Experiments in Fluids ( IF 2.3 ) Pub Date : 2021-10-01 , DOI: 10.1007/s00348-021-03319-z A. D’Aguanno 1 , F. F. J. Schrijer 1 , B. W. van Oudheusden 1
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Abstract
Transonic buffet behaviour of the supercritical airfoil OAT15A was investigated experimentally at flow conditions \(Ma=0.7\) and \(\alpha =3.5^\circ \), using schlieren and particle image velocimetry (PIV). The general behaviour of the buffet cycle was characterised with short-exposure schlieren visualisation and phase-averaged PIV measurements. A spectral analysis showed that the shock oscillation occurs with a dominant contribution at 160 Hz (St = 0.07, in good agreement with the literature) and between 25 and 55 % of the chord of the airfoil. Proper Orthogonal Decomposition (POD) was applied to the PIV data to extract the main modes connected with buffet. It is found that the first three most energetic modes capture around 65 % of the total fluctuating kinetic energy. The first and the third modes have a main frequency peak at 160 Hz and are well representing the separated area and the shock oscillation. The second mode was, instead, associated with an asymmetrical behaviour of the separated area and of the shear layer and displays a main peak at 320 Hz, being double the main buffet cycle frequency. Finally, it was shown that by using the 11 most energetic POD modes, an accurate reduced-order model (ROM) is obtained, which when subtracted from the instantaneous velocity fields allows the visualisation of the small-scale structures present in the flow, such as the upstream travelling waves (UTWs) and the vortex shedding in the separated area near the trailing edge. The analysis allowed to estimate the velocity of the UTWs, obtaining values in good agreement with the literature. In contrast, the analysis of the vortex dynamics in the trailing edge area revealed that vortices shed at the shock foot, which convect downstream in an area detached from the airfoil surface, cannot be considered responsible for the creation of UTWs in view of the mismatch in frequency of the two phenomena.
Graphic abstract
中文翻译：
超临界翼型跨音速抖振循环的实验研究
摘要
在流动条件\(Ma=0.7\)和\(\alpha =3.5^\circ \)下实验研究了超临界翼型 OAT15A 的跨音速抖振行为, 使用纹影和粒子图像测速 (PIV)。抖振周期的一般行为以短曝光纹影可视化和相位平均 PIV 测量为特征。频谱分析表明，冲击振荡在 160 Hz（St = 0.07，与文献非常吻合）和翼型弦的 25% 和 55% 之间以主要贡献发生。将适当的正交分解 (POD) 应用于 PIV 数据以提取与抖振相关的主要模式。发现前三种能量最高的模式捕获了总波动动能的 65% 左右。第一和第三模式在 160 Hz 处有一个主频率峰值，很好地代表了分离区域和激波振荡。相反，第二种模式是 与分离区域和剪切层的不对称行为相关，并在 320 Hz 处显示主峰，是主抖振周期频率的两倍。最后，结果表明，通过使用 11 种能量最高的 POD 模式，可以获得精确的降阶模型 (ROM)，当从瞬时速度场中减去时，可以显示流动中存在的小尺度结构，例如作为上游行波 (UTW) 和后缘附近分离区域中的涡旋脱落。该分析允许估计 UTW 的速度，获得与文献非常一致的值。相比之下，对后缘区域涡流动力学的分析表明，涡流在激波脚处脱落，在与翼型表面分离的区域向下游对流，
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