DNS of Oscillatory Boundary Layer Over a Closely Packed Layer of Sediment Particles

Lack of accurate criteria for onset of incipient motion and sediment pickup function remain two of the biggest hurdles in developing better predictive models for sediment transport. To study pickup and transport of sediment, it is necessary to have a detailed knowledge of the small amplitude oscillatory flow over the sediment layer near the sea bed. Fully resolved direct numerical simulations are performed using fictitious domain approach [1] to investigate the effect of a sinusoidally oscillating flow field over a rough wall made of regular hexagonal pack of spherical particles. The flow arrangement is similar to the experimental data of Keiller et al. [2]. Transitional and turbulent flows at Re_δ = 95,150,200 and 400 (based on the Stokes layer thickness, $δ=2ν/ω$⁠) are explored over a range of non-dimensional sphere sizes. Turbulent flow is characterized in terms of coherent vortex structures, Reynolds stress variation, turbulent kinetic energy budget and PDF distributions. The nature of unsteady hydrodynamic lift forces on sediment grains and their correlation to sweep and burst events is also reported. The dynamics of the oscillatory flow over the sediment bed is used to understand the mechanism of sediment pick-up.