Effects of real trees and their structure on pollutant dispersion and flow field in an idealized street canyon

Trees have a significant impact on the flow field and pollutant dispersion. However, the commonly used porous medium model cannot accurately describe the blocking effect of real trees. In this work, a numerical investigation on the effects of real trees and their different structure (trunk, main branches, lateral branches, and canopy) on flow behaviour in an idealized street canyon is performed. We found that the trunk plays a major role in blocking wind flow by altering the flow 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 of the street canyon to its leeward side. The greater the entire tree structure (trunk, trunk and main branches, trunk and all branches, canopy, whole tree) is, the larger the pollution distribution towards the leeward side and windward side. With no tree, trunk, TMB (trunk and main branches), TAB (trunk and all branches), canopy or whole 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 leeward surface. Moreover, without the influence of trees trunk and main branches, the pollutant concentration at the bottom of the street canyon and leeward side can be underestimated.