River confluences, which are characterized by complex hydrodynamics, are key nodes for flood control and environmental protection. Two field surveys were carried out at the confluence of the Yangtze River and Poyang Lake to investigate the transient character of flow structures, which are often assumed steady. Repeat-transect acoustic Doppler current profile measurements were processed and analyzed, adopting a new method to separate mean flow from turbulence and measurement error based on physics-informed generalized Tikhonov regularization. The two field surveys were characterized by two distinct mixing interface modes: A so-called Kelvin–Helmholtz (KH) mode, and a wake mode. In KH mode, large-scale flow fluctuations were observed. These flow fluctuations exert a substantial influence on the alternation of secondary flows by changing the water surface pressure gradient, affecting the intensity of secondary flows rather than their spatial structure. We infer that temperature-induced stratification is the main cause of this. In the wake mode, multiple vortices in the wake region at the confluence apex also produced flow fluctuations, directly related to the primary velocity gradient. We argue that even for constant incoming flows, secondary flow at river confluences can exhibit channel-scale unsteadiness related to migration of the turbulent mixing interface. Our findings highlight the crucial role of density effects in regulating secondary flow unsteadiness, which is essential for understanding contaminant and sediment dispersal in river systems.

中文翻译：

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大型河流汇流处的失常次流结构


河流汇流具有复杂的水动力学特征，是防洪和保护环境的关键节点。在长江和鄱阳湖的交汇处进行了两次实地调查，以调查流动结构的瞬态特征，这些结构通常被认为是稳定的。对重复样带声学多普勒电流剖面测量进行了处理和分析，采用一种基于物理知情广义 Tikhonov 正则化的新方法将平均流与湍流和测量误差分开。这两次实地调查的特点是两种不同的混合界面模式：一种是所谓的开尔文-亥姆霍兹 （KH） 模式，另一种是尾流模式。在 KH 模式下，观察到大尺度的流量波动。这些流量波动通过改变水面压力梯度对次流的交替产生重大影响，影响次流的强度而不是其空间结构。我们推断温度诱导的分层是造成这种情况的主要原因。在尾流模式下，汇流顶点尾流区域中的多个涡流也产生了流量波动，这与初级速度梯度直接相关。我们认为，即使对于恒定的流入流量，河流汇流处的二次流也会表现出与湍流混合界面迁移相关的河道尺度的不稳定性。我们的研究结果强调了密度效应在调节次生流失稳性中的关键作用，这对于了解河流系统中的污染物和沉积物扩散至关重要。