Coherent structures in compound channel flows at the presence of transverse currents
Structures cohérentes dans des écoulements en lit composé en présence de courants transverses
Résumé
The paper presents results of experiments conducted in a laboratory flume with a compound cross-section consisting of a central main channel (MC) and two adjacent floodplains (FP). The focus of the study has been on turbulent mixing layers and large-scale coherent structures that form at the interfaces between the MC and two FP at non-uniform flow conditions in the streamwise direction. In the experiments, the streamwise non-uniformity is generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance, keeping the total flow rate (Q = 114 L/s) constant. The experiments started with initially uniform flows (8 L/s in each FP and 98 L/s in MC), which then have been modified by redistribution of the water discharge between MC and FPs. Three unbalanced inflow conditions have been tested, with a FP inflow of 4, 12 and 16 L/s at each FP. The discharge imbalance results in a transverse depth-averaged mean flow (termed herein ‘transverse flow’) from MC to FP or vice versa, quantified by the depth-averaged transverse velocity Uyd. Based on ADV measurements with 1 or 2 side-looking probes, it is shown that transverse currents towards the MC with a velocity ratio Uyd / Uxd higher than 5% can entirely displace the mixing layer, shear-layer turbulence, and coherent structures, shifting them into the MC. It is also found that streamwise uniformity is reached faster with a transverse flow towards the FP than with a transverse flow in the opposite direction.
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