Development of a combined Lagrangian / Eulerian approach to understand coherent flow structures in gravel-bed rivers

河流泥沙运动是由流体力驱动的，对河流地貌、生态和管理具有广泛的影响。传统上，水流和泥沙运动是在时间平均的、连续的基础上分析和模拟的，但众所周知，在近床速度中存在巨大的湍流波动，因此，在移动泥沙的阻力和升力中存在巨大的湍流波动。砾石河床中的湍流不是一个简单的随机场：可视化和多点测量表明，可以将复杂的、多尺度的准随机流场分解成基本的有组织的结构，这些结构既具有时空一致性，又被称为涡流或相干流结构。这项研究的目的是加强对砾石床河流中相干流动结构的产生、演变和消散的定量理解。这将通过在英国装备最好的环境流体动力学实验室进行一系列概念验证实验室水槽实验来实现，该实验室使用新的自动湍流测绘技术。这些技术提供了欧拉和拉格朗日相结合的观点，通过向流动中注入示踪剂揭示了相干流动结构，使我们能够监测并因此了解这些相干流动结构的运动学(大小、比例、形状、涡度和能量)和动力学性质(起源、稳定性、增长、形成新形式和对时间平均流动的贡献)，因为它们产生并在复杂的地形上移动，包括砾石床表面。所提出的方法将使我们既可以增加我们对砾石河床河流中相干流动结构的现象学理解，也可以发展和改进这种复杂表面上的流动的随时间变化的数值模型。

Sediment movement in rivers, which has wide-ranging consequences for fluvial landforms, ecology & management, is driven by fluid forces. Flow & sediment movement has traditionally been analysed & modelled on a time-averaged, continuum basis, but it is well known that large turbulent fluctuations exist in near-bed velocity, & therefore in the drag & lift forces that move sediment. Turbulence in gravel-bed rivers is not a simple random field: visualisation & multipoint measurements show it is possible to decompose complex, multi-scaled, quasi-random flow fields into elementary organized structures which posses both spatial & temporal coherence that are called either eddies or coherent flow structures. The aim of this research is to develop an enhanced quantitative understanding of the generation, evolution & dissipation of Coherent Flow Structures in Gravel Bed Rivers. This will be achieved by a series of proof-of-concept laboratory flume experiments using novel automated turbulence mapping techniques in the UK's best-equipped environmental fluid dynamics laboratory. The techniques give a combined Eulerian and Lagrangian view of Coherent Flow Structures revealed by injecting tracers into the flow that allows us to monitor and thus understand both the kinematic (size, scaling, shape, vorticity & energy) & dynamic properties (origin, stability, growth, genesis into new forms & contribution to the time average flow) of these Coherent Flow Structures as they are prduced and move over complex topographies, including gravel bed surfaces. The proposed methodology will allow us to both increase our phenomenological understanding of Coherent Flow Structures in Gravel bed rivers & develop & improve time dependent numerical models of flow over such complex surfaces.