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Quantitative analysis and statistical mechanics of granular pack structures

机译:颗粒包装结构的定量分析和统计力学

摘要

Disordered granular packs present many challenges over regular structures in description, analysis and modelling. Within the granular statistical mechanics, which aims to compute bulk properties from details of the local structure, the quadron description was defined over a decade ago. This quantitative characterisation of disordered systems enumerates the structural degrees of freedom, by dividing the system into tessellating volume elements, quadrons, and assigning each a quantitative structure tensor computed from its shape. Presented in this work is a theoretical derivation of an equipartition principle of the volume, which is the analogue of the well known 3kT/2 in thermal physics. This computation is included in full, for two-dimensional systems, using the quadron description. Following this, quadron tensors and statistics in three dimensional disordered structures are computed and analysed for the first time. A new computer program to achieve this was developed, requiring a new class of solution for cell/pore identification, a custom 3D rendering engine, and several extensions to the quadron description as originally defined. The program was successful in analysing structures from simulations, experimental colloidal suspensions and granular systems of a few thousand grains. By a subsystem-based parallelisation method, much larger data-sets were successfully analysed, including an X-ray tomography experiment with 60,000 non-spherical grains. The quadron statistics of a triaxial shear experiment were computed at multiple stages, and the chirality, a pseudo-vector computed from the structure tensor which measures the deviation from orthogonality of the volume element boundary, develops an anisotropy during the formation of a shear band. While the software performed very well on dense granular packs, its performance in less dense systems or regions, like a shear band, was both slower and less reliable. This was traced to the increased ambiguity in cell structure as the density decreases, and is something that future analysis could improve.
机译:无序的粒状包装在描述,分析和建模方面比常规结构提出了许多挑战。在旨在根据局部结构的详细信息计算体积特性的粒度统计机制中,对四边形的描述是在十年前定义的。通过将系统划分为细分的体积元素,四边形,并为每一个分配由其形状计算出的定量结构张量,无序系统的这种定量表征可以列举结构的自由度。这项工作提出的是体积均分原理的理论推导,它是热物理学中众所周知的3kT / 2的类似物。使用四边形描述,对于二维系统,此计算完整包含在内。此后,首次计算和分析了三维无序结构中的四元张量和统计量。为此,开发了一种新的计算机程序,它需要一种用于细胞/孔识别的新型解决方案,一个定制的3D渲染引擎以及对最初定义的四边形描述的一些扩展。该程序成功地通过模拟,实验性胶体悬浮液和几千个颗粒的颗粒系统分析了结构。通过基于子系统的并行化方法,成功分析了更大的数据集,包括使用60,000个非球形晶粒的X射线断层扫描实验。三轴剪切实验的四边形统计量是在多个阶段进行计算的,而手性是从结构张量计算出的伪矢量,该伪矢量测量了体积元边界的正交性,并在剪切带形成过程中产生了各向异性。尽管该软件在稠密的颗粒状包装上表现出色,但在密度较小的系统或区域(如剪切带)中的性能却较慢且可靠性较低。可以追溯到随着密度的降低,细胞结构的歧义性增加,这是未来分析可以改进的地方。

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