The Role of Coherent Structures in Turbulent Flows

从历史的角度看，湍流中的相干结构是湍流研究中的一个新课题。因此，相干结构在水利工程问题中的作用尚未得到明确和充分的认识。一个全面的研究试图达到一个统一的观点和几个问题之间的关系。本研究以明渠水流为研究重点，分为两部分。它是明渠流的基础部分，目的是找出明渠流的内层和外层的流速结构。另一部分是应用部分，其中讨论了在明渠流动和剪切层中的相干涡的作用。同时应用流动可视化技术和激光多普勒风速仪，获得了明渠流动内层纵向涡的时空变化特征。发现了速度信号的时间序列与纵向涡的横向模式是一一对应的。指定纵向平均持续时间。对于外层的纵向涡旋，采用了旋转谱。在接近底部和水面的地方，垂直速度的两点概率密度具有较强的相关性。在下游面向台阶上，测量了湍流结构，包括压力和雷诺应力。分离气泡的变化和分离涡流的合并是台阶后产生沸腾涡流的两个主要触发因素。阐明了半球前分离涡和马蹄形涡的三维结构。阐明了单涡的特征及其对有利坡度上平均速度场的影响。

Coherent sturucture in turbulent flows is a new topic from historical point of view in the research on turbulence. Therefore, the role of coherent structures in hydraulic engineering problems has not been clarified and fully understood yet. A comprehensive study is sought to reach to a unified view and relationships among several problems. The main emphasis is laid on open channel flows and the study consists of two parts. The one is a fundamental part which aims at finding out cherent structures in inner and outer layers in open channel flows. The other is an application part in which the role of coherent vortices in open channel flows and in a shear layer is treated.With simultaneous application of flow visualization techniques and a laser doppler anemometer, spatial and temporal cherent features of longitudinal vortices in an inner layer of an open channel flow were obtained. One to one correspondence between a time siries of signal of velocity and a transverse pattern of longitudinal vortices was found out. Average duration time in the longitudinal direction was specified. For longitudinal vortices in an outer layer, rotational spectrum was utilized. Near the bottom and the water surface, strong correlation was found in two-point probability density of vertical velocity.At a downstream facing step, turbulence structures including pressure and the Reynolds stress were measured. Variation of the separation bubble and the merging of separation eddied are two major triggers to produce kolk-boil eddies at the rear of a step. Three-dimensional structure of separation eddies and horse-shoe vortices in fornt of a hemisphere was clarified. Features of a single vortex and its effect on average velocity fild over a favorable slope were clarified.