Abstract The effluents produced by reverse osmosis desalination plants are usually disposed under the surface of the sea as inclined negatively buoyant jets (INBJ), to achieve the highest mixing rate. Bed slope in the local environment is one of the contributing factors in the behavior of INBJs. The aim of the present study is to utilize large eddy simulation (LES) to study the effects of the bed slope on the dilution and spreading of INBJs with 30°, 45°, and 60° nozzle angles. In this regard, five down slopes, 0°, 5°, 10°, 15°, and 20° were considered for the bed, and the mixing and geometrical properties of INBJs were studied in both jet plume and spreading layer regions. Present simulations show that the increase in the bed slope for all the three nozzle angles favorably increases the dilution of the INBJs at return point, concentration buildup point (CBP), impact point, and on the bed. For a 20° increase in the bed slope, the relative increase in the dilution at CBP of 30°, 45°, and 60° INBJs is calculated as 167.75%, 81.13%, and 107.43%, respectively. Also, the centerline dilution is the highest for the 60° nozzle angle at each bed inclination. The distribution of the mean concentration on the bed shows that an increase in the nozzle and bed inclination results in a significant decrease in the mean concentration and the area in which the effluent alters the ambient water quality. Present results suggest that the INBJs should be discharged into the ambient water in sloped environments, and the nozzle angle of 60° is preferred to 30° and 45° regardless of the bed inclination.
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