The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.To understand the frequency lock-in mechanism of flow separation control of an airfoil at low Reynolds number, a systematic analysis is performed by extracting the Lagrangian Coherent Structures (LCSs) from the unsteady flow. The actuation is considered via periodic morphing surface, and the dynamical behaviors between morphing surface and unsteady flow are studied from the viewpoint of fluid transport. Attention is drawn to fluid transport and lift improvement when the actuation frequency is locked onto the vortex shedding frequency. The results show that the fluid particle near the actuator is accelerated by the actuation and interacts with the slow fluid particle in boundary layer on the airfoil surface. The so-called stirring jet mechanism is observed, whereby a cusp structure is formed like a jet acting on the flow, which enhances the fluid transport from main stream into separation zone by reducing dead air zone effectively. The results also show that the actuation frequency is found to be the key factor for lift enhancement and determines the cusp structures and the vortex strength on the upper surface of the airfoil
