Dynamics of an oscillatory boundary layer over a sediment bed in Euler–Lagrange simulations

We investigate the dynamics of an oscillatory boundary layer developing over a bed of collisional and freely evolving sediment grains. We perform Euler–Lagrange simulations at Reynolds numbers Re_δ = 200 , 400 and 800, density ratio ρ \!p/ρ \!f = 2.65 , Galileo number Ga = 51.9 , maximum Shields numbers from 5.60 × 10⁻² to 2.43 × 10⁻¹ , based on smooth wall configuration, and Keulegan–Carpenter number from $134.5$ to $538.0$ . We show that the dynamics of the oscillatory boundary layer and sediment bed are strongly coupled due to two mechanisms: (i) bed permeability, which leads to flow penetration deep inside the sediment layer, a slip velocity at the bed–fluid interface, and the expansion of the boundary layer, and (ii) particle motion, which leads to rolling-grain ripples at Re_δ = 400 and Re_δ = 800 . While at Re_δ = 200 the sediment bed remains static during the entire cycle, the permeability of the bed–fluid interface causes a thickening of the boundary layer. With increasing Re_δ , the particles become mobile, which leads to rolling-grain ripples at Re_δ = 400 and suspended sediment at Re_δ = 800 . Due to their feedback force on the fluid, the mobile sediment particles cause greater velocity fluctuations in the fluid. Flow penetration causes a progressive alteration of the fluid velocity gradient near the bed interface, which reduces the Shields number based upon bed shear stress.