Characteristics Of The Development Of Thermocline And Evolution Of Density Currents In Stratified Fluids

In recent years, most source water reservoirs have suffered from the deteriorationof water quality, such as endogenous pollution and eutrophication phenomena,especially for large deep reservoirs in which the vertical thermal stratification easilyoccurs in summer. The themocline, which impedes the material and energy exchangebetween the upper water and the lower water, will be destroyed in early spring, and lateautumn. Therefore, the pollutants released from the bottom sediments of the reservoircan directly pollute the water quality of the whole reservoir and can lead to theendogenous pollution and eutrophication problems. On the other hand, during theflooding periods in summer, large amounts of sand carried from the storm runoff willflow into the stratified reservoirs, and will cause the problem of hugher turbidity andincreased pollutant loads. When the discharge of storm runoff is high, the densitycurrents can form the interflow or sometimes upward circulation and can pollute thesource water under the thermal stratification canditon.Taking Jinpen Reservoir in Xi’an as a case, a numerical simulation method basedon Fluent for thermal structure of a stratified reservoir was developed and validatedagainst the field data, the development and characteristics of the thermoline undervarious conditions of short wave radiation and bulk extinction coefficient for short waveradiation was numerically investigated. The total heat transfer is negative at surface ofthe water reservoir and the proportion of longwave radiation accounts for more than95%and shortwave radiation is one of the most important factors to form the layer. Theeffect of algae on the thermocline layer have the closely relationship on short-wave radiation attenuation coefficient. The study found that the relationship betweenconcentration of algae with the measured values and shortwave radiation attenuationcoefficient have a good positive correlation. With decreasing of shortwave radiationattenuation coefficient, the thickness of thermocline layer increases, the temperaturegradient within the thermocline layer will also decline. The study also found that thereis a limit of shortwave radiation intensity in stratified reservoirs, the temperaturedifference in thermocline layer showed decreasing trend after the first increase withaddition of surface shortwave radiation intensity; However, it is difficult to reachthermal equilibrium to form the stable thermocline layer when the surface shortwaveradiation intensity is too high.Using the multi-phase Euler-Euler model and RNG k-ε turbulence equations, aFluent-based numerical model coupling the hydrodynamics and water quality wasdeveloped, the flow behaviour under different stratification conditons was numericallysimulated with this developed method and validated against the experimental data ofdensity currents by Wuhan University. Effects of temperature gradient on the interflowpatterns were mainly invetigated, and the effects of inflow velocity, sedimentconcentration and sediment diameter on the interflow patterns were also compared. Thesimulated show that separation point reached the deeper position under the largersediment concentration condition, and the thickness of the interflow became thickerunder the higher inflow velocity condition. The research outcomes will establish atheoretical foundation for the further exploration of endogenous pollution problems andwater quality control of source water reservoirs.