Numerical Simulation Of Wind-Blown Sand Saltation

Saltation is the most important transfer and shift way of wind-blown sand. A great number of sand grains gather in the saltation layer, where the interaction between the grains is very complicated. So far we cannot figure out the accurate status of interaction forces, momentum and the energy exchange at the micro level. Most works on wind-blown sand flow use standard drag coefficient formula for single sphere particle to study sand movement in saltation, but failing to concern about the particle swarm effect when more and more grains move together. The natural sand grains from desert have various forms and different diameter sizes. When diameter size distribution changes, corresponding saltation changes too. Yet, most of aeolian physics research in literature focuses on field observation of wind blown sand and numerical simulation for saltation of sand grains with the uniform diameters. Therefore, the numerical study for nonuniform sand grains is very necessary. In order to study the compicated sand saltation in the real world and to provide some reference for further theoretical analysis and numerical research, this paper discusses the influence of grains swarm and several diameter size distributions to sand saltation. The main content includes:1. A modification of drag coefficient by grains'volume density is introduced to the original windblown sand model. Using the modified model, we calculate the mass flux, wind speed profile as well as grains' volume density in steady state of wind-blown sand movements. The numerical results display that both mass flux and grains'volume density calculated with modified model are larger than those calculated by standard model, but the wind speed is insensitive to this kind of modification. In addition, it turns out that when the friction velocity increases, mass flux and grains' volume density increases as well. This means that the effect of particle swarm on saltation is of importance and should be considered in further study.2. A computational method is employed to simulate saltation movements of sand grains with various diameter size distributions. In this paper, we analyzed seven kinds of saltation movements respectively, in case of particles with different particle size distributions. The results show that the air flow has more powerful transporting ability for nonuniform particles than that for uniform particles under the same wind conditions. Meanwhile, when sand saltation approches to steady state there exist different saltation heights and lengths including related saturated saltation layer thicknesses for different particle size distributions.