Turbulent entrainment into a cylinder wake from a turbulent background
Turbulent entrainment into a cylinder wake from a turbulent background
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DOI：
10.1017/jfm.2020.755
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发表时间：
2018-11
影响因子：
3.7
通讯作者：
K. S. Kankanwadi;O. Buxton
中科院分区：
文献类型：
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作者：
K. S. Kankanwadi;O. Buxton
摘要通过对受自由流湍流影响的圆柱远尾迹的研究，研究了背景湍流对夹带过程的影响，以及两个湍流体之间界面区域的性质。同时粒子图像测速和平面激光诱导荧光测量在圆柱体下游40直径处进行。尽管背景中存在湍流和旋转流体，但尾迹和环境流体之间的外部界面表现出类似于湍流/非湍流界面的经典结果的熵跃。即使当背景湍流的强度大于尾流本身的湍流强度时，尾流边界处的这种跃变仍然存在。对尾迹边界结构的分析表明，背景湍流强度的增加导致相对于无湍流情况下界面表面积的增加。然而，随着背景湍流强度的增加，观测到的不是由于表面积增加而增加夹带的直观结果，而是平均夹带质量通量的减少。通过对通量概率密度函数的分析，平均夹带的减少可以归因于在背景湍流存在的情况下，极端夹带和夹带事件在平衡中向夹带侧倾斜。最后，对夹带行为的尺度分析表明，自由流湍流影响所有长度尺度的夹带行为，而不仅仅局限于能量包含尺度。
Abstract The effects of background turbulence on the entrainment process, as well as the nature of the interfacial region between two bodies of turbulent fluid, were examined through an investigation of the far wake of a circular cylinder that is subjected to free-stream turbulence. Simultaneous particle image velocimetry and planar laser induced fluorescence measurements were conducted 40 diameters downstream of the cylinder. Despite the availability of turbulent, rotational fluid in the background, the outer interface between the wake and the ambient fluid exhibits an enstrophy jump akin to the classical result of a turbulent/non-turbulent interface. This jump at the wake boundary persists even when the intensity of the background turbulence is greater than the turbulence intensity of the wake itself. Analysis on the structure of the wake boundary reveals that an increase in background turbulence intensity results in an increased interfacial surface area relative to the non-turbulent case. However, instead of the intuitive result of increased entrainment as a result of increased surface area, a reduction in mean entrainment mass flux is observed with increased background turbulence intensity. Through the analysis of the flux probability density functions, the reduction in mean entrainment can be attributed to a tip in balance of extreme entrainment and detrainment events to the detrainment side in the presence of background turbulence. Lastly, a scale by scale analysis of entrainment behaviour revealed that free-stream turbulence affects entrainment behaviour across all length scales and is not just limited to the energy containing scales.