A direct numerical simulation analysis of coherent structures in bubble-laden channel flows
A direct numerical simulation analysis of coherent structures in bubble-laden channel flows
复制标题
气泡加载通道流中相干结构的直接数值模拟分析
DOI：
10.1017/jfm.2020.780
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发表时间：
2020
影响因子：
3.7
通讯作者：
M. Klein
中科院分区：
文献类型：
--
作者：
J. Hasslberger;P. Cifani;N. Chakraborty;M. Klein
为了研究湍流壁面边界泡状流中的拟序结构，本文进行了局部流拓扑分析。利用速度梯度张量的不变量，对于不可压缩流动，所有可能的小尺度流动结构都可以归类为两个节点和两个焦点拓扑。所分析的自由运动和变形气泡群的直接数值模拟数据库是基于多标记几何流体体积方法。气泡似乎扮演着混合元素的角色，碎裂了大的拓扑结构。由于这两个相的行为有很大不同，以有条件的方式评估了关键量，即分别针对气体分散相和液体载体相进行评估。虽然所研究的槽道流动是湍流的，但与层流蠕动流动的解析解有几个相似之处。结果表明，气泡夹杂导致局部局部流动特征向节流特征的转变，而稳定和不稳定流动类型在两相中所占比例基本不变，约为50%。在气泡夹杂和壁面限制对流动拓扑的整体影响方面，没有明显的相似之处。至少就流动拓扑而言，界面附近的区域根据相的不同表现出明显不同的行为。文中还分析了气泡槽流中相干结构的尺寸分布。实验测得的液体流拓扑尺寸谱可以用一个简单的模型来近似，并具有一定的精度。
To investigate the coherent structures in turbulent wall-bounded bubbly flows, a local flow topology analysis has been performed in this work. Using the invariants of the velocity-gradient tensor, all possible small-scale flow structures can be categorized into two nodal and two focal topologies for incompressible flows. The analysed direct numerical simulation database of freely moving and deforming bubble swarms is based on a multi-marker geometrical volume-of-fluid method. It appears that bubbles are acting as mixing elements and fragmenting the large topology structures. Since the behaviour in both phases is quite different, key quantities were evaluated in a conditional manner, i.e. separately for the gaseous dispersed phase and the liquid carrier phase. Several similarities with an analytical solution for laminar creeping flows have been observed although the investigated channel flow is turbulent. It has been concluded that the inclusion of bubbles results in a partial shift from focal to nodal flow character whereas the share of stable and unstable flow types remains fairly constant at approximately 50 % in both phases. No clear analogy can be discerned in terms of the global effects of bubble inclusion and wall confinement on flow topologies. At least with respect to flow topologies, regions in the vicinity of the interface show a clearly different behaviour depending on the phase. The size distribution of coherent structures in the bubble-laden channel flow has also been analysed. The measured flow topology size spectrum in the liquid phase can be approximated by a simple model with reasonable accuracy.
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