Investigation of sediment transport in an oscillatory boundary layer using Eulerian-Lagrangian simulations.

Areas where the sea bottom is made of loose sediments are particularly prone to erosion caused by the action of waves, which is exacerbated by rising sea levels. The successful modeling of these effects requires an in depth understanding of the interaction between turbulent flow features and sediment grains. In this talk, we discuss the effects of oscillating pressure gradients caused by surface waves on the formation of bedforms and grain entrainment in a sandy seabed using high-fidelity Eulerian-Lagrangian simulations. In this approach, conservation equations for the fluid phase are solved on a Cartesian grid, while each individual sand grain is tracked and updated in a Lagrangian frame. A particle bed is placed at the bottom of the domain with height 25 particle diameters, and a particle diameter of 500[endif]-->m. Four cases are considered where the period of oscillations is maintained constant at 7s and the magnitude of the pressure gradient is varied to yield the Reynolds numbers 100, 200, 400 and 1790. The first three cases yield an oscillatory boundary layer in the laminar regime, whereas the fourth case falls under the intermittently turbulent regime. Statistics from the case at Reynolds 1790 are shown to agree with prior experimental observations, thus validating the numerical approach. The bedforms and particle transport statistics at the four different Reynolds numbers are compared and related to the varying turbulence intensities in the carrier fluid. Furthermore, to understand the role of the sediment grains in modifying flow features, auxiliary simulations with no particles, i.e., simulations of oscillatory boundary layers on a smooth flat wall, are conducted at the four same Reynolds numbers and compared with the simulations with sandy beds. These comparisons quantify the effect of particle dynamics upon the oscillating flow.