The Role of Coherent Structures in Turbulent Flows

湍流拟序结构是湍流研究中的一个新课题。因此，拟序结构在水利工程问题中的作用尚未得到阐明和充分理解。本文试图通过对这几个问题的综合研究，达到统一的观点和相互关系.主要重点放在明渠水流上，研究包括两部分。第一部分是研究明渠水流内外层动力结构的基础部分。另一部分是应用部分，讨论了拟序涡在明渠流动和剪切层中的作用，同时应用流动显示技术和激光多普勒测速仪，得到了明渠流动内层纵向涡的时空变化特征。发现速度信号的时间序列与纵向涡的横向分布有一一对应的关系。规定了纵向的平均持续时间。对于外层的纵向涡，利用旋转谱。在底部和水面附近，垂直速度的两点概率密度具有很强的相关性，在下游台阶处，测量了包括压力和雷诺应力在内的湍流结构。分离泡的变化和分离涡的合并是台阶后产生沸腾涡的两个主要触发因素。阐明了半球前缘分离涡和马蹄涡的三维结构。阐明了单个涡的特征及其对有利坡平均速度场的影响。

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.