雲層中的冰晶粒子是構成雲和降水的重要要素，其運動和碰撞行為對雲層發展和降水形成至關重要。此外，冰晶具有較高的太陽輻射反射率，能調節地表能量平衡並影響氣候系統。在全球氣候變遷逐漸趨向極端化的當下，深入研究冰晶在大氣中的微物理過程對理解天氣模式和氣候變化至關重要。
本研究的目的是探索微小冰晶粒子在紊流流場中與大冰晶碰撞的物理機制與關鍵影響因素。透過數值模擬方法，使用COMSOL Multiphysics®軟體建立二維瞬態流場模型，重現大冰晶周圍的流場特徵，並採用拉格朗日描述法分析小冰晶的運動軌跡。模擬中採用雷諾平均納維-斯托克斯(RANS)方程結合k−ω紊流模型，以模擬紊流流場的特性。同時，加入粒子追蹤功能以詳細探討小冰晶在紊流背景中的運動與碰撞行為，並假設粒子達到終端速度。
研究結果顯示，紊流中形成的渦流對粒子的運動與碰撞效率具有顯著影響。特別是大冰晶周圍流場的波動，與渦流結構的交互作用，對粒子動態行為起著決定性作用。此外，對冰晶粒子初始條件的變化進行分析表明，粒子的慣性以及流場本身的變化對其運動軌跡和碰撞結果存在明顯影響，尤其是渦流的誘發效應更為顯著。

This study focuses on the behavior of minuscule ice crystals in atmospheric flows, focusing on their trajectories and collision efficiency when interacting with larger ice crystals. Using two-dimensional transient flow simulations in COMSOL Multiphysics®, the research examines the flow field around a large ice crystal (10 mm in diameter) and the dynamic movement of smaller ice particles within this field. The particle motions are analyzed using the Lagrangian description.
To effectively simulate turbulent atmospheric conditions, the Reynolds-Averaged Navier-Stokes (RANS) equations, combined with the k−ω turbulence model, are employed to describe the background flow field. Additionally, a fluid particle tracing module (FPT) is incorporated to explore the movement and potential collisions of smaller ice particles relative to the primary ice crystal, with the assumption that each particle reaches its terminal velocity.
The results reveal that vortex shedding within turbulent flow fields significantly impacts the trajectories and collision behaviors of smaller ice crystals around the large ice crystal. Variations in the surrounding flow field, coupled with these vortices, play a critical role in shaping particle dynamics. Furthermore, the study investigates the effects of particle size and initial position, highlighting that particle inertia and flow field fluctuations—particularly those induced by vortex shedding—exert a pronounced influence on particle motion. These findings enhance our understanding of the physical mechanisms underlying ice crystal interactions in atmospheric systems.

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