The effect of the granule velocity on the dilute-dense flow transition in granular system

The effect of granule velocity on the dilute-dense flow transition in granular system is studied both experimentally and by numerical simulations. Our experiments show that high granule velocity above the exit results in a decrease of critical exit size in dilute-dense flow transition if the inflow rate is fixed. When the granular flow becomes the dense flow, the effect of the granule velocity is counteracted by a high density region above the exit, and the outflow rate is almost irrelative with the granule velocity. Similar results are also obtained by molecular dynamics simulations for corresponding two-dimensional systems. With simulation calculations, the space distributions of the density and speed are also obtained. The distributions show that the density and the thickness of the granule accumulation above the exit vary with the incoming granule velocity, and the variation plays a key role in determining the final granular flow state.