Abstract
The structure of free-surface flows is experimentally investigated in a laboratory flume with a compound cross-section consisting of a central main channel (MC) and two adjacent floodplains (FPs). The study focuses on the effects of transverse currents on: (i) mixing layers and quasi-two-dimensional coherent structures at the interfaces between MC and FPs; (ii) secondary currents developing across the channel; and (iii) large and very-large-scale motions that were recently observed in non-compound open channel flows. Transverse currents represent spanwise depth- and time-averaged flow from MC to FPs or vice versa. The study is based on one-point and two-point ADV measurements. Streamwise non-uniform flows are generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance, keeping the total flow rate the same for all scenarios. It is shown that even small transverse currents can be very effective in flow modification, as they can significantly displace the mixing layer, shear-layer turbulence, and coherent structures towards MC or FP, depending on the current direction. They can also alter the distribution and strength of the secondary currents. The interactions of quasi-two-dimensional coherent structures, very-large-scale motions, and secondary currents at different conditions are also part of this study.
Highlights
Overflowing rivers may give rise to streamwise non-uniform flows in compound channels, which consist of the main river channel (MC) and one or two floodplains (FPs)
The study focuses on the effects of transverse currents on: (i) mixing layers and quasi-two-dimensional coherent structures at the interfaces between main channel (MC) and FPs; (ii) secondary currents developing across the channel; and (iii) large and very-large-scale motions that were recently observed in non-compound open channel flows
It is shown that even small transverse currents can be very effective in flow modification, as they can significantly displace the mixing layer, shear-layer turbulence, and coherent structures towards MC or FP, depending on the current direction
Summary
Overflowing rivers may give rise to streamwise non-uniform flows in compound channels, which consist of the main river channel (MC) and one or two floodplains (FPs). Flow non-uniformity is associated with longitudinal changes in flow depth, often accompanied by transverse currents. These currents represent transverse depth-averaged and time-averaged flows from MC to FPs or vice versa, quantified by the depth-averaged transverse velocity Uyd. We present here a part of a laboratory study that focuses on the effects of transverse currents on the flow structure in compound open-channel flows, namely on: (i) mixing. Flow non-uniformity and associated transverse currents are generated in the study by creating an imbalance in the upstream discharge distribution between MC and FPs in a prismatic compound open-channel (see e.g., [2,3]). The paper outlines experimental set up and flow scenarios first, followed by the presentation of the results related to mixing layer parameters, large-scale 2D CSs, and secondary currents
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