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

前人对运动翼型的非定常分离进行了大量的数值和实验研究。然而，由于运动翼型周围的现象非常复杂，非定常分离仍然存在许多悬而未决的问题。近年来，随着微电子机械系统(MEMS)的发展，对低雷诺数区域分离控制的研究越来越受到人们的关注。本文通过流动显示和非定常流体力的测量，揭示了低雷诺数下俯仰翼型非定常分离涡的动力学行为及其所受非定常流体力的特性；在低雷诺数下，在较大攻角和无因次俯仰速率下，前缘翼型上的分离涡瞬间重新附着到吸力面上。再附着对动态升力有很大影响。此时，在俯仰的翼型后面形成了一团漩涡。在较低的无量纲俯仰率下，前缘开始形成一个大尺度涡，由大量的离散涡组成。另一方面，在较高的无因次俯仰速度下，吸力面上的回流涡是由一些离散的涡形成的。在NACA0020中，在较高的无量纲俯仰率下，俯仰运动不仅增加了动态升力，而且还增加了升阻比，因此可以显著地增强流体力特性。

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.