Self-sustaining process of minimal attached eddies in turbulent channel flow
Self-sustaining process of minimal attached eddies in turbulent channel flow
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DOI：
10.1017/jfm.2016.226
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发表时间：
2016-04
影响因子：
3.7
通讯作者：
Y. Hwang;Yacine Bengana
中科院分区：
文献类型：
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作者：
Y. Hwang;Yacine Bengana
最近通过数值实验表明，湍流河道流中的含能运动（即相干结构）以汤森附着涡流的形式存在，该数值实验利用其自持性质仅在规定的展向长度尺度上模拟含能运动（Hwang，J. Fluid Mech.，第767卷，2015年，第254-289页）。在本研究中，对对数区域和外部区域中每个展向长度尺度（即附加涡流）的含能运动的自维持过程进行了详细研究，重点是其与“爆发”的相关性，“爆发”是指运动的能量时间振荡（Flores & Jiménez, Phys. Fluids, vol. 22, 2010, 071704）。结果表明，由条纹和准流向涡结构组成的对数区域和外部区域的附着涡具有与近壁区域非常相似的自持过程：即条纹被准流向涡通过升力效应显着放大；放大的条纹随后经历“快速的流向蜿蜒运动”，让人想起条纹的不稳定或短暂的增长，最终导致条纹的破裂和新的准流向涡流的再生。对于给定展向长度尺度 ${\it\lambda}_{z}$ 在 ${\it\lambda}_{z}^{+}\simeq 100$ 和 ${\it\lambda}_{z}\simeq 1.5h$ 之间的附加涡流，发现自持过程的单周转时间段为 $Tu_{{\it\tau}}/{\it\lambda}_{z}\simeq 2$（$u_{{\it\tau}}$是摩擦速度），它与爆发的时间尺度很好地对应。另外两个分别旨在人为抑制升力效应和条纹蜿蜒运动的数值实验表明，这些过程是对数区域和外部区域的附着涡流自维持过程的重要组成部分，与之前的几项理论研究一致。还表明，对对数区域和外部区域中的附加涡流的提升效应的人为抑制导致湍流表面摩擦的大量减少。
It has been recently shown that the energy-containing motions (i.e. coherent structures) in turbulent channel flow exist in the form of Townsend’s attached eddies by a numerical experiment which simulates the energy-containing motions only at a prescribed spanwise length scale using their self-sustaining nature (Hwang, J. Fluid Mech., vol. 767, 2015, pp. 254–289). In the present study, a detailed investigation of the self-sustaining process of the energy-containing motions at each spanwise length scale (i.e. the attached eddies) in the logarithmic and outer regions is carried out with an emphasis on its relevance to ‘bursting’, which refers to an energetic temporal oscillation of the motions (Flores & Jiménez, Phys. Fluids, vol. 22, 2010, 071704). It is shown that the attached eddies in the logarithmic and outer regions, composed of streaks and quasi-streamwise vortical structures, bear the self-sustaining process remarkably similar to that in the near-wall region: i.e. the streaks are significantly amplified by the quasi-streamwise vortices via the lift-up effect; the amplified streaks subsequently undergo a ‘rapid streamwise meandering motion’, reminiscent of streak instability or transient growth, which eventually results in breakdown of the streaks and regeneration of new quasi-streamwise vortices. For the attached eddies at a given spanwise length scale ${\it\lambda}_{z}$ between ${\it\lambda}_{z}^{+}\simeq 100$ and ${\it\lambda}_{z}\simeq 1.5h$ , the single turnover time period of the self-sustaining process is found to be $Tu_{{\it\tau}}/{\it\lambda}_{z}\simeq 2$ ( $u_{{\it\tau}}$ is the friction velocity), which corresponds well to the time scale of the bursting. Two additional numerical experiments, designed to artificially suppress the lift-up effect and the streak meandering motions, respectively, reveal that these processes are essential ingredients of the self-sustaining process of the attached eddies in the logarithmic and outer regions, consistent with several previous theoretical studies. It is also shown that the artificial suppression of the lift-up effect of the attached eddies in the logarithmic and outer regions leads to substantial amounts of turbulent skin-friction reduction.