Studying Sediment Transport in Oscillatory Sheet Flow by using a Large Eddy Simulation Eulerian Two-phase Model

Coastal morphological evolution is caused by a wide range of coupled cross-shore and alongshore sediment transport processes associated with short waves, infragravity waves and wave-induced currents. However, the fundamental transport mechanisms occur within the thin bottom boundary layer and are dictated by turbulence-sediment interaction and inter-granular interactions. Recently, a turbulence-averaged two-phase Eulerian sediment transport model, called sedFoam, was developed and validated for U-tube sheet flows (Cheng et al., 2016). With closures of particle stresses and fluid-particle interactions, the model is able to resolve full profiles of sediment transport from the immobile bed, to the concentrated near-bed transport layer and up to dilute transport without conventional bedload/suspended load assumptions. In this study, we further extend this model with a 3D large eddy simulation (LES) approach, where substantial amount of the turbulence-sediment interaction is directly resolved. In the present LES model, a dynamic Smagorinsky sub-grid stress closure is adopted for both fluid and sediment phases, and the sub-grid contribution to the fluid-particle interactions is included by a sub-grid drift velocity in drag model (Ozel et al., 2013). The model is validated with high-resolution measurements in a unidirectional steady sheet flow experiment (Revil-Baudard et al. 2015). We further apply the LES Eulerian two-phase model to medium and fine sand in oscillatory sheet flows (O'Donoghue & Wright, 2004). The LES model performance for the medium sand is similar to the turbulence-averaged model, except that the LES model seems to be superior to the turbulence-averaged model during acceleration and decelerating phases. In addition, the LES model is able to capture the enhanced transport layer thickness for fine sand, which may be related to the burst events near the flow reversal. We further confirm that this phenomenon is absent for medium sand.