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MODELING AND SIMULATION OF TURBULENT PENETRATIVE CONVECTION AND POLLUTANT DISPERSION ABOVE THE URBAN HEAT ISLAND IN STABLY STRATIFIED ENVIRONMENT. A.F. Kurbatskiy Institute of Theoretical and Applied Mechanics SB RAS Novosibirsk State University Novosibirsk, Russia L.I. Kurbatskaya
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MODELING AND SIMULATION OF TURBULENT PENETRATIVE CONVECTION AND POLLUTANT DISPERSION ABOVE THE URBAN HEAT ISLAND IN STABLY STRATIFIED ENVIRONMENT A.F. Kurbatskiy Institute of Theoretical and Applied Mechanics SB RAS Novosibirsk State University Novosibirsk, Russia L.I. Kurbatskaya Institute of Computational Math. and Math. Geophysics SB RAS Novosibirsk, Russia NATO ASI Conference, Kyiv-2004
O u t l i n e • Introduction • Objectives • Turbulent Transport Models for Environmental Stratified Flows • Modeling and Simulation of Urban Heat Island Phenomenon and Pollutant Dispersion • Numerical Results • Conclusion NATO ASI Conference, Kyiv-2004
Introduction For stratified atmospheric flows the LES models and third-order closure models should be considered as fundamental research tools because of their large computer demands. A growing need for detailed simulations of turbulent structures of stably stratified flows motivates the development and verification of computationally less expensive closure models for applied research in order to reduce computational demands to a minimum. NATO ASI Conference, Kyiv-2004
Objectives The algebraic modeling techniques can be used in order to obtain for buoyant flows the fully explicit algebraic models for turbulent fluxes of the momentum, heat and mass. The principal object of this work is the development of three-four-parametric turbulence model minimizes difficulties in simulating of turbulent transport in stably stratified environment and reduces efforts needed for the numerical implementation of model. NATO ASI Conference, Kyiv-2004
Governing Equations Governing equations describing the turbulent stratified environmental flows are being written down in the hydrostatic approximation at absence of the Coriolis force and radiation with use a Boussinesq approximation. NATO ASI Conference, Kyiv-2004
Governing Equations in RANS-approach NATO ASI Conference, Kyiv-2004
Transport Equations for heat and mass fluxes NATO ASI Conference, Kyiv-2004
Explicit Algebraic Expressions for Turbulent Fluxes The explicit algebraic models for the turbulent heat flux vector and turbulent mass vector were derived by truncation of the closed transport equations for turbulent fluxes of heat and concentration by assuming weak equilibrium, but retaining all major flux production terms. For turbulent stresses we applied eddy viscosity expression. NATO ASI Conference, Kyiv-2004
CLOSURE:full explicit turbulent fluxes models foractive (heat) and passive (mass) scalars NATO ASI Conference, Kyiv-2004
CLOSURE:three-equation model for active (heat) scalar field NATO ASI Conference, Kyiv-2004
CLOSURE : four-equation model for passive scalar field NATO ASI Conference, Kyiv-2004
Modeling of Urban Heat Island The ability of the proposed full explicit algebraic models for turbulent fluxes of heat and mass to reproduce correctly the environmental flows with a strong thermal stratification was tested on a large-scale circulation flow above an urban heat island NATO ASI Conference, Kyiv-2004
Modeling of Urban Heat Island In the phenomenon of the unsteady turbulent penetration convection above an urban heat island the two remarkable features are shown. The first, due to heating from bellow the interactions between stable and unstable regions occur, because the mixed turbulent ground layer to grow into a stable region. NATO ASI Conference, Kyiv-2004
Modeling of Urban Heat Island The second, there is the entrainment of overlaying non-turbulent fluid into mixed layer causing very step gradients at the interface. These features explain why the phenomenon of urban heat island represents a very challenging test case for turbulent models. NATO ASI Conference, Kyiv-2004
Objectives Thus, the principal aim of this investigation is the modeling and simulation of large-scale turbulent circulation flow above the urban heat island and pollutant dispersion in the stably stratified environment. NATO ASI Conference, Kyiv-2004
Limitations of Laboratory Measurements for Full-scale Simulation There are important limitations utilized in the laboratory experiment and simulation of the real urban heat-island in the nighttime atmosphere: Very large heat fluxes from the heater surfaces Very strong temperature gradients that required to obtain the low aspect ratios (zi/D) and small Froude numbers. NATO ASI Conference, Kyiv-2004
Structure of heat-island circulation The penetrative turbulent convection is induced by the constant heat flux H0 from the surface of a plate with diameter D. It simulates a prototype of an urban heat island with the low-aspect-ratio plume (zi / D « 1) under near calm conditions and stably stratified atmosphere. NATO ASI Conference, Kyiv-2004
NUMERICAL MODELING OF HEAT ISLAND CIRCULATION The problem of development of circulation above a heat island is assumed to be axisymmetric. The domain of integration is a cylinder of a given height . NATO ASI Conference, Kyiv-2004
Numerical Method Fr , Fz – turbulent fluxes of momentum, heat and mass Semi-implicit alternating direction scheme NATO ASI Conference, Kyiv-2004
The numerical method uses a staggered mesh. The difference equations are solved by the three-diagonal-matrix algorithm. Staggered mesh Mesh z r r/2 z z/2 r 0 Ur Uz E, , T, <2>, C, <c> NATO ASI Conference, Kyiv-2004
Main Results of Simulation The results of simulation correspond to a quasi-steady state of circulation over an area heat source in stable stratified environment. Figure (c):shadowgraph picture at t = 240 sec when the full circulation is established. NATO ASI Conference, Kyiv-2004
Calculation of Normal Turbulent Stresses In this problem a simple gradient transport model preserves certain anisotropy of the normal turbulent stresses is turbulent viscosity. NATO ASI Conference, Kyiv-2004
RESULTS: Temperature profiles Calculated temperature profiles inside the plume have characteristic “swelling”: the temperature inside the plume is lower than the temperature outside at the same height creating an area of negative buoyancy due to the overshooting of the plume at the center. This behavior indicates that the plume has a dome-shaped upper part in the form of a “hat”. NATO ASI Conference, Kyiv-2004
CONCLUSION The three-equation model of turbulent transport of heat reproduces structural features of the penetrative turbulent convection over the heat island in a stably stratified environment. This model minimizes difficulties in describing the non-homogeneous turbulence in a stably stratified environment and reduces computational resources required for the numerical simulation.
