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Microscopic origins of shear stress in dense fluid-grain mixtures

机译：致密流粒混合物中剪切应力的微观起源

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摘要

A numerical model is used to simulate rheometer experiments at constant normal stress on dense suspensions of spheres. The complete model includes sphere-sphere contacts using a soft contact approach, short range hydrodynamic interactions defined by frame-invariant expressions of forces and torques in the lubrication approximation, and drag forces resulting from the poromechanical coupling computed with the DEM-PFV technique. Series of simulations in which some of the coupling terms are neglected highlight the role of the poromechanical coupling in the transient regimes. They also reveal that the shear component of the lubrication forces, though frequently neglected in the literature, has a dominant effect in the volume changes. On the other hand, the effects of lubrication torques are much less significant. The bulk shear stress is decomposed into contact stress and hydrodynamic stress terms whose dependency on a dimensionless shear rate-the so called viscous number I-v-are examined. Both contributions are increasing functions of I-v, contacts contribution dominates at low viscous number (I-v < 0.15) whereas lubrication contributions are dominant for I-v > 0.15, consistently with a phenomenological law infered by other authors. Statistics of microstructural variables highlight a complex interplay between solid contacts and hydrodynamic interactions. In contrast with a popular idea, the results suggest that lubrication may not necessarily reduce the contribution of contact forces to the bulk shear stress. The proposed model is general and applies directly to sheared immersed granular media in which pore pressure feedback plays a key role (triggering of avalanches, liquefaction).
机译：数值模型用于模拟在球体的密集悬浮液上恒定法向应力下的流变仪实验。完整的模型包括使用软接触方法的球-球接触，由润滑近似中的力和扭矩的框架不变表达式定义的短程流体动力相互作用，以及由DEM-PFV技术计算出的磁机械耦合所产生的阻力。忽略了某些耦合项的一系列模拟突显了磁机械耦合在瞬态状态中的作用。他们还揭示了润滑力的剪切分量，尽管在文献中经常被忽略，但在体积变化中起主要作用。另一方面，润滑扭矩的影响要小得多。整体剪切应力被分解为接触应力和流体动力应力两个术语，它们都依赖于无量纲剪切速率（即所谓的粘性值I-v）的依赖关系。两种贡献都是I-v的增加函数，接触贡献在低粘度值下占主导地位（I-v <0.15），而润滑贡献在I-v> 0.15时占主导地位，这与其他作者推断的现象学规律一致。微观结构变量的统计数据突出了固体接触与流体动力相互作用之间的复杂相互作用。与流行的想法相反，结果表明润滑不一定会减少接触力对整体剪切应力的影响。所提出的模型是通用的，并且直接应用于剪切沉浸颗粒介质中，其中孔隙压力反馈起着关键作用（雪崩触发，液化）。

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