Particle-based simulation of Aeolian transport: Granular electrification controls on the initiation of particle motion

本研究方案的总体目标是开发一种离散元素法(DEM)，用于以粒子为基础模拟风沙输送和尘埃气溶胶粒子向大气的串联排放。这一模拟将有助于改进沙尘循环在气候模型中的表现，并对包括沿海管理、行星科学和荒漠化研究在内的广泛应用具有根本意义。在这个项目的第一阶段，我们实现了一个数值工具，通过考虑颗粒间的接触和van der Waals力，将大气边界层中湍流风场的流体力学模型与颗粒动力学耦合起来。在该项目的下一阶段，该DEM模拟将扩展到包括接触带电(摩擦带电)过程产生的颗粒-颗粒静电相互作用的影响。沙尘暴伴随着几千伏/米到几百千伏/米的电场，这种电场可以显著降低风沙土壤颗粒(Ut)的直接气动卷吸的最小风切变速度，从而大大增强沙粒的输送、沙丘的迁移和粉尘的释放。然而，在存在电荷的情况下预测Ut是具有挑战性的，因为人们仍然很少了解土壤中颗粒之间的静电相互作用如何影响风沙沉积床的微观(颗粒尺度)动力学。这些微观动力学及其对UT的作用将构成下一个项目阶段的研究主题，使用我们的DEM模拟。最近在杜伊斯堡-埃森大学物理系进行了风洞实验，以估计颗粒大小和电荷的特定分布的Ut。这些实验的数据将有助于我们的模型的验证。这里提出的研究将为颗粒材料中摩擦带电的DEM表示铺平道路--这是一种与环境和工程科学的许多领域基本相关的现象。

The overall goal of the present research programme is the development of a Discrete-Element-Method (DEM) for the particle-based simulation of Aeolian sand transport and the concatenated emission of dust aerosol particles to the atmosphere. This simulation will contribute to improving the representation of the dust cycle in climate models and is of fundamental relevance to a broad range of applications, including the coastal management, the planetary sciences and the desertification research. In the first phase of this project, we achieved a numerical tool that couples a fluid mechanics model for the turbulent wind field in the atmospheric boundary layer with granular dynamics, by considering interparticle contact and van der Waals forces. In the next phase of the project, this DEM simulation will be extended to incorporate the effect of particle-particle electrostatic interactions resulting from contact electrification (tribocharging) processes. Sand and dust storms are accompanied by electric fields of a few kV/m to several hundred kV/m. Such E-fields may significantly reduce the minimal threshold wind shear velocity for direct aerodynamic entrainment of Aeolian soil particles (Ut), thereby contributing to enhance sand transport, dune migration and dust emission rates substantially. However, predicting Ut in the presence of electric charges is challenging because it is still poorly understood how electrostatic interactions between grains in the soil affect the microscopic (particle-scale) dynamics of Aeolian sediment beds. These microscopic dynamics and their role for Ut will constitute the topic of research in the next project phase, using our DEM simulations. Wind tunnel experiments have been conducted recently at the Faculty of Physics of the University of Duisburg-Essen to estimate Ut for specific distributions of particle size and electric charges. The data from these experiments will be helpful, thus, for the validation of our model. The research proposed here will pave the way for a DEM representation of tribocharging in granular materials – a phenomenon of fundamental relevance to many areas of environmental and engineering sciences.