Uniform and gradually varied flows in compound channel versus free mixing layers

Mixing layers associated with uniform and gradually varied flows (GVF) in compound channel are experimentally investigated in two flumes, featuring a rectangular or a trapezoidal main channel (MC). These shear layers are compared with free mixing layers. Starting with uniform flow, the GVF are generated by an imbalance in the upstream discharge distribution between the floodplain (FP) and the MC. The GVF are longitudinally evolving under the influence of four external forcings: (i) a two-stage geometry, (ii) a varying vertical flow confinement (quantified by the relative flow depth, Dr = hf / hm, where hf and hm are the flow depths in the FP and MC); (iii) a variable lateral depth-averaged mean flow; (iv) a variable velocity ratio, l = Us/(2Uc), where Us = Ud2-Ud1 is the velocity difference, Uc = 0.5(Ud1+Ud2) is the mean velocity across the mixing layer, Ud1 and Ud2 are the depth-averaged velocities outside the mixing layer in the FP and MC, respectively. In the case of weakly or moderately sheared flows (l < 0.3 - 0.35), the peak values of scaled depth averaged Reynolds-stress, denoted Max (gd), are independent of l, but increase with a decrease in flow confinement, to reach the maximum values observed for free mixing layers. In the case of highly sheared flows (l > 0.3 - 0.35), the values of Max (gd) become independent of the flow confinement as l increases, reaching values that can be greater than the ones observed for free mixing layers. Despite the flow confinement, the high values of l trigger the development of 2D large coherent structures without interaction with the 3D bed-induced turbulence. Lastly, with nearly constant values of l and Dr, it was found that the scaled shear-layer turbulence was mainly dictated by the lateral flow. For both uniform flows and test cases with a lateral flow to the FP, the Rayleigh&#8217;s inflection point criterion is fulfilled. This gives rise to large 2D structures and high values of Max (gd). By contrast, for test cases with a significant lateral flow to the MC, the convex velocity profiles without inflection-point are associated with low levels of Max (gd). Lastly, the trapezoidal MC was found to enhance the turbulent exchange compared with the rectangular one.