SEGREGATION OF PARTICULATE MATERIALS IN MODEL SHEAR FLOWS

摘要

A longstanding goal of controlling unwanted particle segregation is to uncover which of many competing segregation mechanisms dominate a particular flow regime. To achieve this, it is important to identify underlying flow induced instabilities that can spontaneously lead to inhomogeneities for a single component material. These can dramatically influence microscopic and macroscopic properties through development of high density structures, granular temperature and velocity gradients and sustained vorticity. Thus higher-order segregation of unlike particles may be exacerbated or masked. We numerically examine underlying flow patterns through molecular-dynamic simulations of cohesionless mixtures in high-shear Couette geometry, and support our findings through experimental PIV characterization. By incorporating particle size distributions, we observe significant segregation in these model flows, and relate measured segregation rates and length scales to microscopic mixing processes and local properties. Results are compared to those obtained by extension of kinetic theory for binary materials in bounded shear flows. We find that there exists a critical transition solids fraction at which segregation is reversed. Similar clustering patterns as seen in the MD simulations can be obtained in the lab, and a segregation pattern consistent with large particles accumulating in low granular temperature regions was observed. This suggests Couette flow is suitable for quantifying segregation for a wide variety of materials at different mean densities.