ABSTRACT
Confluences are important junctions in fluvial and artificial networks of open-channels, as they regulate the mixing phenomena of substances transported by the merging flows, as well as the scour and deposition phenomena. This paper aims at contributing to the study of how the bed morphology, such as the presence of a scour hole and a depositional bar, influences the flow and mixing phenomena at a T-shaped open-channel confluence. By means of Large Eddy Simulations (LES) of flow and passive scalar transport, a comparative numerical study is carried out of two fixed bed cases, i.e. a flat one and a degraded one, which were investigated earlier experimentally in the lab and numerically with a RANS based solver by Tang et al. (2018). The spatial focus of the present paper is not exclusively devoted to the Confluence Hydrodynamics Zone, but also extends somewhat further downstream (i.e. up to 10 post-confluence channel widths Wd downstream of the upstream confluence corner). For the investigated flow condition with a dominant incoming flow from the upstream main channel, the recirculation zone in the degraded bed case is found to be significantly shorter and less wide or even absent near the bed, yielding differences in the contraction of the main channel flow and in the secondary flow. Nevertheless, the non-uniformity of passive scalar mixing was found to be very similar up to 3.5Wd . Further downstream, the flat bed case shows a slightly higher mixing rate, which does not seem to be related to differences in secondary flow patterns, but rather to differences in turbulence levels.
