【作者】 白静；

【导师】 方红卫；

【作者基本信息】 清华大学 ， 水利工程， 2013， 博士

【摘要】 紊流在实验室试验水槽和自然河道中普遍存在。紊流中存在着涡系，对泥沙起动和输移有重要的影响，而传统的雷诺时均模型只能给出时均条件下流速和泥沙浓度分布，不能反映瞬时流动特性以及瞬时流动对泥沙输移的影响，因此需要研究更加精细的模拟技术。随着计算机计算速度和存储能力的提高，大涡模拟技术得到了迅速发展，大涡模型可以计算得出水流和泥沙输运的瞬时规律。本文建立了悬移质泥沙输移的大涡模型及边界条件。在大涡模拟中，通过过滤将水沙模型中的变量分解成大尺度量和亚格子量，直接求解大尺度量，引入亚格子应力模型模拟小尺度量。在程序中，基于有限体积法对控制方程进行离散，变量布置在非交错曲线网格上，采用中心差分近似扩散通量，分别采用中心差分和HLPA格式近似水流和泥沙计算中的对流通量，时间推进利用低存储的三步Runge-Kutta法。在Runge-Kutta法的第3步，采用SIP方法求解压强修正方程。针对复杂的几何边界问题，本文构建了基于直接强迫力法的浸没边界法，用于模拟复杂边界下的大涡模拟流动；采用直槽流动、方腔流动和丁坝绕流检验了流速分布、紊动强度分布、涡系分布和切应力分布；分析了丁坝绕流中丁坝长度L、丁坝长度与丁坝距离的比值L/D对流速分布、回流区的流动形式、紊动强度分布、涡量分布及涡系结构分布的影响，并研究了涡体的发展变化过程。针对自然条件的泥沙床面，建立了粗糙边界处理方法和悬沙计算模式；采用经典的净淤积试验对沿程的流速分布和泥沙浓度分布进行验证，获得与实测数据符合较好的结果；引入不同的对比算例，分析了水流的宽深比B/H对横断面上二次流动和涡体结构的影响，讨论了宽深比B/H和二次流动对主流流速、紊动强度和切应力在横断面上分布的影响，并研究了宽深比B/H和二次流动对泥沙浓度在横断面上的分布以及沿程沉积率的影响。针对悬移质泥沙输移的三维特性，建立了悬移质泥沙计算中含有恢复饱和系数的近底部边界条件，用于研究细颗粒悬移质泥沙起动与输移的特性；采用直接模拟结果和Rouse公式分别检验了直槽中的流速分布和泥沙浓度分布；采用循环算例和长槽道算例分析了冲刷平衡的条件下和净冲刷条件下的流速、紊动强度、切应力的分布、泥沙浓度以及垂向泥沙紊动通量的分布，讨论了瞬时流动结构对瞬时泥沙浓度分布的影响，以及猝发与扫射对泥沙浓度分布的影响，给出了紊动扩散系数和Schmidt数的分布，并探讨了近底部泥沙紊动通量的概率分布。更多还原

【Abstract】 Turbulence is very common both in laboratory channels and natural rivers. Vorticesexist in turbulence and have an important impact on sediment transport. ReynoldsAveraged Navier–Stokes Simulation (RANS) is widely used in numerical simulations ofengineering. It can only give time-averaged velocity and sediment concentration, andcould not reflect the effects of unsteadiness and turbulence anisotropy on sedimenttransport. So it needs to introduce some advanced simulation techniques. With theadvances in computer capabilities and large eddy simulation (LES), LES becomespopular in the simulation of water flow and sediment transport, and it can give theinstant distributions of water flow and sediment transport.A LES model, calculating water flow and suspended sediment transport under theconditions of complex topography, has been set up in the thesis. In the model, all thevariables are decomposed into large-scale ones and subgrid ones by filtration. Thelarge-scale variables are solved directly and the subgrid ones simulated by introducingthe subgrid stress (SGS) model. The equations are discretized with the finite volumemethod with no-staggered curvilinear grids. The convective fluxes of momentumequations are estimated by central differences of second order accuracy, while the onesof sediment transport are obtained by HLPA scheme. The diffusive fluxes of momentumand sediment transport equations are approximated using central scheme. Asecond-order accurate Runge-Kutta method is adopted in time discretization. ThePoisson equation for pressure correction is solved via the SIP method.As for the simultions under complex boundaries, an immersed boundary modulebased on the direct forcing method is introduced into the LES model. Firstly the modelis calibrated using channel flow, duct flow and groin flow. And accurate results areobtained including distributions of velocities, turbulence intensities, vortices and shearstresses. A series of cases for flow past rounded-head groins are set up to investigateflow characteristics, involving flow patterns, turbulent intensities, vorticity distributionand vortex dynamics. Groin aspect ratio L/D and groin length L are defined to study theimpact of groin parameters on the flow properties.As for natural boundary in the channel, a module dealing with rough boundary anda module of suspended sediment calculation are added into the LES model, in order to study the impact of the secondary flow on the deposition and transport of fine sedimentin rough channel flow. After validating the model use a classic experiment of netdeposition, different cases are set up to explore the effect of the width to depth ratio B/Hon secondary flow and vortex structures at the cross sections. The influences of thesecondary flow on flow properties like the mainstream velocities, turbulence intensitiesand shear stress distribution at the cross section, are discussed. The results of thedistribution of sediment concentration at the cross sections and deposition ratio alongthe channel are also studied.As for the3D characteristics of suspended sediment transport, a boundary with arestoring saturation coefficient near the bottom is presented, to study the characteristicsof fine sediment entrainment and transport in channel flow. The velocities and sedimentconcentration are validated by Direct Numerical Simulation (DNS) and Rouse equationsseparately. A case with cyclic boundary and a long case with inlet-outlet boundary arecalculated to study the flow and fine sediment transport in the equilibrium andnet-erosion conditions. The results are given out such as velocities, turbulenceintensities, the distribution of shear stresses, sediment concentration, as well asturbulence flux of sediment. The strong relations between instantaneous flow andsediment concentration, and contribution of bursts and sweeps are analyzed throughstatistics. According to the study, turbulent Schmidt number is not constant along thevertical direction. The turbulence flux of sediment near the bottom is discussed at theend.更多还原