Dynamics of a Current Driven by Oscillatory Flow Over a Multiscale Rough Bottom

Generation of currents by interaction of waves with rough bottoms was investigated using numerical simulations. Oscillatory flow over symmetric, asymmetric, and multiscale bottoms weresimulated using Reynolds averaged Navier Stokes simulations. Velocity, turbulence and pressurefields were ensemble-, spatial-, and time-averaged to investigate the momentum balance for thecurrent. For the symmetric bed, the time-averaged force on the flow was negligible and no currentwas generated. The asymmetric bed generated unequal vortex shedding between half-cycles, creating a non-zero time-averaged form drag and a current. Multiscale topographies generated stilllarger currents. Time-averaged bottom pressure forces accelerate the flow in the near-bed canopylayer. Above, in the roughness sublayer, dispersive stress transports time-averaged momentum upwards. In the overlying boundary layer, the turbulent stress accelerates the current. These resultsdemonstrate that purely oscillatory flow over rough bottoms can generate currents if the bed isasymmetric and contains length scales in the drag-dominated regime.