Flow Dynamics and Pollutant Transport at an Artificial Right-Angled Open-Channel Junction with a Deformed Bed

Abstract

Artificial open-channel junctions have sharp corners and a smaller width-to-depth ratio. Despite numerous studies on channel junctions, knowledge on turbulent structures and their role in mixing at artificial junctions is lacking. To fill this research gap, the present study uses a large-eddy simulation (LES) model to investigate the three-dimensional (3D) turbulent structures and their role in pollutant transport at a right-angled laboratory open-channel junction with a deformed bed. The deformed-bed junction represents a quasi-equilibrium condition and consists of a scour zone and deposition bar. The numerical model is validated against experimental data of the velocity field and turbulent kinetic energy. Comparisons of the flow field between the flat-bed condition and quasi-equilibrium deformed-bed condition showed a reduced flow separation zone and less developed recirculating gyre in the latter case because of the strong secondary currents. The coherence of the turbulent structures was drastically disrupted at the deformed-bed junction because the Kelvin-Helmholtz (KH) instability results in more randomly oriented residuals. In contrast, the breakdown of the shear layer at a flat-bed junction displayed the trail of the arch-shaped vortices. The role of turbulent structures in pollutant transport is elucidated by using a neutrally buoyant conservative tracer. Large turbulent structures associated with the KH instability help the tracer evolve faster at the deformed-bed junction. The open-channel junctions of artificial systems (e.g., drainage networks) have sharp corners and a smaller with-to-depth ratio (compared with natural river junctions). This paper describes the detailed three-dimensional (3D) flow field and pollutant transport at a laboratory-scale right-angled channel junction with a quasi-equilibrium deformed bed to elucidate the hydraulics of the artificial channel junction. Right-angled channel junctions are a common feature of artificial channel junctions. Given the limitations of the experimental studies, a 3D numerical model, namely large-eddy simulation, is used in the present study. The quasi-equilibrium deformed bed of the junction was obtained from the initial flat-bed condition. The flow field, including the 3D turbulent structures, was modified remarkably at the deformed-bed junction compared with the field under the initial flat-bed condition. The study shows that bed topography plays a major role in controlling turbulent structures. The role of the 3D turbulent structures in pollutant transport is investigated. The present study improves the understanding of the flow field and mixing patterns at artificial open-channel junctions.