冲击起沙的概念模型

冲击起沙是最重要的起沙方式。鉴于Lu和Shao提出的体积模型的不足,通过分析沙床受到冲击后应力的传递规律,类比于电子的跃迁模型,提出了冲击起沙的跃迁模型来解决冲击起沙问题。该模型的基本思想是,沙粒冲击沙床时,能量会沿冲击点向周围粒子扩散,当某一粒子获得的能量大于跃出所需要的能量时,该粒子即从床面跃出;多余的能量将转化为该粒子的跃出初动能。冲击起沙问题由此变成了一个研究跃出能量的大小和能量传输规律的问题。根据该模型,基于能量传输的吸收系数法和传输方程近似法,给出了两维和三维情形下冲击起沙释放的粒子数、速度分布和冲击坑形状的表达式。两维情形下显示,冲击坑的大小与释放的粒子数和粒径大小正比,而且伴随着冲击坑的形成,可能会发生崩塌现象。吸收系数法显示,两维冲击坑随着吸收系数的增大是先增大,后反而出现了收缩。传输方程近似法显示,两维冲击坑的大小正比于kt。

Transition Model of Saltation Bombardment

Dust emission by saltation bombardment is the most important dust emission way.Lu H and Shao YP ever presented a wonderful model of volume removal to predict dust emission rate.In consideration of its deficiency,we,however,from another point of view,brings forward a transition model to predict dust emission.The transition model of dust emission is an analogy of electron transition,which results from a detailed analysis of the stress transferring process on sand bed.According to this transition model,a particle on the bed will be released if energy transferred to it from the impacting particle through the bed,is bigger than the "transition" energy,which is the least energy to eject a particle into air,and the remained energy of the ejected particle is converted to its initial kinetic energy.Thus the expressions of the number of ejected particles,their velocity distributions and the shapes of bombardment crater in two-dimension and three-dimension situations are then easily obtained based on the transition model.And the dust emission by saltation bombardment then becomes an issue to research into the transition energy and the energy distribution around the point where collision happens.In this paper we introduce two methods,namely,the method of absorption coefficient and the method of transport equation,to approximate the energy distribution.The images of the two-dimension craters by the two methods show that the crater is big(or small) as to big-sized(or small-sized) particles and a big(or small) number of ejected particles,and collapse may appear with the crater’s formation.And of the method of absorption coefficient,the crater expands with the increase of absorption coefficient at the outset,but shrinks when absorption coefficient is bigger than a certain value.Of the method of transport equation,the crater always seems to expand with kt,an important parameter of this method.