Coherence of unsteady wake of periodically plunging airfoil
Coherence of unsteady wake of periodically plunging airfoil
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DOI：
10.1017/jfm.2022.147
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发表时间：
2022-03
影响因子：
3.7
通讯作者：
Burak Turhan;Zhijin Wang;I. Gursul
中科院分区：
文献类型：
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作者：
Burak Turhan;Zhijin Wang;I. Gursul
本文对弦雷诺数Re=20000时NACA0012型翼型正弦俯冲的近尾迹流动结构进行了实验研究，得到了较宽的折减频率k和基于峰峰值幅值的Strouhal数St.Strouhal数，用平均速度场和脉动速度场估算的平均推力系数证实了St2的依赖关系以及k≤1的约化频率的显著影响。一般来说，在实验范围内(k≤3.14和ST≤0.24)，时间平均流量与ST的相关性要好于K。利用两点互相关和本征正交分解对近尾迹的顺流和横流进行了分析，结果表明非定常特性与ST的相关性比平均流量更好。顺流基本尾迹模式和横流拍动模式的能量百分比随ST的增大而增大，但在生阻和生力尾迹中的比例不同。摆动物体背后的尾迹同步也有相似之处。测量域内的展平平均互相关系数对于小ST(在产生阻力的尾迹中)是线性增长的，而对于较大的ST(在产生推力的尾迹中)几乎不变。结果表明，Strouhal数是决定尾迹二维程度的最重要参数，并表明ST≥0.0 5和x/c≤4的展向旋涡是准二维的。文中还讨论了利用振荡翼型实验阵风发生器的意义。
Abstract We present an experimental investigation of the flow structure in the near wake of a NACA0012 airfoil plunging sinusoidally at a chord Reynolds number of Re = 20 000 and for a wide range of reduced frequency k and Strouhal number based on peak-to-peak amplitude St. Estimated mean thrust coefficients using the mean and fluctuating velocity fields confirm the St2 dependence as well as a significant effect of the reduced frequency for k ≤ 1. Generally, time-averaged flow quantities are better correlated with St than k in the range tested (k ≤ 3.14 and St ≤ 0.24). Analysis of the streamwise flow and cross-flow in the near wake using two-point cross-correlations and proper orthogonal decomposition reveals that the unsteady characteristics are even better correlated with St than the mean flow quantities. The percentage energy of the fundamental wake modes of the streamwise flow and the flapping mode of the cross-flow increases with increasing St, but at different rates in the drag-producing and thrust-producing wakes. There are similarities to the wake synchronisation behind oscillating bodies. The spanwise-averaged cross-correlation coefficient in the measurement domain grows linearly for small St (in drag-producing wakes), and is nearly constant at a high value for larger St (in thrust-producing wakes). Results show that the Strouhal number is the most important parameter that determines the degree of two-dimensionality of the wake, and suggest that spanwise vortices are quasi-two-dimensional for St ≥ 0.05 and x/c ≤ 4. The implications for experimental gust generators using oscillating airfoils are discussed.