Published online by Cambridge University Press: 15 June 2009
Stratification patterns are formed when a bidisperse mixture of large roughgrains and smaller more mobile particles is poured between parallel plates toform a heap. At low flow rates discrete avalanches flow down the free surfaceand are brought to rest by the propagation of shock waves. Experiments performedin this paper show that the larger particles are segregated to the top of theavalanche, where the velocity is greatest, and are transported to the flowfront. Here the particles are overrun but may rise to the free surface again bysize segregation to create a recirculating coarse-grained front. Once the frontis established composite images show that there is a steady regime in which anyadditional large grains that reach the front are deposited. This flow istherefore analogous to finger formation in geophysical mass flows, where thelarger less mobile particles are shouldered aside to spontaneously form staticlateral levees rather than being removed by basal deposition in two dimensions.At the heart of all these phenomena is a dynamic feedback between the bulk flowand the evolving particle-size distribution within the avalanche. A fullycoupled theory for such segregation–mobility feedback effects is beyondthe scope of this paper. However, it is shown how to derive a simplifieduncoupled travelling-wave solution for the avalanche motion and reconstruct thebulk two-dimensional flow field using assumed velocity profiles through theavalanche depth. This allows a simple hyperbolic segregation theory to be usedto construct exact solutions for the particle concentration and for therecirculation within the bulk flow. Depending on the material composition andthe strength of the segregation and deposition, there are three types ofsolution. The coarse-particle front grows in length if more large particlesarrive than can be deposited. If there are fewer large grains and if thesegregation is strong enough, a breaking size-segregation wave forms at a uniqueposition behind the front. It consists of two expansion fans, two shocks and acentral ‘eye’ of constant concentration that are arranged in a‘lens-like’ structure. Coarse grains just behind the front arerecirculated, while those reaching the head are overrun and deposited. Upstreamof the wave, the size distribution resembles a small-particle‘sandwich’ with a raft of rapidly flowing large particles on topand a coarse deposited layer at the bottom, consistent with the experimentalobservations made here. If the segregation is weak, the central eye degenerates,and all the large particles are deposited without recirculation.
Movie 1. An animation showing how the stratification pattern shown in figure 1 is built up by the passage of two avalanches. Each avalanche has a coarse rich flow front and is strongly inversely graded behind, with large white sugar crystals on top of smaller more mobile iron spheres. The avalanches are brought to rest by a normal shock (Gray & Hutter 1997; Gray, Tai & Noelle 2003) and the stationary free-surface forms the new slope for the next avalanche to flow down. By placing a ruler along the initial slope of the pile it is possible to visualize the deposition of large particles as the coarse rich front flows past.