Modelling time dependent flow fields over three dimensional dunes.
Modelling time dependent flow fields over three dimensional dunes.
复制标题
对三维沙丘上随时间变化的流场进行建模。
DOI：
10.1201/b17133-141
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
J. Best
中科院分区：
文献类型：
--
作者：
R. Hardy;T. Marjoribanks;D. Parsons;A. Reesink;B. Murphy;P. Ashworth;J. Best
沙丘上的流场已经在实验室条件下进行了广泛的测量，现在对平衡流条件下形成的沙丘上的流的性质有了普遍的了解，然而，河流系统通常经历非稳定流，因此沉积物-水界面在一定的空间和时间尺度上不断响应和重组这种非稳定性。这主要是通过调整河床形态（包括沙纹、沙丘和沙坝形态）来改变流场。本文应用大涡模拟（LES）模型研究了这些粗糙元对总流场的影响以及流态变化时阻力的变化。为了提供边界条件和LES模型的验证数据集，在长16 m，宽2 m的水槽中进行了一系列物理实验，其中细砂（D50为239µm）在一系列非稳态水力条件下进行水处理，产生一系列准平衡三维床形。在实验过程中，流量测量与一系列的声学多普勒测速仪。在四个场合，水槽排水和床地形测量地面激光雷达创建数字高程模型。采用大涡模拟方法对实验中得到的四种静态床面的三维非定常流场进行了数值模拟。采用标准雷诺分解方法和拉格朗日拟序流结构识别方法对数值预报的流动进行了分析。结果表明，沙丘背风面典型分离流区的性质发生了变化，床面形态场随水流条件变化而变化的叠加床面形态是床面形态场的共同特征。特别是，涡脱落的位置和流动再附着点的数量，这影响了随时间变化的剪切应力的预测，被发现有实质性的改变。这对泥沙挟沙和泥沙输移动力学具有重要意义，其结果使人们能够更好地理解水流条件变化对床面形态调整的三维本质。
The flow field over dunes has been extensively measured in laboratory conditions and there isnow a general understanding of the nature of the flow over dunes formed under equilibrium flow conditions.However, fluvial systems typically experience unsteady flow and therefore the sediment-water interface is constantly responding and reorganizing to this unsteadiness, over a range of both spatial and temporal scales. This is primarily through adjustment of bed forms (including ripples, dunes and bar-forms) which then subse-quently alter the flow field. This paper investigates, through the application of a Large Eddy Simulation (LES) model, the influence of these roughness elements on the overall flow and the variation in flow re- sistance during a change in flow conditions. To provide boundary conditions and a validation dataset for theLES model, a series of physical experiments were undertaken in a flume, 16m long and 2m wide, where fine sand (D 50 of 239µm) was water worked under a range of unsteady hydraulic conditions that generated a series of quasi-equilibrium three-dimensional bed forms. During the experiments flow was measured with a series of acoustic Doppler velocimeters. On four occasions, the flume was drained and the bed topography measured with terrestrial LiDAR to create digital elevation models. LES was used to simulate the three-dimensional time-dependent flow fields over the four static bed topographies from the experiments. The numerically pre- dicted flows were analyzed by standard Reynolds decomposition approaches and a Lagrangian coherent flow structure identification method. The results show that superimposed bed forms, that are common to bed formfields adjusting to changes in flow conditions, can cause changes in the nature of the classical separated flow regions in the lee side of dunes. In particular, the number of locations where vortices are shed and the pointsof flow reattachment, which effect the time dependent prediction of shear stress, were found to alter substan-tially. This has significant implications for sediment entrainment and sediment transport dynamics and the re-sults enable improved process understanding of three dimensional nature of bed form adjustment to changesin flow conditions.