Flow Structures in Trapezoidal Compound Channels with Different Side Slopes of Main Channel

Abstract

In a trapezoidal compound channel, the side slope of the main channel has a significant effect on the flow structure and the momentum exchange in the connecting region of the main channel and the floodplain. The three-dimensional compound channel flows with the 90°, 60°, 45° and 30° side slopes of main channel were simulated by solving the Navier–Stokes equations with the Reynolds Stress Model (RSM). The effects of main channel side slopes on the secondary currents, streamwise velocities, bed shear stresses, Reynolds shear stresses, turbulent intensities and the turbulence anisotropy were analyzed. The results show that (1) the secondary currents are modified as a result of the variation of the turbulence anisotropy $$(\overline {{w^{\prime 2}}} - \overline {{v^{\prime 2}}} )$$ , which is caused by the varied momentum exchange between the main channel and the floodplain; (2) as the main channel side slope decreases, the intensity of the secondary currents and their direct influence on the streamwise velocity, the bed shear stress, the Reynolds shear stress and the turbulent intensity becomes weaker. The deceleration of the streamwise velocity in the connecting region becomes less remarkable. The bed shear stresses tend to follow a more uniform distribution and are found to have one drop at the bottom of the main channel sidewall and the other drop at the edge of the floodplain when the side slope of the main channel is not vertical. These are different from the case of the vertical main channel side wall, in which only one minimum and one maximum values are observed at the interface region. The total magnitude of the turbulence intensity near the junction edge increases by a much smaller extent. The discharge ratio of the main channel increases while that of the floodplain decreases. The magnitude difference of $$- \overline {{{{u}}^\prime }{w^\prime }}$$ between the main channel and the floodplain in the interface region decreases, which implies the momentum transport becomes weaker due to the diminished secondary currents. These results will contribute for designing the most optimum cross-section shape for flood discharge of the compound channel.