The unsteady organization of a separated turbulent boundary layer was investigated upstream from the trailing edge of a NACA 4418 airfoil. The angle of attack was $9^{\circ }$ in the pre-stall regime. Two particle image velocimetry fields of view were of interest: a streamwise–wall-normal plane at midspan of the airfoil and a streamwise–spanwise plane parallel to and near the surface of the airfoil. In the near-surface streamwise–spanwise plane, the mean velocity field revealed a saddle point near midspan and a pair of counter-rotating foci at the sides. This pattern is reminiscent of a stall cell, which has been traditionally associated with flow separation on thick airfoils at and slightly beyond the angle of attack of maximum lift. Isolating the low frequencies showed that the instantaneous separation front consisted of several smaller structures that also resembled a stall cell pattern, but they were an order of magnitude smaller than the one found in the mean pattern. These instantaneous stall cells were of two types: forward and backward. The forward stall cells were formed by strong high-speed streaks from upstream, while backward stall cells formed as a result of strong backflow just downstream from the separation front, resulting in a foci pair and a saddle point on their upstream side. In both cases, the foci pairs acted to mobilize high-speed momentum of the associated streak into a rotational motion, causing these streaks to dissipate. Finally, proper orthogonal decomposition revealed that low-order modes were associated with the movement and distortion of the separation front.

中文翻译：

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机翼后缘湍流边界层分离的时间分辨拓扑

在 NACA 4418 机翼后缘的上游研究了分离的湍流边界层的不稳定组织。在失速前的情况下，攻角为 $9^{\circ }$。两个粒子图像测速视野很有趣：翼型中跨处的流向-壁面法向平面和平行于翼型表面并靠近翼型表面的流向-展向平面。在近地表流向-展向平面上，平均速度场显示在跨中附近有一个鞍点，在两侧有一对反向旋转的焦点。这种模式让人联想到失速单元，传统上，失速单元与厚翼型上的气流分离有关，在最大升力攻角处或稍稍超出最大升力攻角处。隔离低频表明瞬时分离前沿由几个更小的结构组成，这些结构也类似于失速细胞模式，但它们比平均模式中发现的要小一个数量级。这些瞬时失速单元有两种类型：向前和向后。前失速单元是由来自上游的强烈高速条纹形成的，而后失速单元是由于分离前沿下游的强烈回流而形成的，从而在其上游侧形成一个焦点对和一个鞍点。在这两种情况下，焦点对都将相关条纹的高速动量调动为旋转运动，导致这些条纹消散。最后，