Flocculation of suspended cohesive particles in homogeneous isotropic turbulence
Flocculation of suspended cohesive particles in homogeneous isotropic turbulence
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DOI：
10.1017/jfm.2021.487
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发表时间：
2021-05
影响因子：
3.7
通讯作者：
K. Zhao;F. Pomes;B. Vowinckel;T. Hsu;B. Bai;E. Meiburg
中科院分区：
文献类型：
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作者：
K. Zhao;F. Pomes;B. Vowinckel;T. Hsu;B. Bai;E. Meiburg
摘要：我们基于单向耦合模拟（包括斯托克斯阻力、润滑、内聚力和直接接触力）研究了均匀各向同性湍流中粘性颗粒的动力学。我们观察到短暂的絮凝阶段，然后是统计上稳定的平衡阶段。我们分析了由于聚集、破碎和变形而导致的絮凝体尺寸和形状的时间演变。较大的湍流剪切力和较弱的内聚力会产生较小的细长絮体。当流体和颗粒时间尺度平衡并且适当定义的斯托克斯数为$O(1)$时，絮凝进行得最快。在过渡阶段，中等强度的内聚力会产生最大尺寸的絮凝体，因为它们的强度足以引起聚集，但又不足以将絮凝体拉成紧凑的形状。小斯托克斯数和弱湍流延迟了平衡阶段的开始。在平衡过程中，更强的内聚力会产生更大尺寸的絮凝物。平衡絮凝物尺寸分布表现出取决于内聚数的优选尺寸。我们观察到，絮凝体在破碎前通常会因湍流应力而被拉长。尺寸接近柯尔莫哥洛夫长度尺度的絮体优先与中间应变方向和涡度矢量对齐。较小尺寸的絮凝体倾向于与拉伸应变方向对齐。更一般地，絮凝物与最强的拉格朗日拉伸方向对齐。科尔莫哥洛夫规模被认为限制了絮凝体的生长。我们提出了一种具有可变分形维数的新絮凝模型，可以预测絮凝体尺寸和形状的时间演变。
Abstract We investigate the dynamics of cohesive particles in homogeneous isotropic turbulence, based on one-way coupled simulations that include Stokes drag, lubrication, cohesive and direct contact forces. We observe a transient flocculation phase, followed by a statistically steady equilibrium phase. We analyse the temporal evolution of floc size and shape due to aggregation, breakage and deformation. Larger turbulent shear and weaker cohesive forces yield smaller elongated flocs. Flocculation proceeds most rapidly when the fluid and particle time scales are balanced and a suitably defined Stokes number is $O(1)$. During the transient stage, cohesive forces of intermediate strength produce flocs of the largest size, as they are strong enough to cause aggregation, but not so strong as to pull the floc into a compact shape. Small Stokes numbers and weak turbulence delay the onset of the equilibrium stage. During equilibrium, stronger cohesive forces yield flocs of larger size. The equilibrium floc size distribution exhibits a preferred size that depends on the cohesive number. We observe that flocs are generally elongated by turbulent stresses before breakage. Flocs of size close to the Kolmogorov length scale preferentially align themselves with the intermediate strain direction and the vorticity vector. Flocs of smaller size tend to align themselves with the extensional strain direction. More generally, flocs are aligned with the strongest Lagrangian stretching direction. The Kolmogorov scale is seen to limit floc growth. We propose a new flocculation model with a variable fractal dimension that predicts the temporal evolution of the floc size and shape.