Dilution characteristics of dual buoyant jets in wavy cross-flow environment

Due to the difference in density between the discharge effluent and coastal water, partially treated wastewater is often discharged into the marine environment as a buoyant jet via submarine outfalls with multiport diffusers. The dilution characteristics of effluent discharge (dual buoyant jets) in a wavy cross-flow environment were studied in a laboratory. The planar laser-induced fluorescence technique was used to obtain the concentration data of the jets. The effects of different environmental variables on the diffusion and dilution characteristics of the jets were examined through physical experiments, dimensional analysis, and empirical formulations. It was found that the dilution process of the dual jets could be divided into two components: the original jet component and the effluent cloud component. The jet-to-current velocity ratio was the main parameter affecting the concentration levels of the effluent cloud. The merging of the two jets increased the jet concentration in the flow field. When the jets traveled further downstream, the axial dilution increased gradually and then increased significantly along the axis. Under the effects of strong waves, the concentration contours branched into two peaks, and the mean dilution became more significant than under the effects of weak waves. Therefore, the dilution of the effluent discharge was expected to be significant under strong wave effects because the hydrodynamic force increased. A dilution equation was derived to improve our understanding of the dilution process of buoyant jets in a wavy cross-flow environment. This equation was used to determine the influences of the jet-to-current velocity ratio, wave-to-current velocity ratio, and Strouhal number on the minimum jet dilution. It revealed that the wave and buoyancy effects in effluent discharges were significant.