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首页> 外文学位 >Three-dimensional unsteady modeling of clear-water scour in the vicinity of hydraulic structures: Lagrangian and Eulerian perspectives.
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Three-dimensional unsteady modeling of clear-water scour in the vicinity of hydraulic structures: Lagrangian and Eulerian perspectives.

机译：水力结构附近的清水冲刷的三维非定常建模：拉格朗日和欧拉观点。

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摘要

The complex interaction between turbulence and sediment dynamics in aquatic environments is the most important mechanism of sediment transport and bed erosion in multiple geophysical, environmental, and engineering flows. Scour around hydraulic structures is an example in which this relation acquires great relevance. The bed erosion in the vicinity of bridge foundations is controlled by the dynamically rich horseshoe vortex system (HSV) that develops in front of the structures and increases the near-bed turbulent stresses by one order of magnitude compared to the approaching turbulent boundary layer flow.;Advances in numerical simulations designed to understand the physical mechanisms of sediment transport and bed erosion in turbulent flows, however, have been limited by the ability of the models to capture the large-scale coherent vortical structures with adequate resolution, and by the level of description and assumptions of the sediment transport models utilized to predict the sediment flux.;In this thesis we develop an advanced computational fluid dynamics (CFD) model to simulate the flow, bed-load transport, and scour in the vicinity of hydraulic structures. To handle arbitrarily complex multi-connected geometries, the numerical solver employs domain decomposition techniques with structured Chimera overset grids. An Arbitrary Lagrangian-Eulerian (ALE) approach is also incorporated to consider the effects of moving boundaries in the flowfield solution. We carry out numerical simulations of the turbulent flow past a cylindrical pier using the detached-eddy simulation (DES) approach as the turbulence model. DES is a hybrid method that combines an unsteady Reynolds-averaged Navier-Stokes (URANS) model in regions of the computational domain near the wall, with large-eddy simulation (LES) in regions away from solid boundaries This numerical method is capable of capturing the dynamics of the HSV and reproducing for the first time, along with the recent study of Paik, Escauriaza, and Sotiropoulos [Phys. Fluids 19, 045107, 2007], all the experimental trends observed in junction flows at high Reynolds numbers.;Two models of sediment transport are developed in the present investigation to study the initiation of motion, transport processes, and clear-water scour by the large-scale vortical structures of the HSV system: (1) A Lagrangian model for sediment grains to simulate the transport of individual particles. The trajectory and momentum of the sediment particles are computed to evaluate the effects of the instantaneous hydrodynamic forces induced by the HSV system. Since the magnitude of the particle stresses are near the threshold of motion, the transport is characterized by intermittent displacement events of varying magnitudes. Groups of sediment grains move continuously, saltating or sliding on the bed, and streaks aligned with near-wall vortices are formed around the cylindrical pier. The global transport of particles past the cylinder is studied by performing a statistical analysis of the flux to reveal scale-invariance of the process and multifractality of particle transport as the overall effect of the flow around the pier. (2) A new unsteady bed-load transport model based on the momentum equation of the sediment in an Eulerian framework. The evolution of scour is obtained from the solution of the Exner equation, computing the bed elevation from the instantaneous flowfield. The model reproduces scour in non-equilibrium conditions, giving information of the spatial distribution and time evolution of erosion and deposition in the vicinity of the pier. A remarkable process captured for the first time by our model is the development of bed-forms along the legs of the HSV system. The interaction of the vortical structures with the wall produces the bed instability that grows and propagates, generating ripples that travel and merge in the downstream direction showing the same dynamic features observed in experiments.^ The model constitutes a powerful simulation tool to investigate the relation between sediment and bed processes with coherent structures in turbulent flows, and it can also serve as a general framework for developing three-dimensional non-equilibrium sediment transport models that can be used in the future for engineering design and optimization. The model also highlights the importance of integrating high-resolution numerical simulations with laboratory experiments to understand and be able to predict the complex physics of sediment transport in nature.
机译：在水生环境中，湍流与沉积物动力学之间的复杂相互作用是多种地球物理，环境和工程流中泥沙输送和河床侵蚀的最重要机制。水工建筑物周围的冲刷就是这种关系具有重要意义的一个例子。桥梁基础附近的床层侵蚀是由动态富裕的马蹄形涡流系统（HSV）控制的，该系统在结构前面发展，并使近床面湍流应力比趋近于湍流边界层的流动增加一个数量级。 ；然而，旨在理解湍流中泥沙运移和床层侵蚀的物理机制的数值模拟研究的进展受到模型捕获具有足够分辨率的大型相干涡结构的能力以及水平的限制。用来预测泥沙通量的泥沙输送模型的描述和假设。本论文中，我们开发了一种先进的计算流体动力学（CFD）模型，以模拟水力结构附近的水流，床荷输送和冲刷。为了处理任意复杂的多连接几何，数值求解器采用具有结构化Chimera重叠网格的域分解技术。还采用了任意拉格朗日-欧拉（ALE）方法来考虑流场解决方案中移动边界的影响。我们使用分离涡模拟（DES）方法作为湍流模型，对绕过圆柱墩的湍流进行了数值模拟。 DES是一种混合方法，它在壁附近的计算域区域中结合了非稳态雷诺平均Navier-Stokes（URANS）模型，并在远离固体边界的区域中结合了大涡模拟（LES）。这种数值方法能够捕获HSV的动态变化和繁殖，以及Paik，Escauriaza和Sotiropoulos的最新研究[Phys。流体19，045107，2007]，在高雷诺数下在汇流处观察到的所有实验趋势。;本研究开发了两种沉积物运移模型，以研究运动引起的运移，运移过程和清水冲刷。 HSV系统的大型旋涡结构：（1）沉积颗粒的拉格朗日模型，模拟单个颗粒的传输。计算沉积物颗粒的轨迹和动量，以评估HSV系统引起的瞬时水动力的影响。由于颗粒应力的大小接近运动的阈值，因此传输的特征是大小不同的间歇位移事件。成群的沉积物颗粒连续运动，盐化或在床上滑动，并在圆柱形墩台周围形成与近壁涡流对齐的条纹。通过对通量进行统计分析，研究了粒子通过圆柱体的整体传输，以揭示过程的尺度不变性和粒子传输的多重分形性，作为围绕墩台流动的整体影响。 （2）基于欧拉框架内沉积物动量方程的新的非稳态床载输移模型。从Exner方程的解中获得冲刷的演变，并根据瞬时流场计算床层高程。该模型再现了非平衡条件下的冲刷，提供了码头附近侵蚀和沉积的空间分布和时间演化信息。我们的模型首次捕捉到了一个非凡的过程，即沿着HSV系统的腿部形成床形。涡旋结构与墙体的相互作用会导致床的不稳定性，该不稳定性会增长和传播，并产生沿下游方向传播和合并的波纹，并显示出与实验中观察到的相同的动态特征。湍流中具有相干结构的沉积物和床层过程，它还可以用作开发三维非平衡沉积物传输模型的通用框架，该模型可在将来用于工程设计和优化。该模型还强调了将高分辨率数值模拟与实验室实验相集成的重要性，以了解并能够预测自然界中沉积物输送的复杂物理过程。

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