粘性泥沙颗粒沉降动力学的颗粒分辨模拟研究
Investigating the Settling Dynamics of Cohesive Silt Particles With Particle-Resolving Simulations
摘要：
粘性沉积物的沉降在水环境中是普遍存在的。在沉降过程中，由于颗粒间cohesion内聚力的影响，粉土颗粒表现出与非粘性颗粒不同的特性。虽然凝聚行为取决于沉积物颗粒的特性（例如，键数，粒度分布），很少有人知道这些特性的确切影响的凝聚行为。本文采用CFD-DEM（Computational流体动力学-Discrete Element Method）对三维沉降过程进行了数值模拟。考虑了颗粒间碰撞力、范德华力和流体-颗粒相互作用力。在前人试验装置的基础上，利用数值模型对粉土的受阻沉降过程进行了模拟。在模拟中得到的结果，包括床层的结构密度，特征线，和颗粒终极速度，是在文献中的实验观察符合得很好。据作者所知，这是第一次分别研究了无因次Bond数和颗粒多分散性对粉土床层结构密度的影响。结果表明，粉土在沉降过程中的粘聚行为是由粉土颗粒间的粘聚作用和颗粒的多分散性共同作用的结果。为了指导粘性粉土沉降的宏观模拟，基于颗粒细观力学，给出了数值模拟中得到的碰撞频率函数。利用CFD-DEM得到的结果表明，二元碰撞理论高估了高固体体积分数下絮凝过程中颗粒碰撞频率。
英文摘要：
The settling of cohesive sediment is ubiquitous in aquatic environments. In the settling process, the silt particles show behaviors that are different from non-cohesive particles due to the influence of inter-particle cohesive force. While it is a consensus that cohesive behaviors depend on the characteristics of sediment particles (e.g., Bond number, particle size distribution), little is known about the exact influence of these characteristics on the cohesive behaviors. In the present work, three-dimensional settling process is investigated numerically by using CFD--DEM (Computational Fluid Dynamics--Discrete Element Method). The inter-particle collision force, the van der Waals force, and the fluid--particle interaction forces are considered. The numerical model is used to simulate the hindered settling process of silt based on the experimental setup in the literature. The results obtained in the simulations, including the structural densities of the beds, the characteristic lines, and the particle terminal velocity, are in good agreement with the experimental observations in the literature. To the authors' knowledge, this is the first time that the influences of non-dimensional Bond number and particle polydispersity on the structural densities of silt beds have been investigated separately. The results demonstrate that the cohesive behavior of silt in the settling process is attributed to both the cohesion among silt particles themselves and the particle polydispersity. To guide to the macro-scale modeling of cohesive silt sedimentation, the collision frequency functions obtained in the numerical simulations are also presented based on the micromechanics of particles. The results obtained by using CFD--DEM indicate that the binary collision theory over-estimated the particle collision frequency in the flocculation process at high solid volume fraction.
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