Numerical comparison of scalar transport over permeable and non-permeable rippled beds using κ-ε RANS models

Sediment boundaries influence the velocity field and transport of scalar quantities in the wave bottom boundary layer through their roughness scales, sediment mobility, and permeability. The occurrence of bedforms in the sediment layer, such as ripples, also introduces a form drag associated with pressure gradients generated by the ripple geometry that significantly affect momentum transfer in the region. The complex interaction of these elements affects the temporal and spatial distribution of momentum fluxes and the transport of substances near the sediment-water interface, yet their individual contributions is difficult to assess. To this end, the effect of bedform geometry was isolated by modelling the rippled coastal bottom boundary as non-permeable and fixed. A model that couples the κ-ε Reynolds–Averaged Navier–Stokes equations with an advection–diffusion solver was applied using the OpenFOAM environment. The model was used to carry out simulations of oscillatory flow with constant scalar sources at various location along the rippled bed to assess the local effects of a boundary-induced pressure gradient on passive scalar mixing. The simulations were replicated using the two-phase model SedFOAM with a Johnson-Jackson particle pressure model to introduce the effects of permeability and sediment mobility. The introduction of a permeable mobile boundary was found to significantly alter the velocity field over the rippled bed, evidenced by increased boundary layer thicknesses and phase differences as high as 45° at ripple flanks. This, combined with sediment mobility and suspension near the scalar sources, led to spatial and temporal differences as high as 30% in both scalar concentration and fluxes. These differences are thus attributed to the sediment mobility and permeability allowed by the two-phase model.