Development of a rheological model for cohesive granular materials across dense and dilute flow regimes

Introduction
Cohesive granular materials are ubiquitous in nature and industrial processes, and exhibit behaviors unseen in non-cohesive materials, including jamming at low packing fractions and agglomerate formation. To describe their flow behavior through a continuum model, one must first formulate a model for particle stress. Based on Discrete Element Method (DEM) simulation results, stress models for cohesive granular materials have been developed for the dense flow regime where the solid volume fraction is high (larger than ~0.4) [1-2]. For the dilute flow regime, many studies [1, 3-6] have found that inhomogeneous flow structures would emerge, rendering model development difficult. Thus, there is a need (1) to investigate the mechanism that leads to inhomogeneous flow structures at low dilute flow regime, and (2) to develop a comprehensive stress model for cohesive granular materials that covers both dense and dilute flow regimes. This talk will address both issues.

Methodology
To study the aforementioned questions, we perform CFD (Computational Fluid Dynamics) - DEM simulations for gas-fluidization of frictional, cohesive particles in a fully periodic domain. By analyzing snapshots gathered from the simulations, quantities of interest in formulating a rheological model are determined. It is found that, unlike previous studies, this approach allows for examination of stresses at both dense and dilute flow regimes.

Results and Discussions
For dense flow regime, it is found that stress results consistent with previous studies [1-2] are obtained, thus validating the approach. For dilute flow regime, it is found that pressure follows a behavior similar to van der Waals equation of state. Specifically, at low granular temperatures, pressure decreases with increasing solid volume fraction for a range of solid volume fractions, which is consistent with a previous analysis by Takada et al. [5]. This behavior explains the formation of inhomogeneous flow structures. We then propose a pressure model for this dilute flow regime. In the end, we bridge the models for both dense and dilute flow regimes to form a comprehensive pressure model for cohesive granular materials. For further studies, we seek to develop an analogous model for particle shear stress.