Dynamics of an oscillatory boundary layer over a cohesionless bed of particles at increasing Reynolds number in Eulerian-Lagrangian simulations.

We investigate the effects of oscillating pressure gradients over a cohesionless particle bed on the formation of bedforms and grain entrainment using high-fidelity Eulerian-Lagrangian simulations. The bed is created by allowing sand particles of size 550[endif]--> to sediment onto a solid wall, forming a sediment layer of height approximately equal to 25 particle diameters. 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 800. 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. The first three cases yield an oscillatory boundary layer in the disturbed laminar, whereas the fourth case falls under the intermittently turbulent regime. The sediment transport modes, i.e saltation and suspension are analyzed in all cases, as well as the bedform generated by the oscillatory flow. With the presence of a bed, the flow at Reynolds 100 is seen to largely behave as a laminar oscillatory boundary layer with a shift in the wall location corresponding to the bed height. Whereas the flow remains laminar at Reynolds 200 and 400 without the particle bed, intermittent vortex shedding is observed when the particle bed is included, caused by small bedforms. At Reynolds 800, vortex shedding intensifies and is observed throughout the period of oscillation. Further, the bedform shows dynamic particle dunes, with the top layer of particles saltating across the bed at phases with the highest vorticity, before returning to rest as the intensity of the vortical structures is reduced.