Numerical Simulations of Canopy Vegetation Influence on Flow, Turbulence and Passive Scalar Dispersion in Street Canyons

摘要

The flow and turbulence inside and above forest canopies and urban street canyons covered by arrays of trees play an important role in studies of the atmospheric flows and for estimations of the air quality in cities. We report on numerical simulations of the turbulent flows over the regions densely populated by trees (forest) and simplified (laboratory size) urban street canyons with dispersion of the passive pollutants. Performances of several variants of the forest canopy models based on an extended version of the linear two-equation (k-□) RANS eddy-viscosity turbulence model with a Durbin's time-scale limiter have been investigated, Durbin. First test cases addressed the air flow inside and above forest - with homogenous or non-homogenous vertical distributions of the surface/volume ratio parameter. The model of Katul et al. with four additional terms in the transport equations of the turbulent kinetic energy and its dissipation rate proved to be the best model in reproducing the forest effects on velocity and turbulence - giving good agreement with available experimental data over a range of Reynolds numbers and surface/volume ratio parameter. The simplified multiple urban street canyons configuration with a point source release of a passive scalar (concentration) is simulated next and obtained results are extensively validated against experimental database of Yee et al. Finally, various scenarios with different orientations and distributions of the arrays of trees along the street canyons are simulated. Effects of the arrays of trees on velocity, turbulence and dispersion of passive scalar are analyzed. It is demonstrated that due to the presence of trees, significant changes in distributions of velocity profiles, turbulence levels and intensity of the local concentrations are observed. This indicates the importance of a proper inclusion and modelling of the trees in simulations of the urban canopies.