Urban aeromechanics: Turbulent circulation and contaminant dispersion above city in stably stratified environment

Abstract

We use a recently developed engineering turbulence model, a so-called explicit algebraic Reynolds-stress (EARS) model, in the context of the turbulent circulation and the contaminant dispersion above a city in the stable stratified environment. The turbulent kinetic energy, its spectral consumption (dissipation) and the dispersion of turbulent fluctuations of temperature and contaminant are from the EARS model that minimizes difficulties in simulating the turbulent transport in a stably stratified environment and reduces efforts needed for the numerical modeling. Numerical simulation of the turbulent structure of the penetrative convection over the urban heat island under the stably stratified atmosphere demonstrates that the EARS model is able to predict the thermal circulation induced by the heat island in accordance with the experimental data. The model describes such the thin physical effects, as a crossing of vertical profiles of temperature of a thermal plume with the formation of the negative buoyancy area testifying to development of the dome-shaped form at the top part of a plume in the form of “hat”. Combined effects of terrain orography and thermal stratification on dispersion of pollutants in an urban heat island are numerically simulated. In this case model was realizes as the four-equation model for the turbulent transport of momentum, heat and mass for simulating a circulation structure and dispersion pollutant over the urban heat island in a stably stratified environment under nearly calm conditions. Turbulent fluxes of momentum uluj¯, heat ulθ¯ and the pollutant concentration ulc¯ determined from the explicit algebraic models and their closure is reached by solving the transport equations for the turbulent kinetic energy (TKE) E=ulul¯2 its dissipation rate ε, turbulent potential energy Eθ=θ2¯2 and the correlation cθ¯. The performance of the E−ε−θ2¯ EARS model was tested by comparing the numerical results with the laboratory measurements of the low-aspect-ratio plume [1]. Good agreements were found.