Scale-dependent anisotropy, energy transfer and intermittency in bubble-laden turbulent flows
Scale-dependent anisotropy, energy transfer and intermittency in bubble-laden turbulent flows
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DOI：
10.1017/jfm.2021.760
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发表时间：
2021-04
影响因子：
3.7
通讯作者：
T. Ma;Bernhard Ott;J. Fröhlich;A. Bragg
中科院分区：
文献类型：
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作者：
T. Ma;Bernhard Ott;J. Fröhlich;A. Bragg
摘要利用垂直通道中分散气泡流动的直接数值模拟数据，研究了气泡对载相湍流的影响。我们开发了一种新的方法，基于质心图方法的扩展，使我们能够量化和可视化任何尺度下流体的各向异性和组件性。由此我们发现，与空载情况相比，气泡在所有尺度下都显著增强了流动的各向异性，并且对于某些气泡情况，非常强的各向异性一直持续到最小的流动尺度。在涉及小气泡的情况下，观察到最强的各向异性。关于流动尺度间的能量传递，我们的结果表明，在有气泡的情况下，与无气泡的情况一样，能量传递是由大尺度到小尺度的。然而，当考虑到与速度场的特定分量相关的能量转移时，有证据表明存在高档转移。尽管在有气泡和没有气泡的情况下，能量传递的方向是相同的，但气泡显著地改变了能量传递的行为，这表明气泡在改变流动中非线性项的活动性方面起着重要作用。速度增量的偏度也揭示了气泡对流动的强烈影响，与单相情况相比，气泡的符号和大小都发生了变化。我们还考虑了四阶结构函数的归一化形式，结果表明气泡的引入在耗散范围内强烈增强了间歇性，但在更大尺度上抑制了间歇性。这种耗散尺度间歇性的增强对于理解气泡如何改变湍流的混合特性具有重要意义。
Abstract Data from direct numerical simulations of disperse bubbly flows in a vertical channel are used to study the effect of the bubbles on the carrier-phase turbulence. We developed a new method, based on an extension of the barycentric map approach, that allows us to quantify and visualize the anisotropy and componentiality of the flow at any scale. Using this we found that the bubbles significantly enhance anisotropy in the flow at all scales compared with the unladen case, and that for some bubble cases, very strong anisotropy persists down to the smallest scales of the flow. The strongest anisotropy observed was for the cases involving small bubbles. Concerning the energy transfer among the scales of the flow, our results indicate that for the bubble-laden cases, the energy transfer is from large to small scales, just as for the unladen case. However, there is evidence of an upscale transfer when considering the transfer of energy associated with particular components of the velocity field. Although the direction of the energy transfer is the same with and without the bubbles, the behaviour of the energy transfer is significantly modified by the bubbles, suggesting that the bubbles play a strong role in altering the activity of the nonlinear term in the flow. The skewness of the velocity increments also reveals a strong effect of the bubbles on the flow, changing both its sign and magnitude compared with the single-phase case. We also consider the normalized forms of the fourth-order structure functions, and the results reveal that the introduction of bubbles into the flow strongly enhances intermittency in the dissipation range, but suppresses it at larger scales. This strong enhancement of the dissipation-scale intermittency has significant implications for understanding how the bubbles might modify the mixing properties of turbulent flows.