Dynamic Behavior of Separation Vortices around a Pitching and Heaving Airfoils and Dynamic Forces

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Many studies on unsteady separation around a moving airfoil have been carried out with the numerical and experimental approaches. Most of them have been performed for high Reynolds number region over Re = 10^6. Recently, a few studies on the unsteady flow at low Reynolds number region have been developed by the interest in the micro-electro-mechanical-systems (MEMS) based on the concept of flow control. On the other hand, the propulsion mechanisms of aquatic animals, birds and insects have also been attracted. However, the relationship between the vortex structure behind the moving airfoil and the dynamic thrust acting on them for low Reynolds number region have been understood sufficiently. In this study, the wake structure behind the moving airfoil and the dynamic behaviors of vortices near the trailing edge of them have been visualized in a water tunnel for low Reynolds number region, and the dynamic thrust acting on them have been measured by using a six-axes sensor.In the case of quarter-chord axis with higher non-dimensional pitching rate and large pitching amplitude, the distance between the vortices in the thrust producing vortex street was much shorter. It was considered that the jet was produced behind a pitching airfoil. As a result, the large dynamic thrust was produced in almost regions during one pitching cycle and the averaged dynamic thrust during one pitching cycle increased as the non-dimensional pitching rate increased. It was necessary to form the thrust producing vortex street with short distance between vortices in order to generate the large dynamic thrust.