Numerical simulations of the wave bottom boundary layer over sand ripples

Surface gravity waves propagating in the nearshore produce an oscillatory freestream potential flow near the seabed and commonly cause regular two-dimensional sand ripples to be formed. The existence of the ripples causes complex turbulent flows to evolve in the boundary layer, which can dissipate significant energy from the surface waves. A time-dependent DNS model is used to solve the Navier-Stokes equations in three dimensions on a transformed curvilinear grid for specified wave parameters. When compared to smooth beds, simulations over wavy topographies demonstrate that turbulence is enhanced due to flow separation in the lee of the ripple crests. Shear instabilities during phases of weak flow and vortex shedding in times of strong flow work together to sustain turbulence throughout the wave period. Resulting flows over ripples exhibit an increase in boundary layer thickness and dissipation rates, as energy is lost to viscous effects in maintaining turbulence.