Investigation Of Density Current In Thermal Stratified Reservoir

With the sunshine on the stagnant water body in reservoirs, the large reservoirs tend to become thermal stratification in vertical direction. This thermal stratification changes with weather and season. Due to the density difference between inflow and ambient water, the density current forms in the reservoir, including overflow, interflow and underflow. The transport process of nutrient substance, contaminant and sediment is influenced by the movement of this density current. The water quality and ecological environment is affected as well. The research of density current in thermal stratified reservoir help to get a better understanding about this movement and provide reference to the management about the water quality of the reservoir and aquatic environment.Based on thermal stratification in laboratory, the behavior of density current into thermal stratified water body is investigated by flume experiments, theoretical analysis and numerical simulation, especially movement characteristics of interflow. Firstly, the influence of inflow conditions and stratification condition on the motion of interflow is investigated. Secondly, the impact of thermal stratification on interflow travel time is theoretically derived. Then experiments and numerical simulations are carried out to validate the applicability of the proposed relation. Finally, the sediment interflow in thermal stratified reservoir is investigated. The main contents and conclusions of the study are as followed:1. The thermal stratification of water body in vertical direction is carried out through multiple heating rods, and this thermal stratification remained the same in the transverse direction. The inflow into homogeneous environment and thermal stratification water body is investigated. The results show that the thermal stratification is the significant factor to produce the interflow.2. In the experiments, the motion of interflow is investigated in various inflow conditions and stratification condition, which is characterized by separation depth and intruding thickness. The inflow conditions are represented by inflow buoyancy flux and stratification condition is represented by buoyancy frequency. The separation depth and intruding thickness increase as inflow buoyancy flux increases, and decrease as buoyancy frequency increases. The intrusion depth is related to the buoyancy flux and buoyancy frequency as well. Compared with the interflow in salinity stratification water body, the intrusion depth of interflow in thermal stratification water body is small. The temperature mixing between inflow and thermal stratified water is the main reason to the decrease of intrusion depth.3. A vertical 2D RNG k-s model is developed to simulate the overflow, interflow and underflow. The different temperature inflows into the same thermal stratification water body are simulated. The results show that mixing between inflow and environment water body, especially temperature mixing, influences the type of density flow in the thermal stratified reservoir. The mixing coefficient ?3 is presented to quantitatively describe the temperature mixing, and a dimensionless parameter Ric is used to quantify patterns of density current. The simulated results show that inflow will form interflow if Ric>1, and inflow will form underflow if Ric<1.4. A quantitative relation between buoyancy frequency and interflow travel time is theoretically derived based on the Bernoulli principle of energy conservation. Experiments and numerical simulations are carried out to validate the applicability of the proposed relation.5. The sediment interflow is investigated in the experiment. In consideration of the influence of particle size on the settling velocity of sediment, a vertical 2D RNG k-s model is developed to simulate the sediment interflow. The results show that the sediment interflow could form if the sediment particle is fine enough. Based on the experiment and numerical results, the mechanism of sediment interflow is analyzed.