A wind tunnel investigation of low speed flow over flapping airfoils and airfoil combinations was performed using flow visualization and laser Doppler velocimetry. Specifically, three cases were studied: A NACA0014 airfoil oscillating in a sinusoidal plunge mode, A NACA0014 airfoil oscillating in a sinusoidal plunge mode near a ground plane, and two NACA0014 airfoils arranged in a biplane configuration and oscillating in counterphase in a sinusoidal plunge mode. The plunge amplitude-to-airfoil chord ratio was 0.4, the reduced frequency of oscillation was 1.0 and the Reynolds number based on airfoil chord was set at 8760. Conditionally sampled measurements of the axial flow velocity were taken at numerous flow field points providing detailed information about the flow features generated by this type of flapping motion. These measurements were complemented by time-averaged flow field data and by visualization of the instantaneous flow field at various points during the flapping cycle. Furthermore, the thrust generated by the sinusoidal plunge motion was measured with a laser range finder. The results shows that vortex shedding occurs both from the airfoil leading and trailing edge.
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