Bedform related macroturbulence: topology and kinematics

所有的河流都沿着河床输送泥沙，并悬浮在水柱中，沿着河床滚动的物质通常被水雕刻成一系列形式：其中最常见的形式之一是沙丘，沙子和砾石沉积物都可能产生沙丘。这样的沙丘几乎总是出现在冲积河道中，是水流输送较粗泥沙的主要方式之一。世界上许多大大小小的河流都有这样的沙丘，在大河流中，沙丘可能高达6米，长度可能达到数十米甚至数百米。这样的沙丘是由湍流产生的，但一旦形成，就开始极大地扰乱在它们上面移动的流动，从而从流动中提取能量，因为它们为水流提供了阻力，并在流动中产生了大范围的湍流涡流，或称“宏观湍流”。这些大尺度的漩涡经常是河流水面上的流体上升或沸腾，对于皮划艇运动员和船夫来说是很熟悉的。这种湍流本身可以负责悬浮和输送相当数量的泥沙，并构成了流体流动、泥沙运动和沙丘发育之间一系列复杂的反馈过程的一部分。事实上，湍流结构和与水面的相互作用之间的联系被认为是限制沙丘高度的机制，因此在河床相控制中发挥着重要作用。此外，由于沙丘是许多河道中水流阻力的主要因素，它们对水位-流量关系以及给定流量量级的洪水水位具有基本控制作用。因此，如果我们要更好地模拟和管理许多天然水道，并更好地预测它们如何以及在哪里输送和沉积泥沙，改善对这些反馈过程的了解是至关重要的。虽然近年来在沙丘产生的宏观湍流的数学模型的某些方面取得了许多进展，并开始阐明其形状和结构的某些方面，但在测量和量化这种湍流方面还没有取得足够的进展。该项目建议使用实验室调查和数值模拟相结合的方法来确定沙丘底床产生的大尺度湍流的拓扑和运动学。最先进的实验室技术，如粒子图像矢量仪、激光诱导荧光和高分辨率热成像，将使我们能够在一系列流动条件下量化固定沙丘形式上的流动。结果将使我们能够回答有关沙丘相关湍流结构的产生、形成和动力学的重要问题，研究这种结构如何平流并与水面相互作用，还将为改进数值模拟程序提供基准数据集。这些目标在今后的河道管理中尤其需要，特别是在洪水计算的水流阻力评估、泥沙输送能力的确定以及更好地了解更大尺度的河道地貌动力学方面。

All rivers transport sediment both along their beds and suspended within the water column, with the material rolling along the bed often been sculpted into a range of forms by the water: one of the most common of these forms are dunes that may be generated in both sandy and gravelly sediment. Such dunes are nearly always present in alluvial channels and are one of the main ways in which the coarser sediment is transported by the flow. Many of the world's small and large rivers possess such dunes, which in big rivers may reach up to 6m high and tens or even hundreds of metres in length. Such dunes are generated by the turbulent flow but, once formed, begin to greatly disrupt the flow that is moving over them, thereby extracting energy from the flow in the resistance they offer to the passage of water, and creating large-scale turbulent eddies, or 'macroturbulence', within the flow. These large-scale eddies are very often visible as upwellings or 'boils' of fluid on the water surface of rivers, and are familiar to canoeists and boatmen. This turbulence itself can be responsible for suspending and transporting appreciable quantities of sediment and forms part of a complex series of 'feedback' processes between fluid flow, sediment movement and the development of the dunes. Indeed, linkage between turbulent structure and interaction with the water surface is theorised as the mechanism that limits dune height and thus plays a significant role in bedform phase control. Furthermore, as dunes are the principal element of resistance to flow in many channels they have a fundamental control on stage-discharge relationships and hence flooding levels for a given flow magnitude. Thus, an improved knowledge of these feedback processes is vital if we are better to model and manage many natural waterways and better predict how and where they both transport and deposit sediment. Whilst recent years have seen many advances in some aspects of mathematical modelling of macroturbulence generated by dunes, and have begun to elucidate some aspects of its shape and structure, this has not been matched by sufficient progress in measuring and quantifying such turbulence. This project proposes to use a combined laboratory investigation and numerical modelling approach to determine the topology and kinematics of large scale turbulence generated by dune bedforms. State-of-the-art laboratory technology, in the form of Particle Image Veocimetry, Laser Induced Fluorescence and High-Resolution Thermal Imaging, will enable us to quantify flows over fixed dune forms for a range of flow conditions. The results will allow us to answer important questions concerning the generation, form and dynamics of dune related turbulent structure, investigate how this is advected and interacts with the water surface and will also provide a benchmark dataset for improvements in numerical modelling codes. Such objectives are particularly needed in the future management of river channels, particularly in assessment flow resistance for flooding calculations, determination of sediment transport capacity and linking to an improved understanding of larger scale channel morphodynamics.