Poor mixing of old and new water in municipal water storage vessels is a well-documented basis for potentially harmful water quality degradation in drinking water distribution systems. This numerical study investigates the effects of inflow and operational variables on mixing in the jet-driven filling process, with a particular focus on the transition from inadequate to sufficient mixing levels. An isothermal unsteady reynolds-averaged-navier-stokes volume-of-fluid (RANS-VOF) simulation is used to model the variable-volume filling process, accounting for the moving free surface following a draw-down in the stored water volume. A low diffusivity tracer is used to mark the old-water volume, and a coefficient of variation (CoV) quantifying the departure from a uniform tracer distribution is used to monitor the time-dependent mixing. The results indicate that adequate mixing does not necessarily follow refills from common draw-down levels. Three distinct mixing regimes are identified by unique CoV transients. Introducing consideration of the mean-flow kinetic energy, the observed mixing behaviors are readily explained by the jet inlet power and the distribution of the mean-flow kinetic energy in the tank. Extending the simulations to periods after cessation of the inflow and to partial refills, the role of residual mean-flow kinetic energy is further highlighted, especially its limited vertical reach. For cases in which a sufficiently mixed condition is achieved, the time-to-mix results are well described by a mixing-time correlation closely matching previously published results.
P. N. Bangalore and K. O. Homan, "Jet-Driven Mixing Regimes Identified In The Unsteady Isothermal Filling Of Rectangular Municipal Water Storage Tanks," Journal of Hydraulic Engineering, vol. 150, no. 2, article no. 04023065, American Society of Civil Engineers, Mar 2024.
The definitive version is available at https://doi.org/10.1061/JHEND8.HYENG-13448
Mechanical and Aerospace Engineering
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01 Mar 2024