On the evolution and form of coherent flow structures over a gravel bed: Insights from whole flow field visualization and measurement
On the evolution and form of coherent flow structures over a gravel bed: Insights from whole flow field visualization and measurement
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DOI：
10.1002/2015jf003753
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发表时间：
2016-08-01
影响因子：
3.9
通讯作者：
Marjoribanks, T. I.
中科院分区：
文献类型：
--
作者：
Hardy, R. J.;Best, J. L.;Marjoribanks, T. I.
砾石河床河流的微地形已被证明产生湍流结构，源于近床区域产生的剪切流。虽然现场和实验室的测量表明，这种流动包含一系列的相干流结构（CFS），起源，演变和湍流结构的特性知之甚少。在这里，我们应用相结合的实验方法，使用平面激光诱导荧光和粒子成像测速（LIF-PIV），同时测量这些CFS的几何，运动学和动力学特性。通过应用标准的雷诺分解和拉格朗日涡检测方法来分析流动结构，以了解它们在边界层中的演化、传播和增长，并表征其内部动力学复杂性。LIF的结果确定大的，个人的，流体包，通过剪切，产生一个爆裂机制在床上启动。当通过直接流动测量分析这些大的单独流体包时，发现它们在一个较大的单独流体包内包含几个较小尺度的流体运动。流动测量表明，近床剪切控制这些CFS的开始和演变，通过合并与涡链起源于床。涡链显示了在形成较大结构时的聚结以及从这些包的边缘脱落的涡，这可能影响CFS在明渠中的寿命和混合。这样的CFS的寿命和几何特征是至关重要的影响的持续时间和强度的近床应力，负责夹带泥沙。
The microtopography of a gravel bed river has been shown to generate turbulent flow structures that originate from shear flow generated in the near-bed region. Although field and laboratory measurements have shown that such flows contain a range of coherent flow structures (CFS), the origin, evolution, and characteristics of the turbulent structures are poorly understood. Here we apply a combined experimental methodology using planar laser-induced fluorescence and particle imaging velocimetry (LIF-PIV) to measure simultaneously the geometric, kinematic, and dynamic characteristics of these CFS. The flow structures were analyzed by applying standard Reynolds decomposition and Lagrangian vortex detection methods to understand their evolution, propagation, and growth in the boundary layer and characterize their internal dynamical complexity. The LIF results identify large, individual, fluid packets that are initiated at the bed through shear that generate a bursting mechanism. When these large individual fluid packets are analyzed through direct flow measurement, they are found to contain several smaller scales of fluid motion within the one larger individual fluid parcel. Flow measurements demonstrate that near-bed shear controls the initiation and evolution of these CFS through merging with vortex chains that originate at the bed. The vortex chains show both the coalescence in the formation of the larger structures and also the shedding of vortices from the edges of these packets, which may influence the life span and mixing of CFS in open channels. The life span and geometric characteristics of such CFS are critical in influencing the duration and intensity of near-bed stresses that are responsible for the entrainment of sediment.