浮力气泡对水平壁面的回弹动力学特性

黏性液体中的气泡浮升运动有趣而又复杂,而气泡与固壁边界的相互作用更是广泛存在于实际工程中.基于轴对称数值计算,模拟了浮力驱动下气泡在黏性液体中上升并与顶部水平固壁面碰撞、回弹的过程.采用考虑表面张力的不可压、变密度Navier-Stokes方程来描述气液两相流流动,并通过基于分级八叉树的有限体积法进行数值求解.为准确捕捉气泡在回弹过程中局部而迅速的拓扑变化,采用了动态自适应网格技术耦合流体体积法(volume of fluid,VOF)来重构气泡的形状. 从气泡对壁面的碰撞和回弹的基本现象入手,研究了伽利略数 Ga和接触速度$U_{a}$对气泡回弹动力学特性的影响, 分析了气泡碰撞过程中涡结构的变化.用回弹高度$H$、回弹周期$T$、长宽比{$A_{r}$}、浮升速度$U$、轴向位置$z$和回复系数$C_{r}$等参数来表征不同条件时气泡的运动和形状特性. 研究结果表明,气泡的回弹运动特性对 Ga十分敏感. Ga的增大可加剧气泡形变, 促进气泡的回弹运动, 增多回弹次数,增大回弹参数($T$和$H)$, 提升回复系数. 然而,接触速度并非决定气泡回弹动力学的控制参数, $U_{a}$的改变并不会改变回复系数. 

BOUNCING BEHAVIORS OF A BUOYANCY-DRIVEN BUBBLE ON A HORIZONTAL SOLID WALL 1)

It is not only interesting but also complex that buoyancy-driven bubbles rising in viscous liquids. In particular, the interactions between bubbles and boundaries (e.g., solid walls) is relevant in practical applications and these interactions may have a significant effect in the global behaviors of the multi-phase fluids. In this work, the rising, collision and bouncing to a horizontal solid wall for a single bubble are studied by axisymmetric computations. The incompressible, density-variable Navier-Stokes equations with surface tension are used to describe the gas-liquid flow and are solved by a tree-based finite volume method (FVM). The evolution of bubble shape is implemented by using a volume of fluid (VOF) approach that combines a balanced surface tension force calculation and a height-function curvature estimation. To finely resolve the local but fast topological evolutions of bubble, the technique of adaptive mesh refinement (AMR) is used. Starting with the basic phenomenon of bubble impacting and bouncing, we explore the effects of Galilei number Ga and approach velocity $U_{\rm a}$. To study the bubble behaviors under different conditions both qualitatively and quantitatively, the evolution of the velocity vector field and a lot of parameters such as bouncing height $H$, bouncing period $T,$rising velocity $U$, axial coordinate $z$and coefficient of restitution $C_{\rm r }$are analyzed. Based on the results, we find that the bubble bouncing behaviors are pretty sensitive to the Galilei number. The increase of Ga promotes the bouncing and signifies the deformation of bubbles, increasing the collisions of bubbles, bouncing parameters and the coefficient of restitution. However, the value of $C_{\rm r}$is nearly unaffected by the variation of the approach velocity, indicating that $U_{\rm a}$is not a governing parameter for the bubble bouncing motion.