Dynamic Behavior of Separation Vortices around a Pitching Airfoil and Dynamic Forces

关于翼型绕流的非定常分离问题，国内外学者进行了大量的数值和实验研究。然而，由于绕动翼型的非定常分离现象非常复杂，因此，有关非定常分离的研究仍有许多问题没有得到解决。近年来，随着微机电系统（MEMS）技术的发展，低雷诺数区分离控制的研究成为热点。本文通过流动显示和非定常流体力测量，揭示了低雷诺数区俯仰翼型非定常分离涡的动力学行为和作用于分离涡上的非定常流体力的特征。在较大攻角和无量纲俯仰速率下，来自前缘的分离涡立即重新附着在吸力面上。再附着对动升力影响很大。此时，在俯仰机翼后面形成了一团涡流。在较低的无量纲俯仰角速度下，由大量离散涡形成前缘大尺度涡。另一方面，在较高的无量纲俯仰角速度下，一些离散涡在吸力面形成回流涡脱落。在NACA 0020中，俯仰运动可以显著地增强在更高的无量纲俯仰速率下的流场力特性，因为它不仅增加了动升力，而且还增加了升阻比。

Many studies on unsteady separation around a moving airfoil have been carried out in the numerical and experimental studies. However, the many unsolved problems on unsteady separation still remain because the phenomenon occurring around moving airfoils is very complicated. Recently, the study on separation control at low Reynolds number region has been interested by developing the micro-electro-mechanical-systems (MEMS). In this study, the dynamic behaviors of unsteady separation vortices around pitching airfoils and the characteristics of unsteady fluid forces acting on them at low Reynolds number region have clarified by the flow visualization and the unsteady fluid forces measurement.In the pitching airfoil at low Reynolds number region, the separation vortices from the leading edge reattaches instantly to the suction surface at higher angle of attack and non-dimensional pitching rate. The reattachment influences much on the dynamic lift. At this time, a cloud of vortices is formed behind a pitching airfoil. At lower non-dimensional pitching rate, a big scale vortex from the leading edge is formed by a lot of discrete vortices. On the other hand, a recirculation vortex shed on the suction surface at higher non-dimensional pitching rate is formed by some discrete vortices. In NACA0020, the pitching motion can enhance considerably the characteristics of fluid forces at higher non-dimensional pitching rate because it increases not only dynamic lift but also lift-drag ratio.