Flow structure and large scale turbulence in an open channel bend of strong curvature with flat and deformed bed

Results of Three-Dimensional (3D) Detached Eddy Simulation (DES) are used to discuss changes in flow structure and large-scale turbulence structures in a high-curvature open channel between conditions present at the start (flat bed) and at the end (equilibrium deformed bed) of the erosion-deposition process. The flow in a 193 degrees bend is simulated, for which the ratio between the bend curvature and the channel width is close to 1.3. For these geometrical parameters, the cross-stream secondary flow and anisotropic effects play a crucial role in the redistribution of the streamwise momentum and influence significantly the distribution of the bed shear stress which, in turn, determines the capacity of the flow to entrain sediment. We also investigated changes in the flow and turbulence structure between Reynolds numbers at which most laboratory experiments are conducted and Reynolds numbers that are closer to those encountered in natural small streams. We show that an energetic thin shear layer containing large-scale eddies develops at the interface between the core of high streamwise velocities and the retarded (flat bed case) or recirculating (deformed bed case) fluid moving close to the inner bank. Highly energetic large-scale Streamwise-Oriented Vortices (SOVs) develop close to the inner bank. The strength of the secondary outer bank SOV cell is enhanced as the Reynolds number is increased. Results show that an increase in Reynolds number by about one order of magnitude enhances significantly the turbulence, the secondary flow, the SOVs, the bed shear stresses and the flow's potential to erode the boundaries.