Shear instabilities in granular flows

Unstable waves have been long studied in fluid shear layers(1-3). These waves affect transport in the atmosphere and oceans, in addition to slipstream stability behind ships, aeroplanes and heat-transfer devices. Corresponding instabilities in granular flows have not been previously documented 4, despite the importance of these flows in geophysical and industrial systems(5-7). Here we report that breaking waves can form at the interface between two streams of identical grains flowing on an inclined plane downstream of a splitter plate. Changes in either the shear rate or the angle of incline cause such waves to appear abruptly. We analyse a granular flow model that agrees qualitatively with our experimental data; the model suggests that the waves result from competition between shear and extensional strains in the flowing granular bed. We propose a dimensionless shear number that governs the transition between steady and wavy flows.