Effect of bed roughness on the mixing layers in a 90° asymmetrical confluence.

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Stephan Creelle, Laurent Schindfessel, Tom De Mulder

Wednesday 1 july 2015

9:45 - 10:00h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 8B. Sediment - River

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Open channel confluences constitute important elements in riverine networks. The complex flow at these confluences is a widely studied topic, since the associated flow features, head losses and mixing properties are of paramount importance for the entire network. In this paper, the mixing layers in a 90° asymmetrical confluence with a schematized section are studied by application of surface PIV measurements in a laboratory flume. The focus is on the influence of bed roughness on the mixing properties (velocity gradient and mixing layer width) in both mixing layers, originating at the upstream and downstream corner of the tributary channel with the main channel, respectively. Bed roughness effects on (shallow) mixing layers have been studied intensively, albeit mostly in the setting of a parallel confluence of two streams separated by a splitter plate. While in those experiments the velocity ratio is identified as the dominant flow forcing, an important effect of bed roughness on the mixing layer properties has been discerned. In this work, it will be investigated whether similar effects are present in an asymmetrical confluence. To facilitate a comparison between the mixing properties of parallel and asymmetrical confluence geometries, the approach by Mignot et al.(2014) will be adopted, making use of a so-called Serret-Frenet frame-axis. The area of interest for the measurements is chosen to cover the inflow sections of both the main and tributary channel, as well as a considerable area downstream of the confluence. As a consequence, not only the mixing layer between the incoming main channel and tributary flows will be studied, but also the mixing layer between the separation zone and the tributary flow. The characteristics of the latter mixing layer will be compared to the special case of a parallel confluence with an infinite velocity ratio. Mignot E, Vinkovic I, Doppler D, Riviere N. Mixing layer in open-channel junction flows. Environmental Fluid Mechanics. 2014;14(5):1027-41