Trailing-edge boundary layer characteristics of a pitching airfoil at a low Reynolds number

The periodic variation of the flow pattern and the boundary layer characteristics near the trailing edge of a pitching NACA (National Advisory Committee for Aeronautics) 0012 airfoil are examined experimentally in this study. The mean pitching angle and pitching amplitude were 0° and 7.5°, respectively, and the reduced frequency varied from 0.094 to 0.157. Static cases with various angles of attack were tested for reference. All the tests were conducted at a low Reynolds number of 66000. Particle image velocimetry was used as the primary measurement for flow visualization and boundary layer analysis. The periodic development of the flow pattern close to the trailing edge within one pitching cycle was monitored. The instantaneous flow patterns acquired at certain phase angles are compared with the static cases, demonstrating the influences of the pitching motion, including the lag effect on the boundary layer. A vortex street is observed as the airfoil achieves certain pitching angles, leading to the largest boundary layer displacement thickness at the corresponding side. As the pitching angle increases, transition of the boundary layer at the trailing edge occurs at a higher incidence compared with the static cases. Increasing the pitching frequency would lead to the decrease in the vortex structure strength and delayed boundary layer transition near the trailing edge.