bends.In this study, a conceptual river bend model with four 90°consecutive bends was built, and a three-dimensional (3D) electromagnetic current meter was used for detailed measurements of the flow turbulence in the channel bends. The point-measurements collected instantaneous time series of the three velocity components across various cross-sections, and both the time-averaged flow and turbulence characteristics were analyzed.Results of the experiment include the longitudinal and lateral water surface profiles, depth-averaged longitudinal velocities at various verticals, location of verti-cals where maximal depth-averaged longitudinal velocities were found, variations of velocity profiles over depth in the longitudinal direction and transverse direction for the three velocity components, respectively, and processes of the formation and development of secondary currents along the longitudinal direction. Some results are different from those of flow in a single bend, especially the surface flow from con-cave bank to convex bank at some sections, the vertical position of the maximal longitudinal velocities, and the diversity of the pattern of secondary currents. These results are very helpful to the study of the flow and transport phenomena in the bends of natural meandering rivers. This thesis also presents detailed analysis of turbulent flow characteristics in the bend, including turbulence spectrum, turbulence intensity, turbulent shear stress, turbulent kinetic energy, the efficiency of turbulent eddies in producing shear, the time scale of the turbulent flow, etc. It is found out that turbulence energy mainly concentrated in fluctuations below 10Hz and the Euler integral turbulent time scale is 0. The highly random nature of turbulence structure in a bend flow was observed. The mutual influence between the different forms of secondary currents and turbulence variables was analyzed, and the following conclusions are reached: 1) the turbulence energy inside the region of secondary currents is weaker than those outside it; 2) the efficiency for turbulent eddies to produce shear stresses is nearly negligible near the bottom and the two side walls of the flume, with the largest efficiency found near the surface of flow, with its exact location related to the pattern of secondary currents; 3) away from the solid boundaries and water surface, local extreme values of the efficiency can also be found near the edge of the secondary currents; 4)near the bottom and side walls of the flume, the time-scale of turbulence structure increase.更多还原