Coherent vortex structures in deforming granular materials

Discrete element models are used to investigate the formation of coherent structures within a deforming granular material. The numerical models predict the formation of coherent vortex-like structures, even when the boundary deformations introduce zero vorticity. We name these structures circulation cells because the particles instantaneously translate and rotate as a rigid body about a common centre. They occur for all the particle shapes and material properties tested. The size of these coherent structures range from approximately 20 to 600 particles, with the largest structures being limited by the test boundaries. Circulation cells are seen to play an important role in granular deformations including the formation of shear bands. (C) 1997 by John Wiley & Sons, Ltd.