Abstract
We study the Aeolian saltation transport problem by analysing the collision of incident energetic beads with granular packing. We investigate the collision process for the case where the incident bead and those from the packing have identical mechanical properties. We analyse the features of the consecutive collision process. We used a molecular dynamics method known as DEM (soft Discrete Element Method) with 20000 particles (2D). The grains were displayed randomly in a box (250X60). A few incident disks are launched with a constant velocity and angle with high random position to initiate the flow. A wind velocity profile is applied on the flowing zone of the saltation. The velocity profile is obtained by the calculi of the counter-flow due to the local packing fraction induced by the granular flow. We analyse the evolution of the upper surface of the disk packing. In the beginning, the saltation process can be seen as the classical “splash function” in which one bead hits a fully static dense packing. Then, the quasi-fluidized upper layer of the packing creates a completely different behaviour of the “animated splash function”. The dilation of the upper surface due to the previous collisions is responsible for a need of less input energy for launching new ejected disks. This phenomenon permits to maintain a constant granular flow with a “small” wind velocity on the surface of the disk bed.
Highlights
Drifting sand is a serious threat to the life of many people and may seriously disturb infrastructure and technical installations
The velocity profile is obtained by the calculi of the counter-flow due to the local packing fraction induced by the granular flow
The problem and its boundaries are defined as shown in figure 1: - Perfectly lateral periodic conditions - The lower part of the packing ((3) in the figure) is defined as a dumping surface in which the two components of the particle velocity are dumped by an amount proportional to the particle altitude: i.e. the particles are less and less mobile as they are deeply located. - A bead becomes an ejecta if its centre moves, from its initial coordinate, more than one diameter in the vertical upward direction. - In order to limit the possible flights of moving disks, we have defined a horizontal upper limit like a perfect mirror for the moving disks. These conditions are biased compared to natural parabolic flights but the number of grains involved in this situation is always very small and avoiding infinite vertical dimension helps a lot in limiting the global time calculation. following some referee comments, incorporation of the results of the cut part of the upper parabolic flight can be made to mimic more precisely the grains acceleration due to the wind
Summary
Drifting sand is a serious threat to the life of many people and may seriously disturb infrastructure and technical installations. It is very important to understand the mechanism of saltation which is the primary way, in which sand grains are propelled by the wind along the surface in short hops [1, 2]. The saltating grains are very energetic and when impact a sand surface, they rebound and eject other particles from the sand bed. The ejected grains, called reptating grains, contribute to the augmentation of the sand flux. The main outcomes of our numerical studies follow. We make a short overview of the previous experimental and numerical results concerning the successive collision between incident beads and a granular packing. We describe briefly our mechanical model based on the Discrete Element Method and the special wind conditions used in this study. We present the main results available from this study
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