Numerical simulations were performed based on the open source computational fluid dynamics (CFD) software OpenFOAM.The numerically obtained velocity profiles over the dune were compared with the field measured results to validate the accuracy of the numerical simulation.On this basis,the velocity profiles at different heights over the dune as well as the velocity vector field were obtained.The sensitivity of the numerical simulation result to the computational mesh was discussed.Furthermore,the effect of the aspect ratio of the dune on the flow behavior was examined.The results show that the standard k-ε turbulence model in OpenFOAM can be used to predict the velocity shift behavior of the flow field over the low sinusoidal dune.In the inner layer close to the dune surface,a phase advance of velocity curve relative to the dune profile is observable within a certain distance over the dune.As the height increases,the phase advance decreases and the velocity begins to flatten.The mesh number has little effect on the velocity profile in the outer layer,while it affects greatly on the velocity profile in the inner layer.It is worth noting that sufficient mesh number is required to attain accuracy information of the flow field over the dune. When the dune width keeps constant,with the decrease in the width-to-height ratio,the phase advance of the velocity profile over the dune increases and the recirculation zone could appear at the leeward.It is found that when the width-to-height ratio becomes smaller,the length and the intensity of the recirculation zone are greater.