Abstract In the present work, experimental tests are conducted to study boundary layer transition over a supercritical airfoil undergoing pitch oscillations using hot-film sensors. Tests have been undertaken at an incompressible flow. Three reduced frequencies of oscillations and two mean angles of attack are studied and the influences of those parameters on transition location are discussed. Different algorithms are examined on the hot-film signals to detect the transition point. Results show the formation of a laminar separation bubble near the leading edge and at relatively higher angles of attack which leads to the transition of the boundary layer. However, at lower angles of attack, the amplification of the peaks in voltage signal indicate the emergence of the vortical structures within the boundary layer, introducing a different transition mechanism. Moreover, an increase in reduced frequency leads to a delay in transition onset, postponing it to a higher angle of attack, which widens the hysteresis between the upstroke and downstroke motions. Rising the reduced frequency yields in weakening or omission of vortical disturbances ensuing the removal of spikes in the signals. Of the other important results observed, is faster movement of the relaminarization point in the higher mean angle of attack. Finally, a time–frequency analysis of the hot-film signals is performed to investigate evolution of spectral features of the transition due to the pitching motion. An asymmetry is clearly observed in frequency pattern of the signals far from the bubble zone towards the trailing edge; this may reflect the difference between the transition and relaminarization physics. Also, various ranges of frequency were obtained for different transition mechanisms.

中文翻译：

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基于热膜传感器的俯仰超临界翼型边界层跃迁检测实验研究

摘要 在目前的工作中，进行了实验测试以研究使用热膜传感器经历俯仰振荡的超临界翼型上的边界层转变。测试是在不可压缩的流动下进行的。研究了三种降低的振荡频率和两种平均攻角，并讨论了这些参数对过渡位置的影响。在热膜信号上检查不同的算法以检测转变点。结果表明，在前缘附近和相对较高的攻角处形成了层流分离气泡，这导致了边界层的过渡。然而，在较低的攻角下，电压信号峰值的放大表明边界层内出现了涡旋结构，引入不同的过渡机制。此外，降低频率的增加导致过渡开始的延迟，将其推迟到更高的迎角，这扩大了上行和下行运动之间的滞后。提高降低的频率会减弱或忽略涡流扰动，从而消除信号中的尖峰。在观察到的其他重要结果中，再层化点在更高的平均攻角中移动得更快。最后，对热膜信号进行时频分析，以研究由于俯仰运动引起的跃迁光谱特征的演变。在远离气泡区朝向后缘的信号的频率模式中清楚地观察到不对称性；这可能反映了过渡物理和再层化物理之间的差异。此外，还针对不同的过渡机制获得了各种频率范围。