Evolution of Lagrangian coherent structures in a cylinder-wake disturbed flat plate boundary layer
Evolution of Lagrangian coherent structures in a cylinder-wake disturbed flat plate boundary layer
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圆柱尾流扰动平板边界层中拉格朗日相干结构的演化
DOI：
10.1017/jfm.2016.81
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发表时间：
2016-04-01
影响因子：
3.7
通讯作者：
Wang, Jin-Jun
中科院分区：
文献类型：
--
作者：
He, Guo-Sheng;Pan, Chong;Wang, Jin-Jun
研究了由圆柱尾流引起的平板边界层转变中拉格朗日相干结构（LCS）的演化。同时使用氢气泡可视化和粒子图像测速（PIV）技术。研究发现，在圆柱体的下游，边界层中的扰动经历了快速增长，然后在过渡过程中缓慢衰减。拉格朗日相干结构通过定性氢气泡可视化和源自 PIV 数据的定量有限时间 Lyapunov 指数 (FTLE) 场来揭示。 LCS 的演变是从过渡一开始一直到边界层变成完全发展的湍流时为止的。 LCS 的平均对流速度和平均倾角首先从 FTLE 场中提取。低速条纹的流向长度似乎增加，而它们的展向距离在边界层过渡中减小。 PIV 数据的本征正交分解 (POD) 显示，与发夹涡和发夹包相关的低速条纹是过渡和湍流边界层中靠近壁面的主要相干结构。 POD 模式还揭示了湍流边界层中的各种尺度。此外，人们发现大尺度相干结构可以调制小尺度相干结构的幅度。
Evolution of Lagrangian coherent structures (LCS) in a flat plate boundary layer transition induced by the wake of a circular cylinder is investigated. Both hydrogen bubble visualization and particle image velocimetry (PIV) techniques are used. It is found that downstream of the cylinder, the disturbance in the boundary layer experiences a fast growth followed by a slow decay in the transition. Lagrangian coherent structures are revealed by qualitative hydrogen bubble visualizations and quantitative finite-time Lyapunov exponents (FTLE) fields derived from the PIV data. The evolution of the LCS is considered from the very beginning of the transition up to when the boundary layer becomes fully developed turbulent flow. The mean convection velocity and average inclination angle of the LCS are first extracted from the FTLE fields. The streamwise length of the low-speed streaks seems to increase, while their spanwise distance decreases in the boundary layer transition. Proper orthogonal decomposition (POD) of the PIV data shows that low-speed streaks associated with the hairpin vortices and hairpin packets are the dominant coherent structures close to the wall in the transitional and turbulent boundary layer. The POD modes also reveal a variety of scales in the turbulent boundary layer. Moreover, it is found that large-scale coherent structures can modulate the amplitude of the small-scale ones.