Effect of geometric parameters on velocity distribution in compound open channel

Compound channels are composed of a main channel and one or two flood plains around it that are relatively shallower than the main channel. Due to the difference between the flow velocity in the main channel and the floodplains, the flow has a complex structure in compound channels. In compound channels, a part of river discharge is carried out by floodplains, thus shear distribution and flow structure becomes complicated due to the transfer of momentum between the main channel and the shallower flow in the floodplains. In the last two decades, the attention of the researchers was devoted to the investigation of flow behavior in compound channels.
In this study, the Fluent 6.3 software was used to solve the governing equations on the open channels with compound cross section. Fluent is a powerful and flexible general-purpose computational fluid dynamics software ideally suited for simulating the flow characteristics in compound channels. In this software, the volume of fluid model (VOF) is used to solve the equations. The VOF formulation relies on the fact that two or more fluids (or phases) are not interpenetrating. The VOF method consists of three ingredients: a scheme to locate the surface, an algorithm to track the surface as a sharp interface moving through a computational grid, and a means of applying boundary conditions at the surface. To generate the mesh as well as to introduce the channel geometry, the Gambit pre-processing software was used. Gambit is an integrated preprocessor program for CFD. It can be used for geometry construction and import, mesh generation with structured and unstructured elements, mesh quality examination and definition of boundary zones. The distance among the meshes was 5 mm, albeit, it was 2.5 mm near the walls and channel bed. For simulating the flow turbulence, the Reynolds stresses model was used. The results of simulations was compared with those of experimental data. The simulated channel was a flume with 12.5 m length, and a 0.4 m * 0.4 m compound cross-section. The flow depth in the main channel was 0.08 m, and the flood plain changed from 0.02 m to 0.06 m. the width of main channel was 0.2 m.
In this study, the effect of the ratio of flow depth at the floodplain to flow depth at main channel on the velocity distribution and the location of the maximum velocity at a compound channel was investigated. In addition, the effect of flood plain roughness on the above parameters was studied. The results showed that by changing the ratio of flow depth at floodplain to flow depth at main channel, the velocity distribution in the channel changes significantly. At the junction of main channel with flood plain, two vortices were appeared, named as flood plain vortex and main channel vortex. Both of the vortices reached to the flow surface and occupied the zone limited by . At the distance of , a secondary flow was formed near the free flow surface from channel wall to the axis of the channel, named as free surface vortex. This vortex is generated because of the anisotropic turbulence. The free surface vortex and the main channel vortex meet together at the central axis of the channel . At the zone , another secondary flow is established known as bottom vortex. The simulated iso-vel curve were in good agreement with the observation of previous researchers. The dip phenomenon was observed in compound channels. As the flow depth over the floodplain decreases, the location of maximum velocity gets further away from the flow surface as well as from the junction of main channel and the flood plain. To investigate the effect of roughness on the velocity distribution at the compound channels, two cases of smooth flood plain and roughened flood plain were studied. The results showed that there are good agreement between the results of Fluent and the experimental data. In this case, the maximum velocity occurred at z/H=1.375, y/H=0.8. The roughness changed the shape of iso-vel curves at the flood plain. As the height of roughness at the floodplain increases, the velocity decreases in this zone and the iso-vel curves move to the main channel. By increasing the height of the roughness, there was no changes in the shape of iso-vel curves at the main channel.