Growing length scale in gravity-driven dense granular flows

We report on simulations of a two-dimensional, dense, bidisperse system of inelastic hard disks falling down a vertical tube under the influence of gravity. We examine the approach to jamming as the average flow of particles down the tube is slowed by making the outlet narrower. Defining coarse- grained velocity and stress fields, we find a length scale and a time scale can be extracted from two-point spatial and temporal correlations of these fields. Both length and time scales are found to grow as jamming is approached \footnote{ S. Tewari, B. Tithi, A. Ferguson and B. Chakraborty, arXiv:0806.2413}. In an ongoing effort to understand the origin of the growing length and time scales, we have been investigating velocity profiles and distributions, and we will report on these results.