Ripples are widely formed in river channels and coastal regions, the bed-forms and vortex dynamics accompanied with them are quite important because they are responsible for frictions and sediment transports. The coherent vortex structure play a key role in the process, so how to model them is needed for understanding the formation process of sand ripples. In this paper, 2D Large-Eddy-Simulation (LES) was used to predict the flow structure and the dynamics of coherent vortex. The numerical results show a complete process of formation, evolvement and disappearance for the cases of unidire ctional and oscillatory flows over symmetic and asymmetric ripples