Separated-Shear-Layer Development on an Airfoil at Low Reynolds Numbers
Separated-Shear-Layer Development on an Airfoil at Low Reynolds Numbers
复制标题
低雷诺数翼型件的分离剪切层开发
DOI：
10.2514/1.36620
复制
发表时间：
2008
期刊：
影响因子：
2.5
通讯作者：
P. Sullivan
中科院分区：
文献类型：
--
作者：
S. Yarusevych;J. Kawall;P. Sullivan
研究了低雷诺数下NACA 0025翼型上表面分离剪切层的流动转变。该研究包括风洞实验和线性稳定性分析。在0、5和10度迎角下，对雷诺数分别为100,000和150,000进行了详细的测量。对于所研究的所有情况，层流边界层分离发生在翼型的上表面。当雷诺数较小时，分离的剪切层不能再附着到翼型表面，但当雷诺数较高时，会发生再附着。尽管在流动发展上存在这种差异，但实验结果表明，两种雷诺数流型都存在相似的转变机制。流动转变是由于在以某一基频为中心的频带内分离的剪切层中自然扰动的放大而发生的。以基频为中心的扰动的初始增长之后是次谐分量的增长，最终导致流动转变。不断增长的扰动还会引起剪切层卷曲和卷曲涡的形成。结果表明，无粘稳定性理论可以很好地估计最大扰动的频率及其传播速度等显著特征。这意味着卷起的涡旋可以归因于无粘不稳定。然而，结果表明，为了有效地模拟分离剪切层中扰动的对流增长，需要考虑粘性和非平行效应。
Flow transition in the separated shear layer on the upper surface of a NACA 0025 airfoil at low Reynolds numbers was investigated. The study involved wind-tunnel experiments and linear stability analysis. Detailed measurements were conducted for Reynolds numbers of 100,000 and 150,000 at 0-, 5- and 10-degree angles of attack. For all cases examined, laminar boundary-layer separation takes place on the upper surface of the airfoil. The separated shear layer fails to reattach to the airfoil surface for the lower Reynolds number, but reattachment occurs for the higher Reynolds number. Despite this difference in flow development, experimental results show that a similar transition mechanism is attendant for both Reynolds number flow regimes. Flow transition occurs due to the amplification of natural disturbances in the separated shear layer within a band of frequencies centered at some fundamental frequency. The initial growth of disturbances centered at the fundamental frequency is followed by the growth of a subharmonic component, eventually leading to flow transition. The growing disturbances also cause shear-layer roll-up and the formation of roll-up vortices. The results show that inviscid stability theory can be employed to adequately estimate such salient characteristics as the frequency of the most amplified disturbances and their propagation speed. This implies that the roll-up vortices can be attributed to inviscid instability. However, the results suggest that viscous and nonparallel effects need to be accounted for to effectively model the convective growth of the disturbances in the separated shear layer.