Mixing and Diffusion in Regular/Fractal Grid Turbulence
Mixing and Diffusion in Regular/Fractal Grid Turbulence
复制标题
规则/分形网格湍流中的混合和扩散
DOI：
10.1007/978-3-319-33310-6_2
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
Y. Ito
中科院分区：
文献类型：
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作者：
Y. Sakai;K. Nagata;H. Suzuki;Y. Ito
湍流中的标量混合在自然界和工业工厂中被广泛观察到。在这一章中，我们将讨论与网格生成湍流中的混合和扩散有关的三个主题。第一个课题（见第一节）是轴对称COjet进入由正方形网格双平面圆杆网格（在第一节中称为规则网格）和正方形分形网格产生的自由流湍流流动的实验研究。COjet的问题，从一个小管道位于这些网格的衰减区域。使用了由两个浓度敏感I型热线传感器组成的复合探头。对于这两种流动，自由流中的网格雷诺数为6，000，基于自由流和射流出口速度之间的相对速度的射流雷诺数为5，000。正方形分形网格和规则网格的泰勒雷诺数分别约为100和35。结果表明，自由流湍流度越大，射流平均速度和浓度的半宽度增加越快，轴向均方根速度和浓度衰减越慢。第二主题（如第2节所示）是由正方形网格双平面方杆网格产生的湍流中高施密特数被动标量混合层的发展（在第2节中称为规则网格）和相同网格雷诺数为2500的正方形分形网格。（罗丹明B）仅从下游供应;因此，具有初始阶跃分布的标量混合层在格栅下游发展。粒子图像测速仪和平面激光诱导荧光被用来调查的速度和浓度统计。再次证实了正方形分形网格比规则网格产生更高的湍流强度。方形分形网格湍流中物质的涡动扩散系数约为规则网格湍流中物质的涡动扩散系数的4.2倍。第三个主题（见第三节）是网格湍流中混合层的直接数值模拟。模拟包括四种网格产生的湍流中的传热：（a）正方形网格双平面方杆网格（在第3节中称为规则网格），（B）正方形网格单平面方杆网格，（c）由平行方杆组成的复合网格和（d）正方形分形网格。两种不同温度的流体分别提供在网格上游的上游和下游，在网格后面产生热混合层。对于网格（a），分别对应于空气和水流，执行具有0.71和7.1的两个不同普朗特数的模拟。结果表明，在网格下游可以模拟出典型的网格湍流和热混合层，当普朗特数较大时，可以观察到较大的垂直湍流热通量。然后对规则分形网格和正方形分形网格下的混合层进行了比较。特别是，最大的分形网格，最小的酒吧的比例的影响进行了研究。结果表明，分形网格湍流比规则网格湍流能更大程度地增强湍流混合，特别是在大尺度上。第四部分是结论和展望。
Scalar mixing in turbulent flows is widely observed in nature as well as in industrial plants. In this chapter, we deal with three topics related to mixing and diffusion in grid-generated turbulence. The first topic (shown in Sect. 1) is experimental research on an axisymmetric COjet issuing into free-stream turbulent flows generated by a square-mesh biplane round-rod grid (referred to as a regular grid in Sect. 1) and a square fractal grid. The COjet issues from a small pipe located in the decaying region of these grid turbulences. A composite probe consisting of two concentration-sensitive I-type hot-wire sensors is used. For both flows, the mesh Reynolds number in the free stream is 6,000, and the jet Reynolds number based on the relative velocity between the free stream and the exit velocity of the jet is 5,000. The Taylor Reynolds numbers are about 100 and 35 for the square fractal grid and the regular grid, respectively. The results show that the half-widths of the mean velocity and concentration of the jets increase more rapidly, and the root mean square velocity and concentration in the axial direction decay more slowly for stronger free-stream turbulence. The second topic (shown in Sect. 2) is the development of a mixing layer of a high-Schmidt-number passive scalar in turbulent flows generated by a square-mesh biplane square-bar grid (referred to as a regular grid in Sect. 2) and a square fractal grid with the same mesh Reynolds number of 2500. A uniform passive scalar (Rhodamine B) is supplied only from the lower stream; therefore, scalar mixing layers with an initial step profile develop downstream of the grids. Particle image velocimetry and planar laser-induced fluorescence are used to investigate the velocity and concentration statistics. It is reconfirmed that the square fractal grid produces a higher turbulence intensity than the regular grid. The eddy diffusivity of the mass in the square fractal grid turbulence is approximately 4.2 times larger than that in the regular grid turbulence. The third topic (shown in Sect. 3) is direct numerical simulation of the mixing layer developed in grid turbulence. The simulations include the heat transfer in turbulent flows generated by four types of grid: (a) a square-mesh biplane square-bar grid (referred to as a regular grid in Sect. 3), (b) a square-mesh single-plane square-bar grid, (c) a composite grid consisting of parallel square-bars and (d) a square fractal grid. Two fluids at different temperatures are provided separately in the upper and lower streams upstream of the grids, generating a thermal mixing layer behind the grid. For grid (a), simulations with two different Prandtl numbers of 0.71 and 7.1, corresponding to air and water flows, respectively, are performed. The results show that the typical grid turbulence and thermal mixing layer can be simulated downstream of the grids, and a larger vertical turbulent heat flux is observed when the Prandtl number is large. Next the mixing layers in regular and square fractal grid turbulences are compared. In particular, the effects of the ratio of the largest to the smallest bar for the fractal grid,, are investigated. The results show that turbulent mixing is enhanced to a greater extent in fractal grid turbulence than in regular grid turbulence, especially at large. In Sect. 4, the conclusion and future prospects are presented.