Streak instability in turbulent channel flow: the seeding mechanism of large-scale motions
Streak instability in turbulent channel flow: the seeding mechanism of large-scale motions
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DOI：
10.1017/jfm.2017.697
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发表时间：
2017-09
影响因子：
3.7
通讯作者：
Matteo de Giovanetti;H. Sung;Y. Hwang
中科院分区：
文献类型：
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作者：
Matteo de Giovanetti;H. Sung;Y. Hwang
经常有人提出，大尺度运动（或隆起）的形成是连续合并和/或近壁发夹涡生长的结果。在本研究中，我们报告了我们的直接观察，即大尺度运动是由外部区域的“放大”条纹运动（即超大尺度运动）的不稳定性产生的。我们设计了一个数值实验，在湍流槽道流高达$Re_{\unicode[STIX]{x1 D 70 F}}\simeq 2000$，其中沿流向均匀的条纹运动是人为驱动的体强迫在外部区域从以前的线性理论计算（Hwang & Cossu，J.流体力学，第664卷，2015年，pp. 51-73）。随着强迫振幅的增加，在流向波长为$\unicode[STIX]{x1 D 706} x}/h\simeq 1{-}5$（$h$为通道的半高）处，出现了一个高能流向涡结构.动态模式分解的应用和湍流统计的检查表明，这种结构是一个后果的曲折模式不稳定的条纹，一个子过程的大规模外部结构的自我维持机制。同时还发现，涡结构的统计特征与外围区域大尺度运动的统计特征非常相似。最后，提出条纹不稳定性的最大流向长度决定了超大尺度运动的流向长度尺度。
It has often been proposed that the formation of large-scale motion (or bulges) is a consequence of successive mergers and/or growth of near-wall hairpin vortices. In the present study, we report our direct observation that large-scale motion is generated by an instability of an ‘amplified’ streaky motion in the outer region (i.e. very-large-scale motion). We design a numerical experiment in turbulent channel flow up to $Re_{\unicode[STIX]{x1D70F}}\simeq 2000$ where a streamwise-uniform streaky motion is artificially driven by body forcing in the outer region computed from the previous linear theory (Hwang & Cossu, J. Fluid Mech., vol. 664, 2015, pp. 51–73). As the forcing amplitude is increased, it is found that an energetic streamwise vortical structure emerges at a streamwise wavelength of $\unicode[STIX]{x1D706}_{x}/h\simeq 1{-}5$ ( $h$ is the half-height of the channel). The application of dynamic mode decomposition and the examination of turbulence statistics reveal that this structure is a consequence of the sinuous-mode instability of the streak, a subprocess of the self-sustaining mechanism of the large-scale outer structures. It is also found that the statistical features of the vortical structure are remarkably similar to those of the large-scale motion in the outer region. Finally, it is proposed that the largest streamwise length of the streak instability determines the streamwise length scale of very-large-scale motion.