Vortex Structure and Scale on an Unsteady Air foil(Wing and Airfoil)

Abstract

A studies on the unsteady flow at the low Reynolds number region have been catalyzed in recent years by an interest in the Micro-Electro-Mechanical-Systems based on the concept of flow control, Micro-Air-Vehicle and micro flight robot. In order to clarify the detailed vortex structure and vortex scale on an unsteady airfoil, such as a pitching airfoil and a heaving airfoil, at low Reynolds number region. We have carried out the flow visualization around unsteady airfoils at Re=4000 using three visualization techniques, such as schlieren visualization, dye flow visualization and PIV measurement. Especially, we report the dynamic behavior of vortices shed from the leading edge, the number of vortex shedding and its scale. The small discrete vortices shed from the leading edge one after another in a pitching airfoil and a heaving airfoil at low Reynolds number region. When the non-dimensional pitching rate in a pitching airfoil and the non-dimensional heaving velocity become high, the clear discrete vortices were visualized. The number of vortex shedding from the leading edge during one pitching cycle on a pitching airfoil was strongly depends on the non-dimensional pitching rate and independent of the airfoil configuration, mean angle of attack and pitching amplitude. At the low non-dimensional pitching rate in a pitching airfoil, a lot of discrete vortices shed from the leading edge and its scale was very small. At the high non-dimensional pitching rate in a pitching airfoil and the high non-dimensional heaving velocity in a heaving airfoil, on the other hand, the number of vortex shedding from the leading edge during one cycle was low. However, its scale was about one fourth of the chord length and it did not depend on the airfoil configuration. Moreover, these vortices consisted of the re-circulation region on the suction surface.