Hydraulics of an Asymmetrical Flume with Sidewall Rib

Open channels with streamwise ribs have been studied for decades, with many relevant applications in chemical engineering, environmental and sanitary engineering, aeronautics, astronautics, biology and geology. Longitudinal ribs along channel walls have been successfully tested for the increased heat transfer rate and mass transfer. In alluvial channels, long-lasting three-dimensional large-scale turbulent vortices may yield the development of longitudinal ridges on the mobile bed with preferential sediment transport in between. Herein a detailed hydrodynamic study was conducted in an asymmetrical rectangular channel equipped with a sidewall streamwise rib. Both free-surface, velocity and boundary shear stress measurements showed strong secondary currents of Prandtl`s second kind. The sidewall rib and channel asymmetry contributed to very-strong secondary motion, associated with turbulent dissipation. A key feature of the channel design was the provision of a well-defined highly-turbulent low-velocity zone (LVZ) beneath the rib. The configuration might be applied to hydraulic structure design, but uttermost care must be considered. A number of practical considerations showed major technical challenges, and in many instances, alternative designs should be preferred. Altogether the study showed how the introduction of a relatively simple streamwise shape, i.e. square rib, placed asymmetrically could induce a major change in hydrodynamic properties, in comparison to a simple rectangular channel flow.