For pitching and plunging airfoils, the lift coefficients are usually higher than those of stationary airfoils, but they are still on the order of one1. In the present experiment, an airfoil placed in an unsteady stream is shown to have a phase-averaged lift coefficient of more than ten.
The unsteady free stream was achieved in a vertical water channel2. The unsteady flow around a stationary NACA 0012 airfoil at post-stall angle (a=20 deg) was visualized by illuminating air bubbles with a thin light sheet (Figure 1a). (Also shown in Figure 1b is the flow visualization in the steady free stream). The measured free stream velocity varies in a sinusoidal form with an amplitude of 70% (Figure 2). At a special frequency2, the maximum phase-averaged lift coefficient (Figure 3) can be more than 10, which is one order of magnitude higher than conventional values. At the beginning of the deceleration (t/T=0), the shear layer is already separated from the leading edge and reattaches further downstream. During the deceleration, this separation bubble grows to a coherent vortex (t/T=0.25) and becomes very strong in the middle of the cycle when the free stream velocity is minimum (t/T=0.5). The phase-averaged lift coefficient increases as the size of the separation vortex increases. At the middle of the cycle, t/T=0.5, the phase-averaged lift coefficient can be as high as 14.
This work is supported by AFOSR.
1. McCroskey, W.J. and Pucci, S.L. Viscous-inviscid interaction on oscillating airfoils, AIAA Paper 81-0051.
2. Gursul, I. and Ho, C.M., 1990 An unsteady airfoil with CL larger than 10, (Submitted to AIAA Journal).
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