Density comparison for Spot Model and DEM
Update (October 2008): the software used to carry out the Voronoi tessellations used in making these plots is now available as an open source library – see the Voro++ website.
DEM Simulation
Spot Model
These two sequences of images track small changes in local density for the Spot Model and DEM simulations. Local packing fraction is computed using the Voronoi cell method on a scale of several particle diameters. Before flow starts, the packing fraction is approximately 63% (colored cyan), and during flow, this drops to around 60% (colored dark blue) or 57% (colored yellow). Density decreases to 50% or lower are colored red and can be seen in the vicinity of the orifice.
While the Spot Model provides a very good match to DEM in many aspects, this sequence of plots highlights a discrepancy between the two simulations. We see that the two simulations agree on how fast the density decrease propagates through the packing. However, the current method of spot motion, based on simple random walks, predicts that the largest density decrease should be in the center of the flow, where the velocity is highest, but it appears that the largest density decrease is on either side of the center, in the regions of highest shear. This suggests the possibility of finding better, more physically realistic spot rules that take into account forces and shear at a mesoscopic scale.
In the future we plan to employ the Voronoi density algorithm to examine granular density changes in other situations, such as in shear or chute flows.