Starting vortex strength in an impulsively started airfoil

This work characterizes the starting vortex and unsteady shear layer generated by an impulsively started National Advisory Committee for Aeronautics 0010 (NACA 0010) airfoil at angles of attack in the range of 3°–7°. Measurements are obtained by using time-resolved particle image velocimetry. The net circulation in the field is accurately predicted by Wagner's function. It is observed that the starting vortex detaches from the shear layer, while the airfoil is surging. Starting vortex circulation at detachment is presented for a range of surge speeds at pre-stall angles of attack for a given surge distance. Kinematic conditions resulting in starting vortex detachment are examined qualitatively through finite time Lyapunov exponents and quantitatively through a non-dimensional velocity ratio. Furthermore, the formation of secondary Kelvin–Helmholtz-type vortices in the shear layer is found to depend on the Reynolds number rather than the detachment of the starting vortex.