Rheology of dense sheared granular mixtures: computational and experimental studies on the effects of particle size distributions

Proposal Number: 0932735

Principal Investigator: Hill, Kimberly

Affiliation: University of Minnesota

The central research goal is to determine the governing features of the rheology of dense granular mixtures. Recent advances in rheological models for monodisperse dense granular materials do not have a mechanism to incorporate the effect of local particle size distributions on the rheology of mixtures. The rheological properties are dependent on species concentration, further complicated by the fact that particulate mixtures tend to unmix. Typically, expressions for the rheology of dense granular flows have an explicit dependence on particle size, though the representative particle size for a mixture is not obvious. In many cases average particle size is not sufficient for predicting general rheological trends. This research will address the question of the dependence of rheology of granular mixtures on particle size distribution. Computational simulations will be performed using the Distinct Element Method (DEM) for soft spheres. Boundary conditions will include free surface gravity-driven flows and constant pressure and constant volume boundary-driven shear cells to investigate the importance of associated boundary effects. Experiments will include Couette shear flow and free-surface gravity-driven flows in small and large rotating drums. Boundary force and digital image analysis will validate many computations. Particle size distributions will include binary mixtures of distinct sizes and concentrations, tertiary mixtures, and, ultimately, multiple sizes. Particle size distribution will be investigated, including the effect of mean, median and standard deviations of the particle sizes. Other considerations are the average Veronoi volume fractions and probability distribution functions of interparticle forces as they depend on particle size distributions and flow conditions. Better understanding of dense particulate mixtures is applicable to powder processing and mixing, prediction and mitigation of debris flows, and environmental restoration of rivers and streams. Including undergraduates from underrepresented groups in research will better train future civil engineers for developing longer-term successful solutions to relevant engineering problems. Related teaching materials will be developed for K-12 teachers and their students at the National Center for Earth-surface Dynamics' (NCED) gidakiimanaaniwigamig (Our Earth Lodge) summer camps and after-school programming at the Fond du Lac Tribal and Community College near Duluth, Minnesota.