Numerical Simulation of Sand Ripple Evolution in Oscillatory Boundary Layers

We perform simulations of sand ripple evolution in an oscillatory boundary layer flow typical of the ripple regime. The simulation framework is a parallel implementation of a three dimensional, variable density, incompressible flow solver, which solves the ensemble averaged Navier-Stokes equations on a fixed, structured grid. The sediment phase is evolved by computing hydrodynamic and inter-particle forces acting on each Lagrangian particle. Particle-particle collisions are treated with a soft sphere model incorporating both normal and tangential collision forces. Realistic and consistent coupling of the sediment to the Eulerian fluid phase is achieved through a typical inter-phase drag force term as well as the effects of volume displacement by the sediment. The Euler-Lagrange computational approach is developed in three-dimensions and its accuracy is verified using two test cases with analytic or empirically known solutions. It is then applied to simulate ripple evolution in oscillatory boundary layers and results are compared with Nielsens ripple predictor model as well as mixture-theory based Eulerian computations.