What happens to the vortex structures when the rising bubble transits from zigzag to spiral?
What happens to the vortex structures when the rising bubble transits from zigzag to spiral?
复制标题
当上升的气泡从之字形转变为螺旋形时，涡旋结构会发生什么？
DOI：
10.1017/jfm.2017.514
复制
发表时间：
2017
影响因子：
3.7
通讯作者：
Mingjiu Ni
中科院分区：
文献类型：
--
作者：
Jie Zhang;Mingjiu Ni
在过去60年中进行的许多实验已经证明，在某些液体中，单个毫米大小的气泡将在不稳定的路径中上升，有时观察到从Z字形转变为螺旋形。在进行了几组直接数值模拟后，本文给出了理论解释，揭示了引起转变的物理机制，结果分为两部分。在第一部分中，其中一个自由上升的气泡进行模拟，等强度的涡对被观察到脱落两次在纯曲折路径的一段时间内，这种类型的运动是由气泡界面上积累的流向涡量触发，当达到临界值。然而，当反向旋转的涡之间的平衡被打破时，在非对称涡对之间诱导出角速度，驱动气泡沿相反的螺旋路径上升，因此，虽然没有实验观察到的螺旋方向的偏好，但它实际上是由更强的涡线的符号决定的。在第二部分中，为了进一步证实涡旋结构与不稳定路径模式之间的关系，我们将外部垂直磁场施加到了振荡上升的气泡上。如我们先前的研究所示（Zhang & Ni，Phys.Fluids，vol.26（10），2014，102102），双螺纹涡对的强度以及它们之间的不平衡将在磁场下减弱。因此，随着涡对变得更加对称，观察到的旋转半径的螺旋上升的气泡减小。我们试图回答Shew等人（2005，预印本，ENS，里昂）提出的问题，“是什么导致泡沫自然地从锯齿形过渡到螺旋形？”
It has been demonstrated by many experiments carried out over the last 60 years that in certain liquids a single millimetre-sized bubble will rise within an unstable path, which is sometimes observed to transit from zigzag to spiral. After performing several groups of direct numerical simulations, the present work gives a theoretical explanation to reveal the physical mechanism causing the transition, and the results are presented in two parts. In the first part, in which a freely rising bubble is simulated, equal-strength vortex pairs are observed to shed twice during a period of the pure zigzag path, and this type of motion is triggered by the amounts of streamwise vorticities accumulated on the bubble interface, when a critical value is reached. However, when the balance between the counter-rotating vortices is broken, an angular velocity is induced between the asymmetric vortex pairs, driving the bubble to rise in an opposite spiral path. Therefore, although there is no preference of the spiral direction as observed in experiments, it is actually determined by the sign of the stronger vortex thread. In the second part, external vertical magnetic fields are imposed onto the spirally rising bubble in order to further confirm the relations between the vortex structures and the unstable path patterns. As shown in our previous studies (Zhang & Ni, Phys. Fluids, vol. 26 (10), 2014, 102102), the strength of the double-threaded vortex pairs, as well as the imbalance between them, will be weakened under magnetic fields. Therefore, as the vortex pairs become more symmetric, the rotating radius of the spirally rising bubble is observed to decrease. We try to answer the question, put forward by Shew et al. (2005, Preprint, ENS, Lyon), ‘what caused the bubble to transit from zigzag to spiral naturally?’