CFD中颗粒破碎的真实表示--泥沙输运的DEM模拟：一种粘结球方法
Realistic Representation of Grain Shapes in CFD--DEM Simulations of Sediment Transport: A Bonded-Sphere Approach
摘要：
在过去的几十年里，算法的发展和计算资源的增长使得泥沙输运过程的模拟具有前所未有的灵活性。计算流体动力学-离散元方法（CFD-DEM）是高保真度的方法之一，它解决了沉积物颗粒的运动、碰撞及其与周围流体的相互作用。在大多数DEM解算器中，由于计算效率和实现复杂性的考虑，颗粒被建模为软球，尽管天然沉积物通常是非球形颗粒的混合物。以前尝试扩展基于球的DEM来处理不规则颗粒，忽略了颗粒上的流体引起的扭矩，并且该方法缺乏处理具有任意颗粒形状混合物的沉积物的灵活性。在这篇文章中，我们提出了一个简单，有效的方法来表示常见的沉积物颗粒形状与结合的球体，计算流体力，并施加在每个球体上。所提出的方法克服了现有方法的上述局限性，并提高了现有方法的效率和灵活性。通过数值模拟，验证了该方法在预测天然泥沙的下落特性、终极速度、起动阈值和输沙率等方面的优越性和有效性。模拟结果表明，该方法是一种很有前途的方法，忠实地代表天然泥沙，从而导致其输运动力学的准确模拟。所提出的方法也打开了基于第一性原理的模拟粘性沉积物的絮凝和沉积动力学的可能性。阐明这些物理机制，可以提供急需的改进的预测能力和物理理解的泥质海岸动力学。
英文摘要：
Development of algorithms and growth of computational resources in the past decades have enabled simulations of sediment transport processes with unprecedented fidelities. The Computational Fluid Dynamics--Discrete Element Method (CFD--DEM) is one of the high-fidelity approaches, where the motions of and collisions among the sediment grains as well as their interactions with surrounding fluids are resolved. In most DEM solvers the particles are modeled as soft spheres due to computational efficiency and implementation complexity considerations, although natural sediments are usually mixture of non-spherical particles. Previous attempts to extend sphere-based DEM to treat irregular particles neglected fluid-induced torques on particles, and the method lacked flexibility to handle sediments with an arbitrary mixture of particle shapes. In this contribution we proposed a simple, efficient approach to represent common sediment grain shapes with bonded spheres, where the fluid forces are computed and applied on each sphere. The proposed approach overcomes the aforementioned limitations of existing methods and has improved efficiency and flexibility over existing approaches. We use numerical simulations to demonstrate the merits and capability of the proposed method in predicting the falling characteristics, terminal velocity, threshold of incipient motion, and transport rate of natural sediments. The simulations show that the proposed method is a promising approach for faithful representation of natural sediment, which leads to accurate simulations of their transport dynamics. The proposed method also opens the possibility for first-principle-based simulations of the flocculation and sedimentation dynamics of cohesive sediments. Elucidation of these physical mechanisms can provide much needed improvement on the prediction capability and physical understanding of muddy coast dynamics.
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