Influence of bed elevation discordance on flow patterns and head losses in an open-channel confluence

Confluences play a major role in the dynamics of networks of natural and man-made open channels, and field measurements on river confluences reveal that discordance in bed elevation is common. Studies of schematized confluences with a step at the interface between the tributary and the main channel bed reveal that bed elevation discordance is an important additional control for the confluence hydrodynamics. This study aimed to improve understanding of the influence of bed elevation discordance on the flow patterns and head losses in a right-angled confluence of an open channel with rectangular cross-sections. A large eddy simulation (LES)-based numerical model was set up and validated with experiments by others. Four configurations with different bed discordance ratios were investigated. The results confirm that, with increasing bed elevation discordance, the tributary streamlines at the confluence interface deviate less from the geometrical confluence angle, the extent of the recirculation zone (RZ) gets smaller, the ratio of the water depth upstream to that downstream of the confluence decreases, and the water level depression reduces. The bed elevation discordance also leads to the development of a large-scale structure in the lee of the step. Despite the appearance of the large-scale structure, the reduced extent of the RZ and associated changes in flow deflection/contraction reduce total head losses experienced by the main channel with an increase of the bed discordance ratio. It turns out that bed elevation discordance converts the lateral momentum from the tributary to streamwise momentum in the main channel more efficiently. (C) 2019 Hohai University. Production and hosting by Elsevier B.V

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

Modelação numérica do escoamento em torno de um pilar

Tese de mestrado. Mestrado Integrado em Engenharia Civil - Especialização em Hidráulica. Faculdade de Engenharia. Universidade do Porto. 201