For many decades,scientists have been aware of the danger of structural failure because of aerodynamically driven oscillations.As a classic case of this phenomenon,flutter is common in aerospace industry.It is well known that an airfoil having pitch and plunge degrees of freedom can easily encounter flutter for certain combinations of the spring stiffness and elastic axis.In the traditional study,researchers focused on suppressing the flutter.Recently,the developments of micro aerial vehicles(MAVs)have resulted in increased interest in developing novel flow control method.Research on the flow induced vibration of airfoil shows that this method could enhance the lift and reduce the drag.However,little work has been done on the nature of high lift generation.This paper presents some simulations of two-dimensional airfoil undergoing self-excited oscillating at low Reynolds number.A two-degree-of-freedom dynamic model is introduced to describe the airfoil oscillation,and Runge-Kutta method is applied to solve the equation of motion.At the same time,the Characteristic Based Split scheme combined with Arbitrary Lagrangian-Eulerian framework is developed for solving the N-S equation to simulate fluid field.Then the loose coupling is employed to deal with the fluid-structure interaction.The results show that the pitching and plunging of airfoil can improve the aerodynamic performance dramatically.The nonlinear phenomena of lock-in and phase-switch are captured.Meanwhile,the limit cycle and bifurcation of the lift coefficient and displacement are analyzed with trajectory,phase portrait and Poincare section mapping.Those nonlinear behaviors reveal the nature of lift enhancement.