Advanced experiments and simulations to model coherent structures in shallow smooth and rough open channels

实验室尺度的光滑明渠流可以认为是水利工程中最简单的流场。然而，这种流动表现出许多发生在复杂湍流中的现象。为了理解复杂的流场，研究人员研究了标准光滑床流对不同形式的边界条件变化的响应（如床面粗糙度的变化）。同样值得注意的是，从实验室尺度上非常简单的流动中建立的经验关系，在更复杂的流场中，经常远远超出其建立的有效性范围。这尤其令人不安，因为使用经验关系的商业计算代码的使用已经激增。例如，有相当多的工作试图将河床形状粗糙度与经典实验中使用的沙粒粗糙度联系起来。大多数实际的明渠流动发生在有或没有渗流（吸入/注入）的粗糙（可移动/不可移动）河床上。虽然平均速度和剪应力的分布有很好的记录，但湍流结构的作用鲜为人知。在对实际流进行建模时，会出现几个关键问题。这些问题包括：粗糙度几何（高度、间距、密度等）如何决定造成大部分运输的较大湍流漩涡的大小和频率？在渗流和床层移动的情况下，传统的规律和作用是如何改变的？本提案将系统地解决这些和其他重要的问题，通过对选定的粗糙度几何形状进行数值模拟和实验室实验，并具有良好定义的流动条件，包括渗透效应。本提案的目的是研究瞬时的、随时间变化的相干结构，以及它们如何受到床/自由表面接近度的影响。速度场将使用适当的正交分解，动量分析和条件象限分析进行检查。这项研究得益于温莎大学最先进的实验设备和数字代码。

Laboratory scale smooth open channel flow can be considered to be the simplest flow field in hydraulic engineering. However, this flow exhibits many of the phenomena that occur in complex turbulent flows. To understand complex flow fields, researchers have studied the response of the standard smooth bed flow to changing boundary conditions which are imposed in many different forms (such as change in bed roughness). It is also noteworthy that empirical relations established from very simple flows at laboratory scales are often employed far beyond their established range of validity in the more complex flow fields. This is particularly disconcerting as there has been a proliferation in the use of commercial computational codes which use the empirical relations. For e.g., there is considerable work which attempts to relate river bed form roughness to sand grain roughness employed in classical experiments. Most practical open channel flows occur on rough (mobile/immobile) beds with or without seepage (suction/injection). While the distribution of the mean velocities and shear stress is well documented, the role of turbulent structures is less known. Several critical questions arise while modeling practical flows. These include: How does roughness geometry (height, spacing, density etc.) determine the size and frequency of the larger turbulent eddies responsible for bulk of the transport? How are the conventional laws and role of coherent structures modified in the presence of seepage and bed mobility? The present proposal will systematically address these and other important questions by conducting numerical simulation and laboratory experiments on selected roughness geometry with well defined flow conditions including seepage effects. The goal of this proposal is to study the instantaneous, time-evolving coherent structures and how they are affected by the proximity of the bed/free surface. The velocity fields will be examined using proper-orthogonal decomposition, momentum analysis, and conditional quadrant analysis. The study is made possible by the availability of state-of-the-art experimental facilities and numerical codes at the University of Windsor.