考虑颗粒间粘结力的雪粒流体起动机理研究

As one of the most important physical processes of wind-blown snow (sand) movement, the aerodynamic entrainment of particles is of great important to the induction and development of the particle flow. However, the mechanism of aerodynamic entrainment is still a controversial issue. Due to the special mechanical properties of snow and cohesive force among particles, the aerodynamic entrainment process of snow as well as its influence factors are much more complex than those of sand particles, and related researches are also rather lacking. In this research, based on the discrete element method that describes inter-particle interactions directly, snow contact model that includes inter-particle cohesive force is developed firstly. And snow bed that can characterize the mechanical properties of snow cover is constructed with the established contact model. Then, with the mechanical model that considers the inter-particle interactions, fluid drag force, and gravity of surface particles, the lift-off process of the static surface particle is reproduced, and the aerodynamic entrainment threshold as well as particle motion features are analyzed. Finally, wind tunnel experiments of the aerodynamic entrainment are designed and performed to confirm the numerical model, and mechanism of the aerodynamic entrainment is discussed based on the analysis of the whole physical process. This research is conductive to the further understanding of the aerodynamic entrainment of snow, and even the whole drifting snow process, which will also provide guidance for the prevention of snow disasters.

颗粒的流体起动过程作为风雪（沙）运动中的关键物理过程之一，对于颗粒流的初始诱发和后续发展至关重要。然而，流体起动的发生机理一直存在争议。由于雪粒的特殊力学性质以及颗粒间粘结力的作用，雪粒的流体起动过程及其影响因素相较于沙粒更加复杂，相关的研究也十分有限。本文基于直接描述颗粒间相互作用的离散单元法，首先开发包含颗粒间粘结力的雪粒接触模型，基于此构建能够表征雪层实际力学性能雪床面模型；其次，考虑表面雪粒受到的颗粒间作用力、流体拖曳力和重力，建立床面颗粒的受力模型，再现床面静止颗粒的整个起动过程，分析雪粒的流体起动临界值和颗粒运动特征；最后，设计并开展相应的流体起动风洞实验，验证数值模型的有效性，并基于整个物理过程的分析讨论雪粒的流体动力学起动机理。本研究有助于加深对于雪粒流体起动机理乃至整个风吹雪动力学过程的认识和理解，也可以为风雪灾害的防治提供指导。

风雪（沙）的跃移运动是典型的由流体起动初始诱发颗粒运动的过程。流体起动表征颗粒由流体直接拖曳诱使其脱离床面的过程。然而，流体起动过程再现在风吹雪（沙）的实验测量和模型预测研究中存在极大挑战，使其发生机理一直存在争议。本项目基于离散元法（DEM）直接模拟再现了地表颗粒的流体起动过程，并给出了完整的流体起动方案。取得了如下重要进展：（1）二次开发了考虑颗粒间粘结力的离散元接触模型，并基于此构建了接近实际特性的雪（沙）床面离散元模型；（2）建立了改进的地表雪（沙）粒受力模型，模拟再现了颗粒物的流体起动过程，发现流体起动临界摩阻风速随粘结力增加而显著增大；（3）定量分析了地表颗粒的响应特征和能量累积规律，证实了流体起动颗粒先滚动并与周围颗粒接触碰撞再起动的假说，澄清了雪（沙）粒的流体起动率随摩阻风速不是二次方而是线性增长的标度关系；（4）给出了流体起动颗粒的起跳合速度和起跳角概率密度分布函数（PDF）服从对数正态而侧向角服从正态分布且不受粘结力的影响，证实了假定颗粒被垂直自由抛出1粒径的不合理性。本项目研究不仅为颗粒物跃移运动的形成和演化提供可靠的途径，还可为风雪（沙）灾害的预测和防治提供指导。项目执行期间，发表SCI论文3篇，2021年入选“陕西省高校科协青年人才托举计划”项目。