Numerical investigation of particle laden flow in oscillatory channel and its implication to wave induced fine sediment transport

Studying particle-laden oscillatory channel flow constitutes an important step towards understanding practical applications such as sediment transport in coastal environments. This study aims to take a step forward in our understanding of the role of turbulence on fine particle transport in an oscillatory channel and the back effect of particles on turbulence modulation using an Eulerian-Eulerian framework. The governing equations are solved by a 3D pseudo-spectral Navier-Stokes solver previously used for direct numerical simulation (DNS) of turbulent flows. The Eulerian-Eulerian modeling framework, chosen in this study, makes the two-way coupled problem computationally feasible without compromising accuracy as long as particles are of small response time (Shotorban & Balachandar, 2006). As a first step, the instantaneous particle velocity is calculated as the superposition of local fluid velocity and particle settling velocity with higher order particle inertia effect is neglected. Correspondingly, the only modulation of carrier flow is due to particle-induced density stratification. In the present investigation at moderately energetic conditions, the volume-averaged concentration and particle settling velocity are varied over a realistic range. The simulation results reveal critical processes due to different degrees of particle-turbulence interaction. Essentially, four different regimes of particle transport for the given Reynolds number are observed within the range of settling velocity (Vs), which represents the particle size, and volume-averaged concentration,: i) virtually no turbulence modulation in the case of very dilute condition, ii) slightly modified regime where slight turbulence attenuation is observed near the top of oscillatory boundary layer. However, in this regime significant change can be observed in the concentration profile with the formation of a sharp gradient in ensemble averaged concentration profile, lutocline; iii) regime where flow laminarization occurs during peak flow, followed by shear instability during flow reversal. Significant reduction of oscillatory boundary layer thickness is also observed; iv) complete laminarization due to strong particle-induced stable density stratification. These regimes revealed by the numerical simulations are consistent with the limited field observations. However, some of the flow features, especially regarding regimes iii and regime iv, cannot be predicted by the conventional Reynolds-averaged models. It is concluded in this study the turbulence resolving simulation tool developed in this study is useful to understand wave-induced fine sediment transport processes. Apart from particle settling velocity and particle concentration, the developed model can be further enhanced to include particle inertia effects and inter-particle influence on the flow.