| Seabeds in shallow coastal environments are composed of different patterns of bedforms, ranging from large sand bars and dunes to small-scale sand ripples. Sand ripples modify the wave-current boundary layer structure and are extremely important because they influence the velocity and pressure fields through flow separation and vortex formation, which in turn enhance physical processes at the sediment-water interface, such as sediment entrainment. Sand ripples and their dynamic properties also modify the hydraulic roughness, and hence new models that consider ripple geometry and dynamics must be developed if their effects are to be accurately represented in under-resolved circulation models.;The model is employed to study dynamics of sand ripples in a combined wave-current flow at the laboratory scale. By means of high grid resolution in the near-wall region, the model allows simulation of detailed turbulence structure in the turbulent boundary layer which in turn accurately simulates bedform initiation. The bedform initiation induced by the turbulence further evolves into rolling grain ripples and vortex ripples, thereby enabling detailed analysis of bedform dynamics, sediment transport and flow turbulence during different stages of ripple evolution. The simulation results are compared to sonar images of laboratory-scale sand ripple evolution, and the results demonstrate the capability of the model to accurately capture real physical features of sand ripples in combined wave-current flows.;To obtain a better understanding of sand ripple dynamics, a numerical code is implemented to simulate the ripple development. The code solves the three-dimensional Navier-Stokes equation in generalized curvilinear coordinates with a dynamic mixed model (DMM) as the sub-grid turbulence model in a large-eddy simulation (LES) framework. Due to the low concentration and the small bulk Stoke's number, the sediment concentration is calculated through the use of the Eulerian approach with a bottom boundary condition that accounts for sediment flux from the bed without requiring specific details of the underlying turbulence model. A consistent discretization of conservative momentum and scalar transport is developed, which allows flow simulations using generalized curvilinear grids that move arbitrarily in three dimensions while maintaining the desired properties on a stationary grid, such as constancy, conservation, and monotonicity. To calculate changes in the bed elevation, a mass-balance equation of bed sediment that accounts for sediment erosion, deposition, and gravity-induced avalanche flow is implemented. Using this approach, motion of the bed affects the flowfield in a coupled hydrodynamic moving-bed simulation in which bed features evolve due to resolved details of the turbulent flow. |
