Mixing of Temperature-Stratified Lakes, Reservoirs or Ponds by Submerged Jets

Abstract

In water quality management and environmental protection of lakes or reservoirs, techniques such as artificial destratification and aeration for water quality improvement and fishkill prevention are used with increasing frequency. The disposal of waste water and heat are major problems in environmental engineering. In both cases the interaction of jet flows and weakly density..-stratified ambients including the mixing is currently not well understood and difficult to predict. In this study 1-D and 2-D numerical simulation models and laboratory measurements are employed to provide information on jet flows and mixing in density-stratified waters. Flow and thermal fields and their evolution are simulated. Jet trajectories are predicted and analyzed especially in low temperature water where strong nonlinearity between water density and temperature exists. Mixing processes which result in destratification and dilution are simulated and analyzed. Laboratory experiments are designed and conducted to observe and measure the basic features of buoyant water jet flows and mixing in a stratified basin. Experiments are conducted with different initial conditions and governing parameters. Data of temperature profiles are analyzed and used to verify the I-D jet-mixing model. The one-dimensional numerical simulation model is developed to describe the gross behavior of jet flows and mixing. The integral I-D model is incorporated into an existing multi-purpose dynamic lake water quality model (MINLAKE) for practical application. This incorporation makes it possible to evaluate effects of jet flows on mixing and water quality of aquatic environments. The temperature submodel in MINLAKE is also extended to winter conditions so that a warm water discharge into an ice-covered lake can be investigated. The model developed for winter predicts the growth and decay of ice and snow covers. and thermal structures of a lake in a cold climate. Based on numerical analysis and experimental data, information on design and operation of artificial mixing (destratification) devices is also provided. Details of flow fields, which would be too complicated to measure in the laboratory and are omitted in the 1-D simulation model, are stp.died in a two-dimensional numerical simulation model. The Reynolds equations are solved with a buoyancy-extended k-E turbulence model asa closure. The flow and thermal fields are modeled as an unsteady phenomenon. The 2-D model is verified against existing data for a vertical jet impinging on a plate and data for offset jets. This 2-D model is applied to simulate the details of flow and mixing of a warm water discharges into a cold lake with ice cover. Buoyancy reversal results in dramatic changes in jet flow behavior.