Morphological influence of sub-scale urban vegetation structures on canopy ventilation and scalar transport based on empirical mode decomposition

Abstract

Vegetation in urban is commonly adopted as shelter from traffic pollutants and soil erosions, however has an uncertain positive or negative effect on various sub-scale structures. Besides, strengthening the introduction of fresh air through the canopy into the pedestrian level requires more aerodynamic mechanisms after the single vegetation element with sub-scale structures. This paper quantifies the contributions of different crown structures to the flow around a solitary fractal tree with different crown structures. The turbulence signals are decomposed into intrinsic mode functions (IMFs) by empirical mode decomposition (EMD), classified into large-scale and small-scale motion. The turbulence intensity proportion of each scale elaborates that the dominant sub-scale structure varies with tree height. The contributions of IMFs to vertical momentum flux u'w' are compared to determine the effect of sub-scale geometries on wind erosion and canopy ventilation, indicating the dominance of the large-scale (IMF3, St = 0.2–0.4, trunk height) near ground and the small-scale (IMF1,2, St = 0.9–2, branch sizes) near the canopy. The tree height and the branch geometries should be considered in soil erosion and canopy ventilation, respectively. The exuberance supplements that large-scale structures promote more canopy ventilation in denser crowns and suggests spacing distance within 1–2 h most capable for suppression of the scalar transport and entrainment. The spanwise momentum flux u'v' emphasizes the dominance of IMF2 on pollutants transport between pedestrians and traffic roads. The dominant sub-scale vegetation structures in different urban issues are revealed to provide a reference for refining the urban environment prediction.