Vortex Dynamics on an Airfoil Pitching at High Angles*

The flow field around a NACA0012 airfoil undergoing large amplitude sinusoidal pitching is investigated using Particle Image Velocimetry (PIV). The airfoil is pitched symmetrically about the quarter chord point with a peak angle of ±40° at reduced frequencies of k=0.2-0.6 and Re_c=12000. Multiple experimental Fields of View (FoV) were phase averaged and combined to provide a domain 2.5c x 1.7c, with Δx= 0.004c and Δt=1/(200f). The dynamics are investigated by tracking the vortical structures induced by the airfoil motion. In all cases a strong leading edge vortex (LEV) is formed along with several weaker vortices in the boundary layer closer to the trailing edge. The LEV forms later in the cycle, and with a higher initial peak vorticity, as k increases.  However, the circulation of the LEV appears to be independent of k initially once it has formed. Later in the pitching cycle, the LEV combines with the vortices that form towards the trailing edge for lower k values, increasing the circulation of the LEV. The cause appears to be the dynamics at the trailing edge and the trajectory of the leading edge vortex, both of which are dictated by the reduced frequency of the airfoil pitching motion.