Nature of flow and turbulence structure around an in‐stream vertical plate in a shallow channel and the implications for sediment erosion

Abstract

Detailed knowledge of the dynamics of large‐scale turbulence structures is needed to understand the geomorphodynamic processes around in‐stream obstacles present in rivers. Detached Eddy Simulation is used to study the flow past a high‐aspect‐ratio rectangular cylinder (plate) mounted on a flat‐bed relatively shallow channel at a channel Reynolds number of 2.4 × 105. Similar to other flows past surface‐mounted bluff bodies, the large amplification of the turbulence inside the horseshoe vortex system is because the core of the main necklace vortex is subject to large‐scale bimodal oscillations. The presence of a sharp edge at the flanks of the obstruction fixes the position of the flow separation at all depths and induces the formation and shedding of very strong wake rollers over the whole channel depth. Compared with the case of a circular cylinder where the intensity of the rollers decays significantly in the near‐bed region because the incoming flow velocity is not sufficient to force the wake to transition from subcritical to supercritical regime, in the case of a high‐aspect‐ratio rectangular cylinder the passage of the rollers was found to induce high bed‐shear stresses at large distances (6–8 D) behind the obstruction. Also, the nondimensional values of the pressure root‐mean‐square fluctuations at the bed were found to be about 1 order of magnitude higher than the ones predicted for circular cylinders. Overall, this shows that the shape of the in‐stream obstruction can greatly modify the dynamics of the large‐scale coherent structures, the nature of their interactions, and ultimately, their capability to entrain and transport sediment particles and the speed at which the scour process evolves during its initial stages.