Bedform related macroturbulence: topology and kinematics

所有的河流都是沿着河床和悬浮在水柱中的沉积物，沿着河床沿着滚动的物质经常被水塑造成各种各样的形式：其中最常见的形式之一是沙丘，它可以在桑迪和砾石沉积物中生成。这种沙丘几乎总是出现在冲积河道中，是水流搬运较粗沉积物的主要方式之一。世界上许多大小河流都有这样的沙丘，在大河中，沙丘可达6米高，数十米甚至数百米长。这种沙丘是由湍流产生的，但是一旦形成，开始就极大地扰乱在其上移动的流动，从而从它们提供给水通过的阻力中提取能量，并在流动中产生大规模的湍流漩涡或“宏观湍流”。这些大规模的漩涡是经常可见的，因为在河流水面上的流体的隆起或“沸腾”，并且是独木舟运动员和船夫所熟悉的。这种湍流本身可以悬浮和输送大量的沉积物，并形成流体流动、沉积物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理论化为限制沙丘高度的机制，从而在床形相位控制中起着重要作用。此外，由于沙丘是许多渠道中水流阻力的主要因素，因此，对于给定的流量大小，沙丘对水位-流量关系以及洪水位具有根本控制作用。因此，如果我们能够更好地模拟和管理许多自然水道，更好地预测它们如何以及在何处运输和存款沉积物，就必须更好地了解这些反馈过程。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形产生的大尺度湍流的拓扑结构和运动学。最先进的实验室技术，以粒子图像测速法，激光诱导荧光和高分辨率热成像的形式，将使我们能够量化流动在固定沙丘形式的一系列流动条件。结果将使我们能够回答有关沙丘相关湍流结构的生成，形式和动力学的重要问题，研究这是如何平流和与水面相互作用，也将提供一个基准数据集，用于改进数值模拟代码。在今后的河道管理中，特别是在洪水计算中的水流阻力评估、泥沙输运能力的确定以及与更好地理解更大尺度的河道形态动力学相联系方面，特别需要这样的目标。

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.