[论文解读] High Fidelity Simulations of Micro-Bubble Shedding from Retracting Thin Gas Films in the Context of Liquid-Liquid Impact

Shahab Mirjalili, Wai Hong Ronald Chan+1 more|arXiv (Cornell University)|Nov 29, 2018

Fluid Dynamics and Heat Transfer参考文献 26被引用 8

一句话总结

本研究采用高保真度的二维和三维扩散界面模拟，研究液体-液体撞击过程中收缩薄气膜的微气泡脱落现象，揭示了三维毛细不稳定性（类似于Rayleigh-Plateau）是气泡形成的主要驱动力。关键发现为：微气泡直径与膜厚呈亚线性关系，在Oh = 0.072时，气泡直径约为膜厚的15倍；而粘性效应会延迟脱落过程，导致Oh数较高时产生更大的气泡。

ABSTRACT

Micro-bubbles are of significant interest due to the long-living signature they leave behind naval ships. In order to numerically model and predict these bubbles in naval applications, subgrid-scale models are required because of the extreme separation of length- and time-scales between the macroscopic geometries and the physical processes leading to the formation of these bubbles. Yet, there is much that is unknown about the mechanism behind the entrainment of such bubbles. Furthermore, quantitative information regarding their size distribution and dependence on flow parameters is very limited. Impact events are hypothesized to be the main contributor to the generation of micro-bubbles. This is due to a phenomenon known as Mesler entrainment, which has been observed in the context of the drop-pool impact problem. Namely, when a water droplet with diameter of $\mathcal{O}(1mm)$ impacts a deep water pool with an impact velocity of $\mathcal{O}(1m/s)$, hundreds of air micro-bubbles are entrained into the pool. These bubbles have been found to be remnants of a very thin air film entrapped between the two liquid bodies. These films have extremely high aspect ratios and after being punctured, shed micro-bubbles while retracting on time scales much shorter than the outer flow time scales. This separation of time-scales, along with a lacking of studies on retracting thin gas films has motivated us to study this fundamental problem with numerical simulations in two and three dimensions. Using a diffuse interface method, we perform two-phase simulations of retracting thin gas films in initially static liquid backgrounds to gain understanding regarding this problem and gather statistics that can be potentially used in a subgrid-scale model to predict micro-bubble shedding from a thin gas film. Coupling a model like this to a model that predicts the thin film characteristics ...

研究动机与目标

• 为了理解液体-液体撞击中收缩薄气膜形成微气泡的基本机制，这是海军尾流中持久性微气泡的关键来源。
• 量化微气泡尺寸与脱落动力学对膜厚及粘度的依赖关系，以Ohnesorge数作为控制参数。
• 阐明三维不稳定性在实现微气泡脱落中的作用，因为二维模拟中未观察到气泡形成。
• 为大规模海军流动模拟中微气泡卷吸的亚网格尺度建模提供数据与尺度律。

提出的方法

• 采用高保真度的二维和三维扩散界面模拟，对水中的薄空气膜收缩过程进行建模，捕捉纳米尺度分辨率下的毛细驱动动力学。
• 模拟中Ohnesorge数（Oh）从0.072变化至0.32，对应膜厚从100 nm至2 μm，以研究粘性效应对收缩和不稳定性的影响。
• 推导出二维收缩前缘的相似性解，显示随时间变化的收缩速度及自相似的边缘演化。
• 应用毛细不稳定性分析，解释收缩边缘上沿展向的波纹形变，进而导致膜破裂与微气泡释放。
• 通过追踪膜体积与时间估算微气泡直径的下限值，并与实验数据进行验证。
• 正在从一系列Oh数范围内的三维模拟中提取微气泡尺寸与脱落时间的尺度律。

实验结果

研究问题

• RQ1 在液体-液体撞击中，薄收缩气膜为何会释放微气泡？为何二维模拟无法产生气泡？
• RQ2 Ohnesorge数如何影响薄气膜的收缩速度、边缘形态及气泡尺寸？
• RQ3 粘性力在多大程度上延迟或抑制薄气膜收缩边缘的毛细不稳定性？
• RQ4 是否可由一种三维毛细不稳定性机制（类似于Rayleigh-Plateau）解释从薄气膜中观测到的微气泡形成？
• RQ5 微气泡直径与脱落时间的尺度律如何随膜厚与粘度变化？

主要发现

• 薄气膜的微气泡脱落由三维毛细不稳定性驱动，而非二维动力学，这一点由二维模拟中未观察到气泡得到证实。
• 薄气膜的收缩速度随时间减小，且二维边缘通过速度的幂律衰减实现自相似演化。
• 当Oh = 0.072时，微气泡直径约为膜厚的15倍，因此对于800 nm厚的膜，最小气泡尺寸约为15 μm。
• 在较高的Ohnesorge数（如Oh = 0.113）下，粘性力抑制毛细波并延迟孔洞形成，导致相对于膜厚而言产生更大的微气泡。
• 模拟结果表明，微气泡直径与膜厚呈亚线性关系，与实验观测结果一致（10–100 μm气泡来自厚度仅为100 nm的膜）。
• 正在进行的三维模拟旨在提取可用于海军流动模拟中微气泡卷吸亚网格尺度模型的定量尺度律。