Mixing downstream a 90° open channel junction.

Nicolas Rivière, Wei Cai, Gislain Liepeme Kouyi, Emmanuel Mignot, Adrien Momplot

Friday 3 july 2015

9:00 - 9:15h at Amazon (level 1)

Themes: (T) Hydro-environment, (ST) Impacts of pollutants on the water environment

Parallel session: 14E. Engineering - River

The dispersion of scalars by a confluence, defined as two upstream free-surface flows merging with a given angle into one downstream flow, is a key point for different aspects of water management. Regarding river management, it concerns flows with different concentrations of scalars (pollutants, minerals and chemicals, salts, gazes, suspended matters, etc.) that merge in a confluence. Regarding urban flood risk management, it concerns the spreading of pollutants from a local releasing point (from overflowing sewers, flooded factories, damaged tankers, etc.) through a crossroad, and then throughout the whole city. Present paper focuses on urban water management (flow within the underground sewer network), where the width to depth ratio is usually one or two orders of magnitude lower than in the two previous cases. The dispersion of scalars (pollutants, suspended matters, bacteria, medicines, etc.) by confluences remains a key point for water quality measurements in sewer networks. The relevance of one-point concentration measurements in the vicinity of junctions, as usually performed for the monitoring of water quality, is directly related to the degree of mixing of the flow. Present work focuses on the mixing downstream a 90° junction of rectangular channels. Laboratory experiments are based on a novel experimental method which permits to measure the 3D scalar concentration field in a closed-loop flume during more than 100 consecutive hours (without changing the water). The experimental data show that the mixing is essentially governed by strong secondary currents promoted by the lateral tributary inflow, leading to complex scalar concentration fields in the measured sections of the downstream branch. Three-dimensional numerical simulations resolving the flow and mass transport equations, once calibrated against the experimental data, allow investigating various flow configurations according to the incoming discharge ratio. Simulation results highlight and enable quantifying the influence of the inflow momentum ratio of the two incoming channels on the length required to obtain a complete mixing in the downstream branch of the junction.