Experimental investigation on hairpin vortex packets in the later stage of bypass transition induced by cylinder wake
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摘要: 基于TR-PIV技术, 通过侧视和俯视两种情况对圆柱尾迹影响下旁路转捩末期发卡涡涡包的结构及特征尺寸进行了实验研究. 结合二维空间子波变换和\lambda_{ci}准则, 运用线性随机估计方法对速度信号进行条件平均. 在侧视情况下, 条件平均结果显示, 在边界层中一系列发卡涡涡头与壁面构成17^{\circ}的倾角, 并且被尾迹涡所占据的低速区域出现在涡包上方的主流区中. 在俯视的结果中, 沿流向方向拉伸(流向尺度3\delta, 展向尺度0.55δ)的低速条带结构出现在法向高度为y/δ=0.2的流向-展向平面中, 并且在该低速条带的两侧对称地出现了沿流向分布的反向旋转的涡结构. 可以得出: 在圆柱尾迹影响下旁路转捩的末期, 由于尾迹涡诱导作用的影响, 发卡涡涡包在形态上显示出了更大尺度的特征.
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关键词:
- 统计尺度
Abstract: The hairpin vortex packets structure and characteristicscale in the later stage of bypass transition induced by cylinder wake areinvestigated by TR PIV technique for both the side-view and top-view cases.The two-dimensional velocity components are correlated with \lambda_{ci} criterion, and linear stochastic estimation is used to estimate theconditional average of the velocity field. For the side-view case, theconditionally averaged structure consists of a series of swirling motionslocated along a line inclined at a larger angle (17^\circ) from the wall, anda low-speed region occupied by cylinder wake appearing right above it in thefree stream. In the (x, z)-plane at the top of the log layer (y/δ=0.2), the dominant motions are shown to be large-scale regions of lowmomentum elongated along the streamwise and within a spanwise width of0.55δ, whichare consistently bordered by small-scale counter-rotating vortex pairsorganized in the streamwise. The results suggest that in the later part ofthe transition zone, due to the upward induction of wake vortex, the hairpinvortex packets statistically perform a skeleton frame of larger scale androbust physique.-
Key words:
- statistical scale
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