Unsteady pressure field and vorticity production over a pitching airfoil

The unsteady pressure field and the accompanying variations in the flux of spanwise vorticity from the surface were measured over a range of dimensionless pitch rates for a two-dimensional NACA 0012 airfoil model undergoing a single pitch-up motion. The results were examined to identify the mechanisms that play key roles in the initiation, development, growth, and movement of the dynamic-stall vortex. The unsteady pressure distribution over the airfoil was dominated by three features, whose emergence and evolution were used to distinguish between two classes of behavior, corresponding to low and high pitch rates. Further, it was found that the flux of vorticity from the surface originated primarily from five concentrated regions or sources, the majority of which were located over the forward portion of the airfoil surface. The behavior of vorticity flux from these sources was related to the interacting mechanisms responsible for the development of the flowfield. The change of these features in the pressure and surface vorticity flux variations with the pitch rate is described.