Eulerian two-phase model reveals the importance of wave period in ripple evolution and equilibrium geometry

Abstract

The evolution of ripple geometries and their equilibrium states due to different wave forcing parameters are investigated by a Reynolds-averaged two-phase model, SedFoam, in a two-dimensional domain. Modeled ripple geometries, for a given uniform grain diameter, show a good agreement with ripple predictors that include the wave period effect explicitly, in addition to the wave orbital excursion length (or wave orbital velocity amplitude). Furthermore, using a series of numerical experiments, the ripple's response to a step-change in the wave forcing is studied. The model is capable of simulating “splitting,” “sliding,” “merging,” and “protruding” as the ripples evolve to a new equilibrium state. The model can also simulate the transition to sheet flow in energetic wave conditions and ripple reformation from a nearly flat bed condition. Simulation results reveal that the equilibrium state is such that the “primary” vortices reach half of the ripple length. Furthermore, an analysis of the suspended load and near-bed load ratio in the equilibrium state indicates that in the orbital ripple regime, the near-bed load is dominant while the suspended load is conducive to the ripple decaying regime (suborbital ripples) and sheet flow condition.