Dense granular flows, characterized by multiple contacts between grains, are common
in many industrial processes and natural events, such as debris flows. Understanding
the characteristics of these flows is crucial to predict quantities such as bedrock erosion
and distance traveled by debris flows. However, the rheological properties of these flows
are complicated due to wide particle size distribution and presence of interstitial fluids.
Models for dense sheared granular materials indicate that their rheological properties
depend on particle size, but the representative particle size for mixtures is not obvious.
Using the discrete element method (DEM) we study sheared granular binary mixtures
in a Couette cell to determine the relationship and rheological parameters such as stress
and effective coefficient of friction and particle size distribution. The results indicate
that the stress does not depend monotonically on the average particle size as it does
in models derived from simple dimensional consideration. The stress has an additional
dependence on a measure of the effective free volume per particle that is adapted from
an expression for packing of monosized particles near the jammed state. The effective
friction also has a complicated dependence on particle size distribution. For these systems
of relatively hard particles, these relationships are governed largely by the ratio
between average collision times and mean-free-path times. The characteristics of shallow
free surface flows, important for applications such as debris flows, are different from
confined systems. To address this, we also study shallow granular flows in a rotating
drum. The stress at the boundary, height profiles and segregation patterns from DEM
simulations are quantitatively similar to the results obtained from physical experiments
of shallow granular flows in rotating drums. Individual particle-bed impacts rather than
enduring contacts dominate the largest forces on the drum bed, which vary as the grain size squared and the 1.2 power of particle-bed impact velocity. In the presence of interstitial
fluids (water + fine particles) these characteristics might change significantly.
Modeling particle-particle and fluid-particle interaction in dense granular flows is still a
challenge. We propose a modification to the DEM to account for specific effects of the
interstitial fluid on the dynamics of certain granular fluid flows. The results from this
simple model are qualitatively similar to results from experiments.