Leading-edge separated flow field over a sharp flat plate is experimentally investigated in Reynolds numbers ranging from 6.2 × 103 to 4.1 × 104, using particle image velocimetry (PIV) and its statistics. It was observed that the average reattachment length is nearly independent of Reynolds number and the small secondary bubble observed near the leading edge was found to shrink with increasing Reynolds number. The wall-normal profiles of the statistical values of kinematic quantities such as the velocity components and their fluctuations scaled well with average reattachment length lR and freestream velocity U∞. Their magnitudes compare well with previous investigations even though the current triangular shaped sharp leading edge is different from previous flat-faced or semi-circular ones. The shear layer was observed to exhibit 2 different linear growth rates over 2 distinct regions. Instantaneous PIV realizations demonstrate unsteady nature of the separation bubble, whose origins in the upstream portion of the bubble are analysed. Bimodal nature of the probability density function (PDF) of fluctuating streamwise velocity at around x/lR = 0.08–0.15 indicates successive generation and passage of vortices in the region, which subsequently interact and evolve into multiscale turbulent field exhibiting nearly Gaussian PDF. Shedding of vortices with wide range of scales are apparent in most of the instantaneous realizations. Proper Orthogonal Decomposition (POD) of the velocity fluctuation magnitude field revealed that the flow structures of the dominant modes and their relative energies are independent of Reynolds number. In each of the dominant modes (first 3 modes), the length scales corresponding to the large scale structures and their spacing are the same for all Reynolds numbers, suggesting that their Strouhal number (observed to be ~ 0.09–0.2 at Reynolds number of 6.2 × 103) of unsteadiness should also be independent of Reynolds number. A single large structure- comparable in size to lR—was apparent well before reattachment in a few instantaneous realizations, as compared to multiple small-scale structures visible in most realizations; at Reynolds number of 6.2 × 103, realizations with such large-scale structures occurred approximately after every 20–30 realizations, corresponding to non-dimensional frequency of 0.4–0.6, which is identified to be the “regular shedding”. It was possible to reconstruct the large-scale structure during the instances from just the first 3 POD modes, indicating that the Strouhal number of regular shedding too is independent of Reynolds number.

中文翻译：

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使用粒子图像测速法对零入射平面上锐利前缘分离的实验研究

使用粒子图像测速法 (PIV) 及其统计数据，在雷诺数范围从 6.2 × 103 到 4.1 × 104 的情况下，对锐利平板上的前沿分离流场进行了实验研究。观察到平均再附着长度几乎与雷诺数无关，并且发现在前缘附近观察到的小的二次气泡随着雷诺数的增加而收缩。运动量统计值的壁法向剖面，例如速度分量及其波动，与平均再附着长度 lR 和自由流速度 U∞ 成比例良好。尽管当前的三角形锋利前缘与以前的平面或半圆形前缘不同，但它们的大小与以前的研究相比很好。观察到剪切层在 2 个不同的区域表现出 2 种不同的线性生长速率。瞬时 PIV 实现证明了分离气泡的不稳定性质，分析了气泡上游部分的起源。在 x/lR = 0.08-0.15 附近波动的流向速度的概率密度函数 (PDF) 的双峰性质表明该区域中涡旋的连续产生和通过，随后相互作用并演变成多尺度湍流场，表现出接近高斯 PDF。在大多数瞬时实现中，具有广泛范围的涡旋脱落是明显的。速度波动幅度场的适当正交分解（POD）表明，主要模式的流动结构及其相对能量与雷诺数无关。在每个主要模式（前 3 个模式）中，对应于大尺度结构的长度尺度及其间距对于所有雷诺数都是相同的，这表明它们的 Strouhal 数（在雷诺数为 6.2 时观察到约为 0.09-0.2） × 103) 的不稳定性也应该与雷诺数无关。与在大多数实现中可见的多个小规模结构相比，单个大型结构 - 大小与 lR 相当 - 在重新连接之前在一些瞬时实现中很明显；在雷诺数为 6.2 × 103 时，这种大规模结构的实现大约在每 20-30 次实现之后发生，对应于 0.4-0.6 的无量纲频率，这被确定为“规则脱落”。