The detailed behaviour of the unsteady boundary layer and stall events occurred on a sinusoidally oscillating NACA 0012 airfoil at Re = 1.35 x 105 was investigated experimentally by using closely-spaced multiple hot-film sensor arrays. The hot-film measurements were supplemented by surface pressure measurements, hot-wire wake velocity surveys and smoke-flow visualizations. Three typical oscillation parameters: attached flow, light stall, and deep stall were tested. Special attention was focused on the non-intrusive identification of the spatial-temporal progression of the locations of the boundary-layer transition and separation and reattachment points for a range of oscillation frequencies and amplitudes both prior to, during, and post the stall. The results show that for an unsteady airfoil, the reduced frequency of the oscillation was found to be highly significant and only small values of reduced frequency were required to delay the onset of the various boundary-layer events, and to produce significant variations and hysteresis in the peak values of lift and drag forces and the pitching moments, which are fundamentally different from their static counterparts. Lift stall was observed to occur when the leading-edge vortex reached 90% of the chord, while moment stall occurred at the end of the upward spread of the trailing-edge flow reversal. The convection speed of the leading-edge and the secondary vortices were also reported. Dynamic stall was found to be caused by an abrupt turbulent separation near the leading-edge region and not with the bursting of the laminar separation bubble, as is commonly observed for a static NACA 0012 airfoil; the result being the initiation, growth and convection of an energetic leading-edge vortex. Moreover, the detection of the sudden turbulent breakdown could serve as an indicator for dynamic stall detection and control. The results reported on here provide a deepened insight into the detailed nature of the unsteady boundary-layer events as well as the stalling mechanisms at work at different stages in the dynamic-stall process.
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机译：使用紧密间隔的多个热膜传感器阵列，通过实验研究了Re = 1.35 x 105时正弦振荡的NACA 0012机翼上不稳定边界层和失速事件的详细行为。通过表面压力测量，热线唤醒速度调查和烟流可视化来补充热膜测量。测试了三个典型的振荡参数：附加流量，轻停顿和深停顿。特别关注的重点是在失速之前，之中和之后的一系列振荡频率和振幅的边界层过渡，分离和重新附着点的位置的时空变化的非侵入性识别。结果表明，对于不稳定的机翼，发现降低的振荡频率非常显着，并且仅需要较小的降低频率值即可延迟各种边界层事件的发作，并产生明显的变化和滞后现象。升力和拖曳力的峰值以及俯仰力矩与静态静力根本不同。当前缘涡旋达到弦的90％时，观察到升力失速，而后缘流逆转的向上扩展结束时出现了失速。还报道了前沿涡旋和次级涡旋的对流速度。发现动态失速是由前沿区域附近的湍流分离引起的，而不是由层流分离气泡的破裂引起的，正如静态NACA 0012机翼通常会观察到的那样；结果是充满活力的前沿涡旋的开始，生长和对流。此外，湍流突然破坏的检测可以作为动态失速检测和控制的指标。此处报告的结果提供了对不稳定边界层事件的详细性质以及动态失速过程中不同阶段工作的失速机制的深入了解。
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