Numberical Simulation Of Density Currents In Thermally Stratified Reservoir And Water Quality Prediction

The storm runoff carry plenty of sediment and pollutants,and can develop to density currents when it enter a stratified reservoir.In this study,a small scale self-designed model reservoir was used,the density currents and distributions of sediment concentration in stratified reservoirs were numerically simulated with a commercially-available software-Fluent.Main conclusions are as follows:(1)Fixing the upper outlet,inflow velocity of 0.005 m/s ? inflow sediment concentrations of 1.25g/L and 3.75g/L,density currents were numerically simulated with two-dimensional(2D)and three-dimensional(3D)Euler model and Miture model,simulated and measured sediment concentration and water temperature were compared at different times under various conditions.Under these operational conditions,density currents could be well simulated with both Euler model and Miture model.Within a certain range,increasing grid number can improve the precision of simulated results,but oversize grid may increase the simulation cost.3D simulated results are better than 2D ones,and the motion patterns of different gravity flows can be observed from multiple views when the three-dimensional model is used,which is closer to the actual environments.Considering the precision and cost of calculation,Mixture model is suitable for the simulation of density currents in this study.(2)Simulated results showed,fixing the inflow velocity and outlet position,the temperature stratification could be maintained when the density currents propogated as the type of interflow.The temperature stratification could be partly destroyed when the density currents propogated as the type of underflow.The head flow velocity decreased as the density currents propogated.When the inflow sediment concentration increased,the interflow spread widely upwards,the separation point declined and the depth of the interflow increased.(3)Fixing the inflow sediment concentration,the position of the interflow will move down with the decline of the outlet position.When the outlet position is at the upper,the position of the thermocline is basically unchanged.When the outlet position is at the middle and bottom,upper,the thermocline moves down,the lower the position of the outlet,the stronger the moving.With the movement of the density currents,heat exchange happened continuously between the density currents and the background water in the reservoir,which caused the temperature of the upper water body keep dropping,and the lower water temperature keep rising.The outlet position has little influence on the depth of separation point and the thickness of the interflow.The head flow speeds were the highest for the upper outlet,and were the lowest for the bottom outlet.