Identification of Flow Patterns in T-Shaped Open Channel Bifurcations

Bifurcations in natural rivers and man-made open-channel systems are commonly encountered and pose major engineering challenges. Water and sediment flows through these features can be described by the Bulle-Effect, i.e. the bulk of such sediments are transported into the lateral (or branch) channel. As there is a lack of a complete understanding of the phenomenon, especially with related processes such as secondary flows and vortices, research is still ongoing to this day. In this paper, experiments conducted in the laboratory flume at LMFA, INSA Lyon to further examine the findings of (Momplot et al. in J Hydraul Res 55:63–71, 2017) and to distinguish the two distinct flow structures, i.e. standard 2D closed recirculation and complex 3D helical patterns, are presented and discussed. From trial runs for equal-width channels with outlet weirs fixed at the same heights, flow visualisation techniques (ink injection and thread/strip behaviour) were observed to be erratic and impractical in distinguishing the 2D and 3D flow structures. Upon analysis of data and results for additional completed runs, the key finding is the introduction of alternative non-visual approaches in identifying such flow characteristics, namely the transformation of standard 2D to complex 3D recirculation patterns in T-bifurcations. This can be achieved firstly by identifying the distinctly steeper curves for 3D in plotting upstream Froude number versus aspect ratio as compared to 2D. Secondly, these patterns could also be separated by a certain limit or line when the power loss percentage is graphed against flow discharge. These are potentially useful methods to differentiate the divided flow structures and should be further investigated.