Large-Eddy Simulation of Turbulent Oscillatory Flow Over Three-Dimensional Transient Vortex Ripple Geometries in Quasi-Equilibrium

Vortex ripples induced by oscillatory flow display straight crests at equilibrium, but they develop transient three-dimensional (3-D) bedform geometries when transitioning from one equilibrium state to another due to changes in the oscillatory flow. Large-eddy simulations were performed of oscillatory flow over 3-D bedform defects associated with an increase of the orbital motion amplitude, similar to what might happen during the waxing phase of a storm. The objective is to determine the intercorrelation between flow processes and evolution of 3-D bedform defects toward an equilibrium two-dimensional (2-D) state of vortex ripples. Results are presented for oscillatory flows over fixed ripples with sinusoidal crests in the spanwise direction, which introduce slope asymmetries on opposite sides of the crests, during different transient states. It was found that steep-sloped regions of the bedforms are associated with thicker recirculation areas, whereas in more gently sloped regions thinner recirculation vortices exist. Furthermore, streamwise vortical structures and significant secondary flow develop in the vicinity of sinusoidal crests, relative to straight crests. These flow structures might be responsible for the gradual reformation of the rippled bed, as they induce higher bed shear stresses, that is, increasing bed sediment transport, and they diffuse turbulent kinetic energy to higher levels above crests, that is, increasing suspended sediment transport, relative to equilibrium 2-D ripples. Finally, the period-averaged currents around the sinusoidal crests exhibit higher velocities on the steep-sloped side than on the mild-sloped one, also likely contributing to sediment transport and bed reformation toward a 2-D configuration.