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Open-channel confluences are important junctions in fluvial and artificial networks, which regulate the mixing phenomena of substances transported by the merging flows. The present research aims at investigating how the bed morphology, like the presence of a scour hole and a depositional bar, influences the flow patterns and mixing phenomena at a T-shaped open-channel confluence with a wider downstream channel. Large Eddy Simulations (LES) are applied to compare the flow and passive scalar transport processes at two different bed morphologies: the flat bed and the degraded bed. The results clearly show that in the degraded bed case, the reverse flow zone is shorter and narrower, even absent near the bed, due to the complex interaction of the flow with the scour hole and the depositional bar. Moreover, the degree of contraction of the main flow is less in the degraded bed case. Despite the differences in flow patterns, the degree of mixing in the confluence zone is first very similar for the degraded and the flat bed cases. Only when going more downstream, there is a higher degree of mixing for the flat bed, which may be attributed to the differences in the turbulence levels and the mixing interface.

Confluences of open-channel flows are common in nature as well as in urban drainage networks and in hydraulic structures. The complex hydrodynamics is often studied in schematized, right-angled confluences. In this paper, the influence of the downstream channel width onto time-averaged and turbulent flow features will be investigated numerically, based on Large-Eddy Simulations. For one flow situation, i.e. flow ratio and downstream Froude number, two geometries will be compared: a discordant width case, which was studied experimentally by Yuan et al. (2016) in a flume with a wider downstream channel than the upstream mean and tributary channels, and the corresponding concordant width case, in which the downstream channel has the same width as the confluent channels. The widening of the downstream channel turns out to reduce the backwater effects, the flow contraction and the associated water surface depression. Moreover, the three-dimensionality of the recirculation zone in the mean flow is enhanced due to complex flow patterns, resulting in a reduced width and length of the recirculation zone in the lowest third of the water column. Finally, the respective cores of high values of the dimensionless TKE and Reynolds shear stress, that persist over the water column, have lower peak values in the discordant width case and the shape of those cores is more distorted, especially near the bed.

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.

In the present work, a numerical model based upon the Large Eddy Simulation approach has been set up for predicting the three-dimensional flow around a cylindrical pier, mounted on a flat and fixed bed, a generic case that is relevant for the study of flow and scour around bridge piers. This turbulent flow configuration was studied experimentally by Nogueira et al. (2008) with Particle Image Velocimetry (PIV). The main goal of this paper is a first validation of the numerical model, based upon the available data. The numerical tool is capable to qualitatively reproduce the characteristic flow features around the pier, like e.g. the horseshoe vortex system and the vortex shedding in the wake. The predicted extent of the initial scour hole, based upon the bed shear stress magnitudes, agrees well with the observations at the onset of the souring process during the lab experiments. Further quantitative validation of the numerical model will benefit from additional measurement efforts in the experiments.

Tese de mestrado. Mestrado Integrado em Engenharia Civil - Especialização em Hidráulica. Faculdade de Engenharia. Universidade do Porto. 2012