Direct numerical simulations are performed to investigate the multiscale flow physics of binary droplet collision over a wide range of Weber numbers and impact factors. All possible collision outcomes, including bouncing (both head-on and off-center), coalescence, reflexive separation, and stretching separation, are considered. The theoretical formulation is based on a complete set of conservation equations for both the liquid and gas phases. An improved volume-of-fluid technique, which is augmented by an adaptive mesh refinement algorithm, is used to track the liquid/gas interface. Several local refinement criteria are validated and employed to improve the computational accuracy and efficiency substantially. In particular, a thickness-based refinement technique is implemented for treating cases involving extremely thin gas films between droplets. The smallest numerical grid is ∼10 nm, which is on the order of 10⁻⁵ times the initial droplet diameter. A photorealistic visualization technique is employed to gain direct insights into the detailed collision dynamics, including both the shape evolution and mass relocation. The numerical framework allows us to systematically investigate the underlying mechanisms and processes, such as gas-film drainage and energy and mass transfer, at scales sufficient to resolve the near-field dynamics during droplet collision. The nonmonotonic transition of bouncing and merging outcomes for head-on collision is identified by varying the Weber number over two orders of magnitude. A geometric relation defining the droplet interactions is developed. Analytical models are also established to predict the mass transfer between colliding droplets.

中文翻译：

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二元液滴碰撞多尺度流物理学的直接数值模拟

进行直接数值模拟以研究在广泛的韦伯数和影响因子范围内二元液滴碰撞的多尺度流物理学。考虑所有可能的碰撞结果，包括弹跳（正面和偏心），合并，反射性分离和拉伸分离。理论公式基于一套完整的液相和气相守恒方程。一种改进的流体体积技术（通过自适应网格细化算法进行了增强）用于跟踪液/气界面。验证并采用了几种局部改进标准，以显着提高计算精度和效率。特别地，实施基于厚度的细化技术以处理涉及液滴之间极薄的气体膜的情况。初始液滴直径的^-5倍。采用逼真的可视化技术来直接了解详细的碰撞动力学，包括形状演变和质量重定位。数值框架使我们能够以足以解决液滴碰撞过程中近场动力学的尺度系统地研究潜在的机制和过程，例如气膜排水以及能量和质量传递。通过在两个数量级上改变韦伯数，可以确定正面碰撞的弹跳和合并结果的非单调过渡。建立了定义液滴相互作用的几何关系。还建立了分析模型以预测碰撞液滴之间的质量转移。