Low-Reynolds-number effect on the aerodynamic characteristics of a pitching NACA0012 airfoil

The effect of a low Reynolds number in the range of 2.0 × 10 4 < Re c < 5.0 × 10 4 on the aerodynamic characteristics of a pitching NACA0012 airfoil was investigated. Reverse flow and near wake were visualized by smoke-wire flow visualization, and lift and pressure drag coefficients were estimated by measuring unsteady pressure through pressure distortion correction. A NACA0012 airfoil sinusoid-pitched at quarter chord was employed, and its mean angle of attack and oscillation amplitude were α = 0 ° and 6° respectively. The test Reynolds numbers were Re c = 2.3 × 10 4 , 3.3 × 10 4 and 4.8 × 10 4 with a fixed reduced frequency of K = 0.1 . Through reverse flow visualization, the first and second trailing-edge vortices and mushroom structure depending on the Reynolds number were observed. In lift and pressure drag coefficients, hysteresis loops were comparatively varied with the Reynolds number. As a result, the phase angle, at which boundary-layer events occurred, was in inverse proportion to the increase in Reynolds number. This result implies that the increase in Reynolds number promotes the occurrence of boundary-layer events such as laminar separation and transition.