Transition of transient channel flow after a change in Reynolds number
Transition of transient channel flow after a change in Reynolds number
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DOI：
10.1017/jfm.2014.698
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发表时间：
2015-02-01
影响因子：
3.7
通讯作者：
Seddighi, M.
中科院分区：
文献类型：
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作者：
He, S.;Seddighi, M.
先前已经示出，在雷诺数从2800阶跃增加到7400（基于通道半高和体积速度）之后的通道中的瞬态流有效地是层流-湍流旁路过渡，即使初始流是湍流（He & Seddighi，J. Fluid Mech.，第715卷，2013年，pp. 60-102）。在本文中，它表明，瞬态流结构表现出强烈的对比特性，在大，小流量扰动的情况下。当雷诺数增加较大时，流动的特征是在初始阶段出现强烈的细长条纹，随后出现孤立的湍流斑并扩展，如前所示。相比之下，流动似乎逐渐演变，湍流再生过程在很大程度上保持不变，当雷诺数比较低时，在流动瞬态，条纹似乎没有发挥重要作用。尽管在流动结构上有明显的差异，但在所有考虑的条件下，瞬态流动的特征无疑是层流-湍流转变，这在各种流动统计中清楚地表现出来。在预过渡期间，在所有的情况下，时间发展的边界层显示出很强的相似性，彼此密切遵循的瞬态层流边界层的斯托克斯解。流向波动速度也表现出良好的相似性，在各种情况下，无论外观的细长条纹或没有，和最大能量增长表现出类似的线性速率在空间发展的边界层。在所有情况下，使用最小摩擦因子，可以清楚地定义过渡的开始，因此定义的临界时间与幂律形式的自由流湍流密切相关。
It has previously been shown that the transient flow in a channel following a step increase of Reynolds number from 2800 to 7400 (based on channel half-height and bulk velocity) is effectively a laminar-turbulent bypass transition even though the initial flow is turbulent (He & Seddighi, J. Fluid Mech., vol. 715, 2013, pp. 60-102). In this paper, it is shown that the transient flow structures exhibit strong contrasting characteristics in large and small flow perturbation scenarios. When the increase of Reynolds number is large, the flow is characterized by strong elongated streaks during the initial period, followed by the occurrence and spreading of isolated turbulent spots, as shown before. By contrast, the flow appears to evolve progressively and the turbulence regeneration process remains largely unchanged during the flow transient when the Reynolds number ratio is low, and streaks do not appear to play a significant role. Despite the major apparent differences in flow structures, the transient flow under all conditions considered is unambiguously characterized by laminar-turbulent transition, which exhibits itself clearly in various flow statistics. During the pre-transition period, the time-developing boundary layers in all the cases show a strong similarity to each other and follow closely the Stokes solution for a transient laminar boundary layer. The streamwise fluctuating velocity also shows good similarity in the various cases, irrespective of the appearance of elongated streaks or not, and the maximum energy growth exhibits a linear rate similar to that in a spatially developing boundary layer. The onset of transition is clearly definable in all cases using the minimum friction factor, and the critical time thus defined is strongly correlated with the free-stream turbulence in a power-law form.