Behaviors of vortices emitted from the laminar separation bubble near airfoil stall when leading-edge is vibrated actively

Behaviors of laminar separation bubbles formed over an airfoil strongly affect airfoil stall characteristics. When the angle of attack is near the stall angle, it has been reported that the flow around a NACA0012 airfoil oscillates between an attached flow with the laminar separation bubble and a largely separated flow at the chord Reynolds number of 1.3 105. This characteristic flow is called quasi-periodical oscillation. Near airfoil stall, attempts are being made to control flow separation. As one of the active flow control methods, installing a dynamic bubble burst control plate, which is a protruding flat plate placed near the leading-edge, and vibrating it actively is effective at suppressing an airfoil stall. However, if active vibration is only applied without protruding the plate, the stall suppression effects are unknown. In this paper, instantaneous surface pressure and velocity near the leading edge were measured when the NACA0012 airfoil surface near the leadingedge is vibrated actively. Flow visualization and time-resolved surface pressure measurements were also conducted. The Reynolds number based on the airfoil chord length was 1.3 105. It was indicated that there is a stall suppression effect when the leading-edge is vibrated, while maintaining the quasi-periodical oscillation of the flow near stall. It was also found that active vibration near the leading edge changed behaviors of the vortical structure emitted from the laminar separation bubble and decreased the thickness of the laminar separation bubble formed on the airfoil.