Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Civil Engineering

Committee Chair/Advisor

Qiushi Chen

Committee Member

Ronald D. Andrus

Committee Member

M.Z. Naser

Committee Member

Yidong Xia


Granular materials, such as biomass feedstocks, agricultural grains, pharmaceutical pills, and geomaterials, are widespread in nature, industrial systems, and everyday life. Fundamentally, the bulk mechanical behavior of granular materials is governed by particle-level attributes such as particle morphology, surface roughness, and contact behavior. Among various numerical methods developed for modeling granular materials, the particle-based discrete element method (DEM) is particularly suited and effective in modeling the mechanical, flow, and failure behavior of granular materials.

Focusing on one specific type of granular material (i.e., biomass feedstocks), the main objective of this dissertation is to develop and validate novel DEM models that can effectively capture complex particle shapes and the history-dependent contact behavior of biomass particles. Revolving around the main objective, four studies have been conducted:

In the first study, the computed tomography-informed DEM models are proposed for modeling complex-shaped biomass particles, in which particle surface geometries are approximated by a polyhedral model and a sphero-polyhedral model. These models are applied to simulate compressibility tests of biomass particles, where the polyhedral model demonstrates convincingly better suitability than the sphero-polyhedron model. The polyhedral model is then applied in the simulation of the friction test. Remarkably, the polyhedral model is capable of predicting both the compressive and frictional behavior of the pine particles when evaluated against experimental data.

In the second study, a set of hysteretic nonlinear DEM contact models are developed and calibrated to capture the history-dependent and the strain-hardening behavior of granular biomass feedstocks. The developed models are applied to simulate axial compression tests of biomass pine particles. Results show that the proposed models can reproduce the bulk stress-strain profiles of the physical samples and that the predicted bulk compressibility and constrained modulus under repeated compression agree reasonably with the experimental data.

In the third study, the exponential form of the proposed hysteretic models is applied to granular hopper flow simulations. Numerical studies are conducted to predict the potential processing upsets and their relationships to hopper design parameters. A detailed analysis of the granular hopper flow has been provided in cross-validation of the experimental flow tests over wide ranges of the processing parameters of the hopper and material attributes of pine particles.

In the fourth study, the exponential form of the proposed hysteretic models is applied to simulate the quasi-static and dense flow along an inclined plane. The effect of irregular shapes is approximated by a motion (rolling) resistance model, and the impact of particle shapes on bulk flowability is then investigated. DEM studies have verified the strong influence of inter-particle motion resistance (equivalent to particle interlocking) as critical material attributes on determining the flowability in the dense flow regime.



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.