Regime analysis of the rheology of spherical and non-spherical particles

Abstract

In the early stages of granular rheology, the majority of analytical studies were based on granular assembly consisting of spherical particles. This was due to geometric simplicity and feasibility when calculating dynamic variables. Furthermore system limitation emerged as a problem when investigating more complex and realistic considerations. However, in the contemporary research field, with the steadily increasing ability to perform more complex computations and with available resources, attention has focused on non-spherical particles because of their deeper relevance to practical applications. In this work, a 3D shear cell model is developed based on the Discrete Element Method using the commercial software platform "PFC" to study non-spherical particles' flow characteristics. A comparison is made with those of spherical assemblies. Firstly, the simulation model of annular shear cell consisting of spherical particles is tested with PFC and this agreed well with previous results, thus justifying the use of this tool to analyse the nonspherical level. Then the effect of platen roughness is investigated on spherical particle assembly from the microdynamic perspective, in order to establish a correlation between platen roughness and granular flow dynamics. This is undertaken in terms of particle size that is used to construct the platens. It is found that linearity and non-linearity of gradient profile across several important parameters are distinguishing features affected by variations in platen texture. The externally applied load is the most important aspect that bridges studies where gravity is considered and yet often overlooked. This point is established through in-depth investigation of granular flow in presence and absence of gravity where comparison of an number of flow characteristics is presented. Following this, the effects of particle shape are microdynamically investigated with reference to aspect ratio of non-spherical (ellipsoidal) particles and compared with spherical particles. The following key properties - particle linear velocity, angular velocity, contact normal force, contact shear force, total contact force, total contact moment and porosity - are 4 analysed to explain the effect of variation of the above-mentioned geometric properties on each of these parameters. Then, macrodynamic analysis is performed in a comparative study between spherical particles and ellipsoidal particles of varying aspect ratios with focus on the variables that are important in general constitutive model such as velocity, density and stress tensors. Physics underlying the observation is discussed to highlight effect of particle aspect ratio. Finally and most importantly, regime transition of ellipsoidal particle assembly is contrasted with the findings for spherical particles. In this study, the techniques that are generally used to identify regime transition for granular rheology of spherical particles are tested on flow of non-spherical (ellipsoidal) particles of varying shapes (aspect ratios). This includes correlation between elastically scaled force, kinetically scaled force, coordination number, apparent coefficient of friction and porosity. Some observations are found to be similar and useful for non-spherical particles while others found not suitable for nonspherical particles.

Date of Award	2018
Original language	English