Large eddy simulation of bed formation in subaqueous bedload

Large eddy simulation of incompressible turbulent flow over a loose bed of spherical particles are investigated in an open channel. Eulerian and Lagrangian point-particle methods is applied to solve the Navier-Stokes equations and particle motion respectively and the particle-flow interaction is also considered. A new method solving particle-particle collisions is utilized for the first time to reduce the computational time spending on calculating the pairwise distances between particles. A turbulent fluid condition from the experiment of Robert and Uhlman (2001) are chosen of which the corresponding sediment patterns are ‘ripple’. Flow over the formed bed is considered and it is found that double-averaged Reynolds stresses including shear stress and three normal stresses reach their peak values near the bed. However, affected by the movable bed, they decay quickly as the height increases. The flow direction slightly rises over the stoss-side of ripples and falls after the crest and the velocity magnitude of time-averaged flow accelerate and decelerate before and after the crest of ripples as well. Hence, recirculation zones and clockwise vorticity appear at the trough of the bed where kolk boil vortices like hairpins and elongated streamwise vortices is also evident. Coherent structures, in the form of high- and low-speed streaks near the bed are also affected by the bed formation. The near-bed low-speed streaks entrain into the main flow domain over the stoss-side of ripples and the high-speed fluid streaks from the main flow rush toward the bed over the leeside of ripples. The bedload transport rate is well represented by previous empirical formulas. The bed surface elevation changes from upstream to downstream with time and there is a difference in the direction of sediment transport which is obvious three-dimensionality. In addition, it is also found that the bed surface elevation shows a positive correlation with particle streamwise velocity and entrainment rate which means higher bed elevation leads to larger bedload transport rate.