题目 Title
On the role of the Froude number on flow, turbulence, and hyporheic exchange in open-channel flow through boulder arrays
期刊 Journal
Journal of Physical Chemistry B (IF=4.1)
作者 Author
Liu, Y.; Tang, Z. D.; Huang, L.; Stoesser, T.; Fang, H. W.
摘要 Abstract
In this paper, the results of numerical simulations of open-channel flow through boulder arrays at varying Froude numbers are reported. The simulations aim at clarifying the role of the Froude number on flow, turbulence, and hyporheic exchange. At low and intermediate Fr, the boulder top is above the water surface, and time-averaged streamwise flow velocity, Reynolds shear stresses, and the turbulent kinetic energy (TKE) are relatively low in the wake of boulders. Conversely, at high Fr values, the boulders are submerged, hence the flow separates at the boulder crest, creates vertical recirculation, and reattaches on the bed downstream, resulting in an area of elevated Reynolds shear stresses and TKE downstream of the boulders. Two dominant turbulence structures are observed: (i) flapping of boulder wakes with a characteristic length of 2.1 times the boulder diameter (D) at low and intermediate Fr and (ii) an upstream oriented hairpin vortex with a length scale of 1.0D at high Fr. These turbulence structures influence hyporheic exchange downstream of boulders within a limited region of x/D＜2.0. In other locations, hyporheic flow is driven by downwelling flow immediately upstream of boulders with a wavelength larger than 2.9D. Finally, the normalized time-averaged hyporheic flux increases with increasing Fr, but it decreases at higher Fr values once the overtopping flow disrupts the formation of the boulder wake.
简介 Brief introduction
本文报告了不同弗劳德数条件下通过卵石阵列的明渠流动数值模拟结果。模拟旨在明确弗劳德数对流动、紊流结构和潜流交换的作用。在中低弗劳德数下，卵石顶部露出水面，卵石尾流的时均流向速度、雷诺切应力和湍动能（TKE）较低。相反地，在高弗劳德数下，卵石被水淹没，流动在卵石顶部发生分离，形成垂向跌水，导致卵石下游区域的雷诺切应力和湍动能增加。研究观察到两种主要的紊流结构：（i）在中低弗劳德数下，卵石尾流的横向摆动具有2.1倍卵石直径（D）的尺度；（ii）在高弗劳德数下，形成指向上游的发卡涡，具有1.0D的特征长度。这些紊流结构在卵石下游x/D＜2.0的有限区域内影响了潜流交换。而在其他区域，潜流交换主要由卵石上游的大尺度地表下降流主导，其波长超过2.9D。最后，归一化时均潜流交换通量随弗劳德数增加而增加，但在更高的弗劳德数时，卵石顶部跌水破坏了卵石尾流的形成，通量反而减少。
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