Leading-Edge Flow Structure of a Dynamically Pitching NACA 0012 Airfoil

The leading-edge flow structure of the NACA 0012 airfoil is experimentally investigated under dynamic-stall conditions (; , 22.4 deg; ) using planar particle image velocimetry. The airfoil was dynamically pitched about the one-fourth chord at a reduced frequency, . As expected, on the upstroke, the flow remains attached in the leading-edge region above the static stall angle, whereas during downstroke, the flow remains separated below the static stall angle. A phase-averaging procedure involving triple velocity decomposition in combination with the Hilbert transform enables the entire dynamic-stall process to be visualized in phase space, with the added benefit of the complete phase space composed of numerous wing oscillations. The formation and complex evolution of the leading-edge vortex are observed. This vortex is seen to grow, interact with the surrounding vorticity, detach from the surface, and convect downstream. A statistical analysis coupled with instantaneous realizations results in the modification of the classical dynamic-stall conceptual model, specifically related to the dynamics of the leading-edge vortex.