Large-eddy simulations of wind flow and pollutant dispersion in a street canyon

The wind flow and pollutant dispersion phenomena in urban streets of different aspect ratios (h/w) and relative canyon height ratios (h2/h1) are studied using large-eddy simulations (LES). The concerned large eddies are computed by the filtered Navier–Stokes equations in LES and the unresolved small eddies are modelled using Smagorinsky subgrid scale model. The domain is discretised into uneven staggered grids using marker and cell (MAC) method. The objective of this work is to demonstrate the various flow regimes and their threshold values in urban street canyon using LES for various canyon geometries and Reynolds numbers. All cases are investigated with Reynolds number 400 primarily to obtain information of the three regimes of canyon flow and the Reynolds number is then increased incrementally to 2000 to study the consequent flow fields and pollutant dilution patterns. In low Reynolds numbers, results presented agree with the generally obtained threshold values for different flow regimes. Increase in Reynolds number has smeared the flow regimes boundary. The ease of pollutant dispersion is mainly promoted by better mixings inside the canyon, formation of unstable circulations and higher Reynolds numbers. All results show that the flow regime and pollution pattern demarcations depend on both the varying canyon geometry and Reynolds number.