Numerical Simulation of Ripple Evolution under Turbulent Flow Using a Coupled LES and DPM Model

Abstract

The processes of ripple evolution are studied through numerical simulation using a coupled computational fluid dynamics (CFD)–discrete particle method (DPM) model with focus on discussing the effect of the size of the computational domain on ripple evolution. Ripple-induced form resistance and bed load transport rate are also discussed. Fluid movement is simulated using the CFD computation with the introduction of large eddy simulation for turbulent closure. The movement of sediment particles is simulated using the DPM. It is found from the results of simulation that for a two-dimensional case the ripple evolution involves three stages from wavelet, merging of wavelets to equilibrium. The ripple sizes increase during the merging process and reach a stable state at the end of the merging process. The ripple sizes obtained in the final equilibrium stage are closely related to the streamwise size of the computational domain and have an upper bound for given sediment and flow conditions. If the streamwise size of the computational domain is set to approximately 6 times the ripple length or beyond, the discrepancies among the equilibrium ripple lengths obtained from using different streamwise size of computational domain can be below 9.2%. The ripple lengths modeled in the wavelet stage agree well with the experimental results. During the process of ripple merging, an abrupt reduction in the form resistance and an increase in the bed load transport rate are observed.