The orientational dynamics of deformable finite-sized bubbles in turbulence
The orientational dynamics of deformable finite-sized bubbles in turbulence
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DOI：
10.1017/jfm.2021.69
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发表时间：
2021-03
影响因子：
3.7
通讯作者：
A. U. M. Masuk;Ashwanth K. R. Salibindla;R. Ni
中科院分区：
文献类型：
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作者：
A. U. M. Masuk;Ashwanth K. R. Salibindla;R. Ni
摘要：我们提出了可变形的有限尺寸气泡和周围湍流的同时三维测量。气泡的方向与驱动气泡变形的两个关键机制有关：湍流应变速率和两相之间的滑移速度。气泡和滑移速度之间的优先排列最强，表明后者起主导作用。我们还将实验结果与无滑移速度和表面张力的理想材料单元的变形进行了比较。没有这些，材料元素表现出高度不同的取向，进一步证实了滑移速度在气泡取向中的重要性。除了变形外，当气泡开始破裂时，它们的相对方向也会发生显著变化。虽然严重变形的气泡与湍流应变率的特征向量的对齐变得更强，但气泡的半长轴通过几乎$90^{\circ}$的转弯与滑移速度对齐（而不是垂直于）。之所以出现这种令人费解的方向变化，是因为滑移速度包含了气泡和背景流的贡献。由于气泡发生强烈的变形，其半长轴的快速伸长导致气泡速度很大，这支配了滑移速度，并迫使其与气泡的半长轴对齐。因此，当气泡接近破裂时，滑移速度从驱动机制转变为驱动结果。结果强调了气泡方向与周围流动之间的复杂耦合，在模拟湍流中的气泡变形和破裂时应考虑到这一点。
Abstract We present simultaneous three-dimensional measurements of deformable finite-sized bubbles and surrounding turbulent flows. The orientations of bubbles are linked to two key mechanisms that drive bubble deformation: the turbulent strain rate and slip velocity between the two phases. The strongest preferential alignment is between the bubbles and slip velocity, indicating the latter plays a dominant role. We also compared our experimental results with the deformation of ideal material elements with no slip velocity or surface tension. Without these, material elements show highly different orientations, further confirming the importance of the slip velocity in the bubble orientation. In addition to deformation, when bubbles begin to break, their relative orientations change significantly. Although the alignment of the severely deformed bubbles with the eigenvectors of the turbulent strain rate becomes much stronger, the bubble semi-major axis becomes aligned with (rather than perpendicular to) the slip velocity through an almost $90^{\circ }$ turn. This puzzling orientation change occurs because the slip velocity contains the contributions from both the bubble and the background flow. As the bubble experiences strong deformation, the rapid elongation of its semi-major axis leads to a large bubble velocity, which dominates the slip velocity and forces its alignment with the bubble's semi-major axis. The slip velocity thereby switches from a driving mechanism to a driven result as bubbles approach breakup. The results highlight the complex coupling between the bubble orientation and the surrounding flow, which should be included when modelling the bubble deformation and breakup in turbulence.