题目 Title

Insights into the enhanced flow resistance due to coarsened riverbed sediments via large-eddy simulations

期刊 Journal

FLOW-Application of Fluid Mechanics (IF=2.7)

作者 Author

Liu, J. C.; He, G. J.; Wang, K.; Liu, Y.; Fang, H. W.

摘要 Abstract

In this study, the method of large-eddy simulation (LES) is applied to investigate the impact of patches of coarsened riverbed sediments on near-bed hydrodynamics and flow resistance. Six simulations are performed with riverbed coverage ratios of coarser particles (Ac/At, where Ac and At are the riverbed area covered by coarsened sediments and the total riverbed area, respectively) ranging from 0 % to 100 %. By ensuring identical crest heights for all particles, the influence of heterogeneous roughness height is eliminated, allowing for an isolated investigation of heterogeneous permeability effects. Results reveal distinct high- and low-flow streaks above coarsened and uncoarsened sediments, associated with elevated and reduced Reynolds shear stress, respectively. These streaky patterns are attributed to time-averaged secondary flows spanning the entire water depth, that converge toward coarsened sediments and diverge from uncoarsened areas. Elevated Reynolds shear stress, up to 1.9 times the reach-averaged bed shear stress, is observed in the interstitial spaces between coarser particles due to intensified hyporheic exchange at the sediment–water interface. Upwelling and downwelling flows occur upstream and downstream of coarsened sediments particles, respectively, driving dominant ejection and sweep events. At Ac/At = 16 %, ejections and sweeps contribute maximally to Reynold shear stress, increasing by up to 130 % and 110 %, respectively – approximately double their contributions in the uncoarsened case. The study identifies two mechanisms driving increased flow resistance over coarsened riverbeds: water-depth-scale secondary flows and grain-scale hyporheic exchanges. Consequently, the reach-averaged friction factor increases by 29.8 % from Ac/At = 0 % to 64 %, followed by a 15.8 % reduction in the fully coarsened scenario.

简介 Brief introduction

本文利用大涡模拟（Large-Eddy Simulation, LES）方法，系统研究了河床粗化对水流阻力的影响机制。通过构建不同粗化比例和分布形式的非均匀颗粒床，研究并揭示了粗化区域在非均匀河床中不仅显著改变了近床区的流动结构，还导致了床面切应力与湍流强度的局部增强。特别是在粗化区与非粗化区的边界附近，水流发生扰动增强与流速剪切，形成了更为复杂的二次流与垂向水流输移结构，进而强化了床面摩阻。研究指出，这种由河床表面粒径空间变化引发的结构性扰动是导致整体水流阻力上升的关键因素。此外，文章还量化了不同粗化比例下的整体阻力系数变化，表明随着粗化面积的增加，整体阻力呈现非线性增长趋势。本研究不仅丰富了对河床粗化与水流动力学相互作用的理解，也为实际河道整治及生态修复提供了理论支撑。

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