It is well known that heterogeneous granular flows exhibit collisional, dense and creep regimes that can coexist in space. How to correctly predict and control such complex phenomena has many applications in both mitigation of natural hazards and optimization of industrial processes. However, it still remains a challenge to establish a predictive granular rheology model due to the lack of understanding of the internal structure variation across different regimes and its interaction with the boundary. In this work, we use DEM simulations to investigate the internal structure of heterogeneous granular flow developed at the center of rotating drum systems. By systematically varying the side wall conditions, we are able to generate various heterogeneous flow fields under different levels of boundary effects. Our extensive simulation results reveal a highly relevant micro-structural quantity \(\delta \theta = |\theta _c - \theta _f|\), where \(\theta _c\) and \(\theta _f\) are the preferred direction of inter-particle contacts and the preferred direction of inter-particle force transmissions, respectively. We show that \(\delta \theta \) can characterize the internal structure of granular flow in collisional, dense and creep regimes, and its variation can identify the transition between them. In particular, in dense and collisional regimes, the classical rheological relation between bulk friction \(\mu \) and inertia number I holds, while in the creep regime, such relation breaks down and \(\mu \) instead depends on \(\delta \theta \). Our findings hold for all investigated flow fields regardless of the level of boundary effect imposed, and regardless of the amount of shear experienced. \(\delta \theta \) thus provides a unified micro-structural characterization for heterogeneous granular flow in different regimes, and lays the foundation of establishing microstructure-informed granular rheology models.

中文翻译：

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识别异质颗粒流中的空间过渡

众所周知，异质颗粒流表现出可以在空间中共存的碰撞，稠密和蠕变状态。如何正确地预测和控制这种复杂现象在减轻自然灾害和优化工业过程中都有许多应用。然而，由于缺乏对跨不同状态的内部结构变化及其与边界的相互作用的了解，建立预测的颗粒流变学模型仍然是一个挑战。在这项工作中，我们使用DEM模拟来研究在转鼓系统中心产生的非均质颗粒流的内部结构。通过系统地改变侧壁条件，我们能够在不同水平的边界效应下产生各种异质流场。\（\ delta \ theta = | \ theta _c-\ theta _f | \），其中\（\ theta _c \）和\（\ theta _f \）是粒子间接触的首选方向和粒子间接触的首选方向-粒子力传递。我们证明\（\ delta \ theta \）可以描述碰撞，稠密和蠕变状态下颗粒流的内部结构，并且其变化可以识别它们之间的过渡。特别是在稠密和碰撞状态下，体摩擦\（\ mu \）与惯性数I之间保持着经典的流变关系，而在蠕变状态下，这种关系破裂了，而\（\ mu \）则取决于\（ \ delta \ theta \）。我们的发现适用于所有调查的流场，而不管施加的边界效应水平如何，也不受剪切力的大小影响。因此，（\ delta \ theta \）为不同状态下的非均质颗粒流提供了统一的微观结构表征，并为建立具有微观结构信息的颗粒流变模型奠定了基础。