Unsteadiness of flow over airfoil has received more and more attention.The motivation of these studies is to avoid unsteadiness,which may induce undesirable effects,such as vibration,flutter,or even stall.It has been shown that open-loop external forcing can reduce flow unsteadiness.The frequency of most effective external forcing appears at the sub-and super-harmonics of the frequency of separated shear layer,separation bubble and wake.The interaction between shear layer,separation bubble and wake dominates the flow development from laminar to turbulent.The optimal external forcing frequency is difficult to find.This is due to highly complex flow behaviours at the region around the trailing edge.When separation bubble detaches,the dominant frequency are due to the instability of shear layer and the instability of wake.When flow reattaches,a third frequency comes in to play,which is due to the shedding of the separation bubble.The optimal external forcing frequency is dependent on these different frequencies and the coupling between them.In present work,unsteady numerical simulation based on Navier-Stokes equation is used to calculate flow field around an NACA 0012 airfoil.Three different angles of attack are investigated,which are 20,10,and 5 degree.These cover the detached and reattached flow conditions.In order to remove the uncertainty of turbulence model and avoid expensive computing,Reynolds number is chose to be small so that flow is laminar.Spectral analysis is used to find the dominant frequency at each flow condition.Transitional spectral analysis is limited by its spatial dependence.The signals from different probes at different locations may give different spectral features.Considering different major frequencies may exit at different regions,a spectral analysis of the whole domain is demanding.Dynamic mode decomposition(DMD)is proposed for this purpose.This method uses flow snapshots of the domain and calculates dominant frequency.To understand flow behaviours better,proper orthogonal decomposition(POD)is used to find the most energetic mode.Comparing the flow structure associated with dominant frequency and most energetic mode,it is found that flow structure of the dominant frequency is similar to flow structure of the most energetic mode.