Particle based Large Eddy Simulation of vortex ripple dynamics using an Euler–Lagrange approach

A volume-filtered Large Eddy Simulation (LES) of oscillatory flow over a mobile rippled bed is conducted using an Euler–Lagrange approach. The modeling is done to complement the experimental work, which shows quasi-steady state ripples in a sand bed under oscillatory flow, as observed in unsteady marine flows over sedimentary beds. Simulations approximate the experimental configuration with a sinusoidal pressure gradient driven flow and a sinusoidally shaped rippled bed of particles. The LES equations, which are volume-filtered to account for the effect of the particles, are solved on an Eulerian grid, and the particles are tracked in a Lagrangian framework. A Discrete Particle Method (DPM) is used, where the particle collisions are handled by a soft-sphere model, and the liquid and solid phases are coupled through volume fraction and momentum exchange terms. Comparison of the numerical results to the experimental data show that the LES-DPM captures the mesoscale features of the system. The simulations reproduced the large scale shedding of vortices from the ripple crests observed in experiments. Likewise, the simulations exhibited quantitative agreement between the wall-normal flow statistics, and qualitative agreement in ripple shape evolution and sand particle entrainment behavior above the fluid-bed interface. The numerical data provide insight into the formation of shear layers and eddies within the flow field above and through the ripple and their influence on the particle transport and the dilation of the ripple mobile layer thickness during an oscillatory period. The resolution of flow structures within the particle bed in the LES-DPM simulations make the framework a unique tool for investigating ripple driven sediment transport and distribution of momentum over the ripple.