【作者】 张金凤；

【导师】 张庆河；

【作者基本信息】 天津大学 ， 港口、海岸及近海工程， 2007， 博士

【摘要】 粘性泥沙絮凝过程对粘性泥沙运动及河口海岸许多物理过程起着十分重要的作用,目前尚缺乏对絮凝机理进行有效描述的直接数值模拟手段。为此,本文建立了粘性泥沙运动的三维格子Boltzmann模型,对粘性泥沙絮凝过程进行数值模拟,研究了静水中与水流剪切作用下的絮团沉降规律,揭示了不等速沉降引起的粘性泥沙絮凝机理,对实际絮凝现象从微观角度进行了阐释。论文的具体研究内容和结论如下:1、建立了粘性泥沙运动全尺度模拟的三维格子Boltzmann模型。泥沙颗粒沉降引起的层流和过渡区流场的格子Boltzmann模拟结果与理论解比较吻合,说明本文建立的模型可以对层流和过渡区范围内颗粒沉降进行直接数值模拟。2、在格子Boltzmann方法中引入大涡模拟,对大粒径球体在静水中沉降引起的水流紊动进行了模拟。模拟结果与颗粒图像测速系统测定的颗粒沉降速度和紊动流场结果进行了比较,趋势基本一致,说明利用格子Boltzmann方法与大涡模拟技术相结合可以合理模拟紊流区泥沙沉降。3、利用扩散受限聚集体模型生成分形絮凝体近似代表河口海岸粘性泥沙絮团,应用格子Boltzmann方法对粘性泥沙三维分形絮团的静水沉降进行了模拟。模拟结果与得到广泛验证和应用的Winterwerp絮团沉速公式有较好的一致性,从微观角度揭示了三维分形絮团的静水沉降水动力特性。4、应用格子Boltzmann方法对粘性泥沙三维分形絮团在水流剪切作用下的沉降、破裂过程进行了模拟,结果表明:水流流速较小时,絮团将不发生破裂而沉积到海床;在水流流速较大的情况下,强度较小的絮团将在水流剪切作用下发生破裂,破裂后的单个泥沙颗粒或小絮团随着水流一起运动,不会沉降到床面,而强度较大的絮团在经受近底剪切变形后仍可沉降到床面。5、应用格子Boltzmann方法对粘性泥沙不等速沉降絮凝过程进行了全尺度模拟,结果表明:密度相同,大小不同,质心在同一铅垂线上的两个泥沙颗粒可以由于不等速沉降碰撞、粘结形成絮团;不等速沉降时不同泥沙浓度条件下形成的絮团若大小相近则沉降速度接近,但泥沙浓度对泥沙总体平均沉速有明显影响,泥沙浓度越高,泥沙平均沉速越大,主要是因为泥沙浓度越大,颗粒之间越容易发生碰撞而形成更大、更多的絮团,絮凝时间也越短。6、絮团在不等速沉降过程中会快速捕捉其他颗粒或小絮团而在较短时间内形成更大絮团,造成泥沙较快沉积,这种现象在潮流憩流期是有可能出现的。更多还原

【Abstract】 The flocculation processes play an important role in cohesive sediment transport and some physical phenomena in estuaries and on coasts. There is few effective ways for direct numerical simulation of flocculation mechanism. Therefore, a three-dimensional numerical model of flocculation dynamics of cohesive sediment via the Lattice Boltzmann method was presented. The flocculation process and settling behavior of cohesive sediment were explored in still water and shear flow. The flocculation mechanism due to differential settling was disclosed. Some physical phenomena in the field were explained from the mesoscale view. The main results are summarized as follows:1. A fully resolved numerical model of flocculation dynamics of cohesive sediment via the Lattice Boltzmann method was developed. The simulated results of the laminar and transitional flows induced by single particle settling, agreed with those analytical solutions. It was shown that the present model could be applied to the simulation of the particles settling in the laminar and transitional flows.2. The turbulent flows induced by the large settling particle were obtained through the Lattice Boltzmann method combining with the large-eddy simulation method. The computational results were basically in accordance with the settling velocity and flow field measured by the Particle Imaging Velocimetry. It was concluded that the Lattice Boltzmann method and the large-eddy simulation could be used to simulate the turbulent flow induced by particle settling.3. The fractal mud flocs formed by using the diffusion limited cluster-cluster aggregation model resembled the mud flocs in the field. The simulated settling velocities of the fractal mud flocs via Lattice Boltzmann method agree with the calculated results of Winterwerp’s settling velocity formula adequately. The settling behaviour of three-dimensional fractal mud floc in still water was disclosed from the mesoscale view.4. The processes of settling and breakup of three-dimensional fractal mud flocs under the shear flows were simulated. Under low shear conditions, flocs will not break and deposit to the bed. Under high shear conditions, the flocculated aggregates with weak strength will break up, and the single particles and small flocs after breakup could be suspended in the flow without settling into the bed. However, the flocs with great strength could fall through the shear zone near the bed and deposited to the bed though undergoing large deformation.5. The flocculation processes due to the differential settling of cohesive sediment were fully simulated via Lattice Boltzmann method. Two particles of the same density and different sizes, located in the same vertical line, could collide and aggregate. For the differential settling, flocs with similar sizes formed in different sediment concentrations have similar settling velocities, which show that the sediment concentrations exert few effects on the settling velocities of flocs themselves. Otherwise, the sediment concentrations have obvious influences on the bulk mean settling velocity of suspended sediment. The higher is the sediment concentration, the larger the bulk mean settling velocity will be, which can be attributed that the particles in the higher concentration water are easy to collide and form larger flocs quickly.6. Flocs quickly capture other single particles or smaller flocs and aggregate into larger flocs during the process of the differential settling, which will lead to the fast deposition of suspended sediment. This phenomenon is likely to occur during the slack water.更多还原