Numerical simulations of oscillatory flow over rough beds with different shapes and angularity*

The seabed hydrodynamics in coastal areas is characterized by the so-called wave bottom boundary layer, an oscillatory motion generated by free-surface gravity waves. The wave bottom boundary layer affects the whole seabed ecosystem, from remodeling the bed morphology to the transport of nutrients and substances dispersed in water. Despite significant research progress made over the years, a complete understanding of this flow is still lacking. A key challenge is that the seabed is typically rough and made of particles (sediments) that, depending on the location, have different shapes and angularities. Most of the previous work in the literature has focused spherical particles (representative of non-cohesive silica sands), while less attention has been devoted to other non-spherical sediment shapes that are common, for example, in tropical areas. Nevertheless, the particle geometry and spatial distribution are likely to have a significant impact on the bottom hydrodynamics. In this work, we consider beds with particles in the shapes of either ellipses, disks, or pyramids under the same oscillatory flow conditions. The study is conducted using direct numerical simulations coupled with an immersed boundary method to treat the bed. The oscillation is imposed through a sinusoidal velocity boundary condition on the rough bed, which mimics the shear-driven flow commonly observed in oscillating tray rigs experiments. The objective of this study is to investigate the dependence of turbulent flow statistics on the particle shape and angularity towards an improved understanding of the wave bottom boundary layer dynamics.