The unsteady flow field around a moving airfoil has attracted significant attention in bio-hydrodynamics, micro-air-vehicles and micro flight robots. Recently, a number of studies have been performed on the flow field around airfoils with unsteady motion in low Reynolds number regions using both experiment and numerical analysis. On the other hand, it is well known that insects and aquatic animals fly or swim by skillfully controlling their wings or fins, which deform elastically, and vortices are generated around their bodies. The flow around an elastic body is treated as a coupled problem between the fluid and structure. There have been only a few reports on the experimental evaluation of vortex flow structures around an elastic moving airfoil and their fluid dynamical properties. In this study, we investigate the wake structures behind the moving elastic airfoils and the characteristics of the dynamic thrusts acting on them. The thrust producing vortex streets are clearly formed behind the combination airfoils for all phase differences. The dynamic thrust acting on the moving elastic airfoil depends strongly on the Strouhal number based on the maximum trailing edge deformation and is independent of the moving motion and phase difference. The maximum thrust efficiency of the combination airfoil is higher than that for the pure pitching and heaving airfoils and become about 0.5 at φ = 90 deg. around St = 0.3.

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