Experimental analysis of shock oscillations during shock-boundary layer interaction in transonic flow with artificially introduced sound waves

在绕翼型的跨音速流动中，导致自持激波振荡的机理尚不完全清楚。最初的恒定流动变得不稳定，并引起阻力和升力的变化。这种现象，也被称为抖振，可能导致翼型结构的临界状态。因此，考虑到虚拟试飞和支承结构重量优化的重要性，如何更好地预测导致抖振的流场扰动就显得尤为重要。然而，在文献中可以找到一些关于自助餐背后的物理机制的理论，而且在某种程度上是相互矛盾的。因此，这项实验研究的总体目标是深入了解抖动的产生和维持机制，遵循前缘噪声导致抖动不稳定的假设。为此，利用高速层析粒子图像测速仪和非定常压力测量对超临界二维翼型的跨音速绕流进行了分析。为了确定声波的影响以及激波位置、边界层、自由剪切层和声场之间的相互作用，在流场中人工引入特定性质的声波，并通过引入多孔尾缘来操纵在尾缘自然产生的声场。而具有非多孔尾缘的翼型周围的流场表现出自维持的激波振荡，而当声场被多孔尾缘操纵时，流场预计不会表现出任何激波振荡。随后，将分析是否可以通过人工引入声波来重新启动自助餐。通过对两种后缘构型的研究，可以明确尾缘产生的声学扰动对抖振现象的影响。因此，这些基础研究的目的是论证发展低噪声尾缘的必要性，不仅涉及到高升力性能，而且涉及到跨音速流型。

The mechanisms that lead to self-sustained shockwave oscillations in the transonic flow around airfoils are not fully understood, yet. The initially steady flow becomes unstable and induces variations in drag and in lift. This phenomenon, also known as buffet, can lead to a critical state for the wing structure of an airfoil. Therefore, regarding the increasing importance of virtual flight test and weight optimization of supporting structures, it is of outstanding importance to reach a better predictability of the disturbances in the flow field that lead to buffet. However, several and to some extent contradictory theories concerning the physical mechanisms underlying buffet can be found in literature. Hence, the global objective of this experimental study is to gain insight into the mechanisms that cause and maintain buffet, following the hypothesis that it is the trailing-edge noise that leads to the unsteadiness. For this, the transonic flow field around a supercritical, two-dimensional airfoil is analyzed using high-speed tomographic Particle-Image Velocimetry and unsteady pressure measurements. To determine the impact of the sound waves and of the interaction between shock position, boundary layer, free shear layer, and acoustic field, sound waves of specific properties are artificially introduced into the flow field, and the sound field naturally generated at the trailing edge is manipulated by introducing a porous trailing edge. Whereas the flow filed around the airfoil with the non-porous trailing edge shows self-sustained shock oscillations, it is expected that the flow field does not exhibit any shock oscillations when the sound field is manipulated by a porous trailing edge. Subsequently, it will be analyzed whether or not buffet can be reinitiated by the artificial introduction of sound waves. By investigating the two trailing-edge configurations, the impact of the acoustic disturbances originating at the trailing edge on the buffet phenomenon can be clarified. Hence, these fundamental investigations aim at demonstrating the need for the development of low-noise trailing edges, not only concerning the high-lift performance but also concerning the transonic flow regime.