Numerical simulations of the flow field and pollutant dispersion in an idealized urban area under different atmospheric stability conditions

Abstract

This study simulated the flow and near-field plume dispersion in an urban-like environment under unstable, neutral and stable atmospheric stratification using the steady Reynolds-averaged Navier-Stokes (RANS) methodology. First, a validation study for two trials of the Mock Urban Setting Test (MUST) experiments is performed to examine the predictive performance of the computational fluid dynamics (CFD) model, Fluidyn-PANACHE. The effects of atmospheric stability on the flow structure in street canyons under perpendicular incident flow conditions are investigated. In addition, the patterns of urban dispersion in different cases of stability are also analysed under perpendicular and oblique wind direction conditions. The results show that in the urban environment, the influence of atmospheric stability on the canyon vortex intensity, flow structure and plume dispersion is apparent; intense thermal turbulence enhances the vortex intensity and plume dilution in the street canyon under unstable conditions; when the atmospheric conditions are stable, the vertical profile of the streamwise velocity is significantly decreased by the obstacles, and the concentration level and spread of pollutants increase in the street canyon due to relatively weak turbulent motions; plume deflection within the obstacle array is noteworthy when the incident flow is oblique; in particular, the transport of the plume is basically independent of the wind direction very near the ground.