Browsing by Author "Stefanovic, Dragoslav L."

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    Certain Computational Aspects of Modeling Stratified Environmental/Geophysical Flows Pertaining to Lakes
    (St. Anthony Falls Laboratory, 2000-03) Stefanovic, Dragoslav L.; Stefan, Heinz G.
    Two intricate issues reiated to environmental hydrodynamics and water quality numerical modeling, with specific application to lakes, are addressed in this report: turbulent closure for stratified flows and transformation of the physical domain into a computational domain. Both problems are very important for the development of an accurate and efficient numerical algorithm intended to simulate thermo-hydrodynamics and transport processes in a lake (pond) cross section. The first section of the report considers the state-of-the art in turbulence modeling of environmental/geophysical flows (e.g. lakes, oceans, atmosphere) particularly in stratified ambiences, where the vertical turbulent transport is significantly hIndered by density stratification. The findings and recommendations stemming from this investigation are reported herein. In the second section of the report, the numerical treatment of irregular lake geometry is described in detail and a simple, efficient mapping transformation is proposed to facilitate the computation in a lake cross section of an arbitrary form.
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    Simulation of Water Temperature, Flow and· Dissolved Oxygen Exchange Processes in Holland Lake
    (St. Anthony Falls Laboratory, 2000-03) Stefanovic, Dragoslav L.; Stefan, Heinz G.
    As a part. of the study of dissolved oxygen (DO) dynamics in Holland Lake, the underlying flow, temperature and material.transport were simulated. In this report the simulation results are being presented. The simulation was two-dimensional to capture the interactions between the shallow and deep subbasins of the lake in addition to the water/air heat and momentum exchange. The vertical cross section in which flow veloCities, water temperatures and· material concelltrations were simulated extended from the westerly shallow ~ubbasin through Jhe deep basin to the northeasterly shallow subbasin. Flow velocities were induced by wind shear on the lake surface and· by buoyancy forces due to temperature differences. Temperatures were in response to heat exchange through the water surface by radiation, convection and evaporation. The two-dimensional model developed for Holland Lake.computed velocities and water temperatures at 5 minutes intervals. Weather conditions (data) observed during the period from July 1 to August 1, 1999 were imposed as boundary conditions. The flow field calculated for the month of July 1999 was then used to investigate. how a material such as dissolved oxygen or dissolved organic carbon is transported from the shallow subbasin to the deep subbasin. These simulations show the intrusion of bottom waters from the shallow subbasins to the metalimnion of the deep basin. This mechanism can explain the rapid DO. depletion of metalimnetic waters in the summer because the bottom waters in the shallow subbasins are known to be low in DO and rich in detrital carbon. The simulation results also show that the low water temperatures associated with groundwater intrusion are largely responsible for the movement of oxygen-poor water from the shallow subbasins into the deep subbasin. The selection of alternative supplemental aeration techniques will benefit from the simulations results. The depth at which aeration systems should be placed, and their location will be guided by the model results.
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    Two-Dimensional Water Quality Model for Unsteady Advection-Diffusion of Nonconservative Substances
    (St. Anthony Falls Laboratory, 1997-12) Stefanovic, Dragoslav L.; Stefan, Heinz G.
    A numerical model for calculation of advective-diffusive transport of nonconservative substances in two-dimensional environments was developed. The numerical method is based on the splitting-operator approach, in which the advection, the diffusion and the chemicallbiological kinetic processes are calculated separately in one time step. Special attention was paid to the advection operator, which introduces essential difficulty in many numerical methods, and to the linearized source term which, in many cases, has proven to cause instability problems. The model calculates pure advection by the explicit Holly-Preissmann method of characteristics, and diffusion plus source/sink terms by an extended implicit alternate-direction (ADI) method. By comparison with analytical results for fronts and discrete mass releases it is established that numerical separation of differential operators does not induce significant errors in the solution or the physical realism of the results. The numerical scheme is accurate, stable and efficient because it eliminates the need to solve a pentadiagonal algebraic systems, replacing it with two tridiagonal ones. The computational method is intended for further use in the, study of a two-dimensional lake hydrodynamic and transport field, driven either by forced (wind induced) or natural (buoyancy induced) convection.