Numerical investigation of flow patterns in an open-channel confluence with a wider downstream channel

Open-channel confluences are common in nature as well as in hydraulic structures and urban drainage networks. In contrast to natural channels, artificial channels are usually prismatic, with a simple cross-sectional shape (e.g. rectangular) and a smaller width-to-depth ratio. Two confluent artificial channels are often of similar dimensions, followed by a wider post-confluence. Since the flow hydrodynamics in a confluence highly depends on the geometry (angle of confluence, bed elevation discordance, channel width discordance, width-to-depth ratios, etc.), changing either geometrical parameter can have important consequences on the flow patterns and turbulence characteristics. Previous studies have shown that the widening of the post-confluence channel distorts the mixing layer and therefore might suggest the width discordance as the source of distinct secondary cells orientation. Nevertheless, given some recent controversy on the subject, specific research on this subject is needed to confirm the stated hypothesis. A more detailed investigation is presented here, by means of Large-Eddy simulations (LES), aiming at a description of the secondary flow and turbulence of open-channel confluences in the discordant channel width case of Yuan et al., as well as in the corresponding concordant width case. Knowledge of the hydrodynamics and turbulence characteristics in confluence zones, such as near the mixing layer, is key to better understand the dispersion of transported matter (pollutants, nutrients, etc.), which might be important for the design of, for example, sewage outlets, water intakes or fish passages.