An unsteady flow in a low Reynolds number region attracts attention in recent years. Various experiments/numerical analyses have been conducted in wake flow fields of objects with unsteady motions in low Reynolds number regions have been studied and reported recently. The authors clarified vortical structures in a wake of a rigid airfoil (NACA0010) with pitching, heaving and an unsteady motion with these motions combined quantitatively. The purpose of this study is to evaluate quantitatively the vortex flow formed in the wake of a flat airfoil whose edge deforms elastically by a PIV measurement by giving a heaving motion to the flat plate in a low Reynolds number region. A clear thrust producing vortex street equivalent to the airfoil shape can be formed by giving elasticity to the latter part of a rigid flat airfoil. vortices forming the thrust producing vortex street is extremely large and their vorticity is equivalent to that of NACA0010. In the mainstream of a vortex street forming a propulsion power generating vortex street, the vortex interval in the vertical direction is comparatively large and an interference of the vortices rolling up from the pressure and suction sides is small therefore the accelerating flow induced by these vortices becomes as large as the mainstream. The dynamic thrust acting on the elastic flat plate is greater than that of NACA0010. Because vortices with high vorticity roll up and these vortices are aligned with a comparatively large vortex interval in the vertical direction therefore a large accelerating flow is induced in the wake.

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