Mixing and spreading of inclined dense jets with submerged aquatic canopies

Abstract

While the current practice in designing brine outfalls for seawater desalination plants has provided a level of protection to the coastal environment that is deemed acceptable, additional research can be pursued to further improve the state-of-the-art understanding as well as enable the integration of beneficial features introduced by the brine outfall. The present study performed both numerical simulations and laboratory experiments to investigate the mixing and spreading of an 45º inclined dense jet in coastal regions with submerged vegetation canopies at the seabed. The numerical simulations utilized the Reynolds-Averaged Navier-Stokes (RANS) equations with the standard k-ε turbulence closure and non-interactive species transport model. The experiments were performed using the technique of Planar Laser Induced Fluorescence (PLIF) for verification. The overall results showed that the mixing of the inclined dense jet is enhanced just above the top of the submerged canopy due to the presence of stem-scale wakes and increased dispersity promoted by the spatial inhomogeneity near the canopy interface. A threshold value of 0.6 for the characteristic parameter, ϕFr, where Fr is the densimetric Froude number of jets and ϕ is the canopy density, has been observed above which the dilution coefficients reach a plateau and are no longer influenced by the canopy density. Within the canopy, however, the barrier effect prevails in the mixing process leading to constrained dilution particularly with denser canopies. In particular, the concentration at the impingement region within the canopy near the bottom is observed to increase with the increase in the general canopy density.