Abstract Channel meanders in rivers induce complex three-dimensional (3D) flow characteristics such as secondary flows and flow recirculation. Helical secondary flows promote transverse dispersion, and flow recirculation zones trap tracers. As a result, these meander-driven flows may cause anomalous transport manifested by unusually elevated levels of tracer concentration at early and late times of breakthrough curves (BTCs). In this study, we perform 3D numerical simulations in meandering channels across a wide range of channel sinuosity to investigate the impact of meander geometry on flow and transport. We solve 3D Reynolds-averaged Navier-Stoke equations blended with a SST k − ω turbulence model to obtain velocity and turbulence fields. We then incorporate the obtained flow fields into a Lagrangian particle tracking model to simulate solute transport. Sinuosity higher than 1.5 leads to the onset of horizontal recirculating flows along the apex outer banks, and these recirculation zones expand as sinuosity increases. The analysis of the transport simulations elucidates that the interplay between the secondary flows and recirculation zones induces anomalous transport. The helical secondary flows bring particles into the recirculation zones by promoting transverse dispersion, and the recirculating flows delay particle transport by the trapping effect. We show that the tail power-law slope and truncated time of BTCs as well as Lagrangian tortuosity distributions change dramatically with the emergence of recirculation zones. These analyses demonstrate that the recirculation zones are acting as the primary driver of anomalous transport. Finally, we successfully predict the observed anomalous transport with a Spatial Markov Model (SMM), which is an upscaled transport model that incorporates Lagrangian velocity distribution and spatial velocity correlation. The successful predictions show that the Lagrangian velocity statistics effectively capture the underlying mechanisms of anomalous transport in meandering open-channel flows.

中文翻译：

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曲折明渠流中循环流引起的异常输运

摘要 河流中的河道曲流会导致复杂的三维 (3D) 流动特征，例如二次流和流动再循环。螺旋二次流促进横向扩散，流动再循环区捕获示踪剂。因此，这些曲流驱动的流动可能会导致异常传输，表现为在突破曲线 (BTC) 的早期和晚期示踪剂浓度异常升高。在这项研究中，我们在各种渠道曲率的蜿蜒渠道中执行 3D 数值模拟，以研究蜿蜒几何形状对流动和运输的影响。我们求解与 SST k − ω 湍流模型混合的 3D 雷诺平均 Navier-Stoke 方程以获得速度和湍流场。然后我们将获得的流场合并到拉格朗日粒子跟踪模型中以模拟溶质传输。高于 1.5 的曲率导致沿顶点外岸的水平再循环流的开始，这些再循环区随着曲率的增加而扩大。传输模拟的分析阐明了二次流和再循环区之间的相互作用导致了异常传输。螺旋二次流通过促进横向分散将颗粒带入再循环区，再循环流通过捕获效应延迟颗粒传输。我们表明，随着回流区的出现，BTC 的尾幂律斜率和截断时间以及拉格朗日曲率分布发生了显着变化。这些分析表明，再循环区是异常运输的主要驱动因素。最后，我们使用空间马尔可夫模型 (SMM) 成功预测了观察到的异常传输，该模型是一种结合拉格朗日速度分布和空间速度相关性的放大传输模型。成功的预测表明，拉格朗日速度统计有效地捕捉了蜿蜒明渠流中异常传输的潜在机制。