Particle-resolved direct numerical simulation of the oscillatory flow and sediment motion over a rippled bed

The flow field and the sediment motion generated over a rippled bed by a uniform and oscillating pressure gradient are evaluated by means of a direct numerical simulation of Navier-Stokes and continuity equations and assuming that small spherical particles, free to move, mimic the sediment grains. The numerical approach determines the flow around each sediment particle, along with the force and torque that the fluid exerts on it. Then, the particle trajectories are computed by means of Newton-Euler's laws. The parameters of the problem are the same as those of one of the laboratory experiments described by Blondeaux and Vittori (1991a) and the initial ripple profile is similar to that observed during the experiment. The numerical results are qualitatively compared with the experimental observations and support the reliability of the numerical simulation. Moreover, the results provide further valuable information on sediment dynamics under surface waves which propagate over a rippled bed. In particular, the obtained results allow a detailed analysis of the dynamics of the vortices generated by flow separation at the ripple crest and of the mechanism through which the sediments are lifted up from the bottom and carried into suspension.