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

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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.