Author(s): Jeonghu Lee; Van Thinh Nguyen

Linked Author(s): Jeonghu Lee, Van Thinh Nguyen

Keywords: Coherent structure; Dune development process; Flow reattachment; Lee side angle; Sediment transport

Abstract: Dune development process including migration and growth of dunes in natural rivers has been widely investigated due to its substantial effect on flow resistance, energy expenditure, and local scour on erodible beds. Lee side angle, which is one of the important parameters, affects the dune development process. The energetic vortices, so-called kolk-boil vortices, are generated in the flow reattachment zone occurred just behind the lee side of dunes; and these vortices are known to entrain much amount of sediment toward the water surface. This study is to investigate the change of flow behavior and dominant sediment transport process near the bed as the bed profile evolves from a flatbed under conditions of lower-regime. All the numerical simulations are conducted using 3D LES (Large Eddy Simulation) approach to reproduce the instantaneous flow behavior near the bed and 3D coherent structures. Since the lee side angle is characterized by backward facing step (BFS), the relationship between the lee side angle and flow reattachment over dunes is investigated over various inclined BFSs associated with the reattachment length, in order to find out the effect of flow separation on dune evolution processes in rivers. The effect of coherent structures in the reattachment zone on sediment suspension is estimated by quadrant analysis with various step angles. The change of distance needed for the flow to be fully recovered behind the reattachment zone with various step angles is also investigated. The contribution of each bed load and suspended load at each dune evolution stage is quantitatively examined with an emphasis on the kolk-boil vortices and the coherent vortices along the separated shear layer.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221041

Year: 2022