Coherent Flow Structures and Pollutant Dispersion in a Street Canyon

Coherent flow structures and pollutant dispersion in a spanwise-long street canyon are investigated using a parallelized large-eddy-simulation model. Low- and high-concentration branches, starting from the downwind top corner and upwind bottom corner, respectively, are detected in the time-averaged field of pollutant concentration, and detailed structures of in-canyon flow and pollutant dispersion following the two branches are demonstrated. When turbulent eddies impinge on the upper downwind wall, low- and high-concentration blobs with U-shaped flow structures appear and move downward. The downdrafts tilt away from the downwind bottom corner and impinge on the canyon bottom, driving horizontally diverging flows. Cellular structures of low-concentration centres and high-concentration edges are induced by the downdrafts and diverging flows. The diverging flows push low-concentration air toward the downwind and upwind building walls, resulting in local divergence and convergence of pollutants on both walls. Time series of pollutant concentration at multiple points illustrate that pollutant concentration at the pedestrian level is highly sensitive to the diverging flows. The multiresolution spectra show that time scales of variations of pollutant concentration and vertical velocity component increase from the canyon top to the pedestrian-level centre, indicating longer time-scale flow structures are dominant inside the street canyon. The multiresolution cospectra also show that the time scale of vertical turbulent transport of pollutants increases from the canyon top to the pedestrian-level centre. At the two bottom corners, however, short and long time-scale transports occur together, confirming that the low-concentration diverging flows transport pollutants downward while short time-scale turbulence transports pollutants upward.