Unsteady overflow behavior of polydisperse granular flows against closed type barrier

Abstract

The overflow behavior of polydisperse granular flows against closed type barrier is a crucial yet poorly understood aspect of multiple-barrier design. We use a robust numerical model based on the three-dimensional discrete element method (DEM) to simulate polydisperse granular flows and identify fundamental mechanisms. The morphology of the dead zone behind the upstream barrier significantly influences the overflow behavior of granular flows. This implies that aside from the case of a gentler barrier (<70°), the launch direction is controlled by the inclination surface of the dead zone. We identified a three-stage evolution process of the overflow velocity. The velocity component in the z-direction controls the first stage, whereas the latter two stages are controlled by the velocity component in the x-direction. The launch angle is usually larger than 45° during the initial overflow stage. Such a high launch angle is maintained for about 0.2–0.3 s. Boulders are transported downstream by the overflow process owing to particle-size segregation and a more concentrated load is received by the downstream barrier, which can lead to localized barrier damage. A different overflow mechanism would result in a different launch distance. Steeper (135°) and gentler (70°) barrier configurations are dangerous because the normalized launch distance is up to 20% larger than that of a barrier with a medium inclination. And a point-mass based assumption is used to estimate the maximum launch distance with the use of maximum overflow velocity and a single launch angle. In addition, we discuss some limitations of numerical modelling, including the effect of fluid phase on overflow, as well as the direct application of our results in engineering design.