液滴动力学耦合稀薄气体效应的多尺度计算与分析

Latest high-speed imaging experiments demonstrated that there might be a gas cushion layer between a droplet and a solid wall or between two colliding droplets. This gas cushion layer plays an important role in droplet dynamics. Conventional theoretical analyses and computations build on continuous medium hypothesis and neglect rarefied gas effect, and thus they cannot be applied to explain such kinds of experiments. In this project, we propose to develop multiscale computational platform based on OpenFOAM frame, and employ multiscale computation coupling direct simulation Monte Carlo method, molecular dynamics, and macroscopic computational fluid dynamics method to investigate droplet dynamics. Our aim is to achieve the following targets: for a droplet impact on a solid wall and two droplets collision, we aim to derive the key factors for determining the characteristics of droplet dynamics such as spreading, splashing, and bouncing, clarify the rarefied gas effect on droplet dynamics, and find out the optimal condition for smooth spreading; for a droplet migration on a solid wall, we aim to determine the dominant condition for either the thermal creep effect of gas or the Marangoni effect of liquid, and hence realize the control of the direction of droplet migration. The outcome of this project can provide theoretical guide for increasing the accuracy and efficiency of ink-jet printing and 3D printing. This project couples the convectional droplet dynamics based on continuous medium hypothesis with rarefied gas dynamics based on molecular kinetic theory. This coupling will benefit the development of cross discipline.

最近的高速成像实验表明，在液滴与壁面之间或者两个液滴之间，往往存在一层微纳米尺度的气膜，对液滴动力学行为产生重要影响。传统的理论研究和计算建立在连续介质假设之上，未考虑稀薄气体效应，无法解释此类实验现象。本项目拟发展基于OpenFOAM框架的计算平台，采用耦合直接模拟Monte Carlo方法、分子动力学和宏观计算流体力学方法的多尺度计算研究液滴动力学问题。预期达到以下目标：针对液滴撞击壁面和两个液滴碰撞问题，得到决定液滴动力学行为如铺展、飞溅或者反弹的关键因素，揭示稀薄气体效应的影响，找到液滴铺展的最优条件；针对液滴在壁面迁移问题，得到气体热迁移效应和液体Marangoni效应各自占优的条件，实现对液滴移动方向的控制。本项目的成果对提高喷墨打印和3D打印等的精度和效率可提供理论依据。本项目将传统的基于连续介质假设的液滴动力学与基于分子动理论的稀薄气体动力学有机结合，有助于学科交叉发展。

最近的实验研究表明，稀薄气体效应会显著影响液滴的动力学行为，该现象在时间和空间上的多尺度特性给实验观测带来了较大难度，针对该现象的数值计算存在巨大挑战。本项目围绕液滴撞击、润湿、结冰等动力学过程中的气体效应，在多尺度计算方法、气固相互作用建模、液滴动力学耦合气体效应的研究等方面取得了系统性的进展。针对液滴动力学耦合稀薄气体效应的多尺度计算需要，发展和完善了分子模拟方法，优化了耦合分子模拟和计算流体力学的多尺度方法，提出了基于分子模拟数据得到宏观模型的数据驱动建模新范式，分析了液滴接触角的修正杨方程和线张力，确定了稀薄条件下气固相互作用引起的滑移边界条件。在此基础上，研究了液滴-壁面间气体薄层对液滴撞击壁面动力学的影响，实现了对液滴润湿后封闭气膜动力学行为的精确仿真，并针对液滴撞击壁面和两个液滴碰撞问题，得到了决定液滴动力学行为的关键因素。本项目的结果对液滴动力学数值计算的发展以及喷墨打印等工程应用的精细控制具有重要意义。