Toward modeling flocculation in turbulence-resolving simulations for cohesive sediment transport

Direct Numerical Simulation (DNS) is the most accurate technique that provides detailed information for studying the fundamental physics of turbulent flows. This makes DNS an attractive tool to integrate with fine sediment transport equations to investigate the mechanisms of transport in the bottom boundary layer. However, most existing models assume a constant settling velocity for the sediment by neglecting the flocculation processes. Flocculation can change the settling velocity of cohesive sediment depending on sediment properties, sediment concentration and turbulent energy dissipation. The settling velocity further affects the vertical distribution of sediment and turbulence attenuation due to sediment-induced density stratification. As a first step, we evaluate a weakly coupled method between a Eulerian turbulence-resolving fine sediment transport model (Yue et al., 2020, Journal of Geophysical Research) and a floc size class-based flocculation model (Verney et al., 2011, Continental Shelf Research) for a turbulent statistically-steady open channel flow in dilute conditions of an oscillatory bottom boundary layer. The chosen statistically-steady flow case has a Reynolds number Re=180 based on the friction velocity and channel height. Results show that the turbulent shear rate G is higher close to the bed compared to the value obtained at the free surface. Since the concentration profile follows the same trend, the flocculation model reaches the equilibrium state more quickly close to the bed. Moreover, the vertical profile of weighted average floc properties suggests that coarse flocs are located at the bed due to higher sediment concentration that encourages aggregation, while fine flocs are close to the free surface. Nevertheless, all the flocs are microflocs based on the Manning et al. (2010, Ocean Dynamics) classification. The same weak coupling is applied to oscillatory flow with a Stokes boundary layer thickness is 0.0018 m and free-stream velocity amplitude of 0.4484 m/s , giving a Stokes Reynolds number of 800. Preliminary results for the wave-bottom boundary layer will be presented at the conference.