Transient Behavior of Granular Material

Citation

Lin, Han-Hsin (2023) Transient Behavior of Granular Material. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/jjrv-x616. https://resolver.caltech.edu/CaltechTHESIS:06012023-222631854

Abstract

This PhD thesis focuses on the flows on granular materials, such as sand, glass beads, and powders, which are sheared at low speeds with gravity perpendicular to the flow direction. The study is conducted using a combination of experiments, simulations, and theory, with the goal of developing a unifying theory of granular materials that can be described by continuum models. The main objective is to understand how microscale physics propagate to macroscale phenomena and to address issues related to setting boundary conditions and predicting timescales from unsteady to steady states. This research primarily aims to investigate stress variations in granular materials as a function of shear rate, encompassing both steady and unsteady states. Additionally, the thesis examines the phenomena of wall force anomalies and vortex flows. In Couette cell experiments and vertical plane shear simulations, granular material demonstrates a downward flow near the vertical shearing wall and an upward flow adjacent to another static vertical wall. Interestingly, this vortex flow causes a change in the direction of vertical shear stress when wall shearing commences, contradicting the prevalent assumption that particles consistently apply a downward force on the vertical wall.

The study concludes with key findings, including the observation that normal and shear stresses on the shearing wall increase slowly after the initiation of shearing, and that steady-state values for these stresses are independent of the shearing speed within a certain range. The study also found that the height of particles near the shearing wall decreases gradually with the presence of vortex flow, and that the shear rate near the moving wall is initially high and decreases slowly to reach a steady state. Additionally, we used a non-local constitutive model and Boussinesq approximation to predict the downward flow that is driven by gravity and variations in the solid fraction near the shearing surface, as well as the decay profile of velocity in an infinitely wide box for the steady state.

Overall, this thesis contributes to our understanding of granular materials in the slow flow regime, providing insights into their behavior under shear. The non-local model accurately predicts the downward flow and velocity decay profile, indicating its potential as a valuable tool for future research.

Thesis Files