Effects of real trees and their structure on pollutant dispersion andflowfieldin an idealized street canyon

Özet

Trees have a significant impact on theflowfield and pollutant dispersion. However, the commonly used porousmedium model cannot accurately describe the blocking effect of real trees. In this work, a numerical in-vestigation on the effects of real trees and their different structure (trunk, main branches, lateral branches, andcanopy) onflow behaviour in an idealized street canyon is performed. We found that the trunk plays a major rolein blocking windflow by altering theflow pattern. The velocity magnitude and the turbulent kinetic energy(TKE) are low around trees due to this blocking effect. Pollutants tend to accumulate from the middle-bottom ofthe street canyon to its leeward side. The greater the entire tree structure (trunk, trunk and main branches, trunkand all branches, canopy, whole tree) is, the larger the pollution distribution towards the leeward side andwindward side. With no tree, trunk, TMB (trunk and main branches), TAB (trunk and all branches), canopy orwhole tree, the dimensionless average pollutant concentrations are 77.2, 90.2, 90.5, 88.3, 87.73, and 98.2,respectively. The anisotropic feature of a tree canopy has little effect on the pollutant distribution on the leewardsurface. Moreover, without the influence of trees trunk and main branches, the pollutant concentration at thebottom of the street canyon and leeward side can be underestimated.