Three-dimensional wake transition of a square cylinder
Three-dimensional wake transition of a square cylinder
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DOI：
10.1017/jfm.2018.104
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发表时间：
2018-03
影响因子：
3.7
通讯作者：
Hongyi Jiang;Liang Cheng;H. An
中科院分区：
文献类型：
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作者：
Hongyi Jiang;Liang Cheng;H. An
采用直接数值模拟方法研究了垂直和平行于来流的方柱绕流的三维尾迹转捩。二次尾流不稳定性，即模式A不稳定性，发生在雷诺数（$Re$）为165.7。在Re$从185到210的范围内观察到从模式A*（即具有涡旋位错的模式A）到模式B的逐渐尾流转变，在此范围内涡旋位错发生的概率随着Re$的增加而单调减小。分析了Strouhal-Reynolds数关系的特点。在发病的模式A*，突然下降的3-D Strouhal数从其二维对应观察，这是由于亚临界性质的模式A* 不稳定性。连续的3-D Strouhal-Reynolds数曲线在模式交换区域被观察到，因为模式A* 和模式B具有非常接近的旋涡脱落频率，因此在频谱中仅观察到单个合并峰。研究了模式A和模式B尾流不稳定性的滞后现象。无约束模式A和模式B尾流不稳定性分别是滞后和非滞后的。然而，展向限制模式A可以是非滞后的。有人提出，滞后在尾流不稳定的存在下，可以通过检查突然/逐渐变化的3-D流特性的发病时的尾流不稳定，与突然和逐渐变化对应的滞后（亚临界）和非滞后（超临界）流，分别确定。
Three-dimensional (3-D) wake transition for flow past a square cylinder aligned with sides perpendicular and parallel to the approaching flow is investigated using direct numerical simulation. The secondary wake instability, namely a Mode A instability, occurs at a Reynolds number ( $Re$ ) of 165.7. A gradual wake transition from Mode A* (i.e. Mode A with vortex dislocations) to Mode B is observed over a range of $Re$ from 185 to 210, within which the probability of occurrence of vortex dislocations decreases monotonically with increasing $Re$ . The characteristics of the Strouhal–Reynolds number relationship are analysed. At the onset of Mode A*, a sudden drop of the 3-D Strouhal number from its two-dimensional counterpart is observed, which is due to the subcritical nature of the Mode A* instability. A continuous 3-D Strouhal–Reynolds number curve is observed over the mode swapping regime, since Mode A* and Mode B have extremely close vortex shedding frequencies and therefore only a single merged peak is observed in the frequency spectrum. The existence of hysteresis for the Mode A and Mode B wake instabilities is examined. The unconfined Mode A and Mode B wake instabilities are hysteretic and non-hysteretic, respectively. However, a spanwise confined Mode A could be non-hysteretic. It is proposed that the existence of hysteresis at a wake instability can be identified by examining the sudden/gradual variation of the 3-D flow properties at the onset of the wake instability, with sudden and gradual variations corresponding to hysteretic (subcritical) and non-hysteretic (supercritical) flows, respectively.