Cooling Effects of Tree Transpiration: A Cfd Simulation Study on Heterogeneous Tree Canopy Configurations (Tccs)

Abstract

Tree transpiration enhances cooling and humidity (RH), improving thermal comfort, but quantitative studies on its cooling mechanisms and influencing factors are needed. This study adopts a porous model in computational fluid dynamics to simulate the transpiration of six tree canopy configurations(TCCs) under various environmental conditions (ambient temperature Ta=20℃,30℃, RH=30%,60%). We comprehensively evaluated their cooling intensity using multiple thermal comfort indices. We found that the heterogeneous TCCs affect wind field distribution, which is categorized into periodic/non-periodic variation (PV&NPV), while the average wind-blocking efficiency is similar at 81.0%. The cooling and humidifying capacity of tree transpiration is affected by background climates (Ta&RH) and heterogeneous TCCs (tree coverage ratio(TCR), canopy size, leaf area density). For NPV/PV cases, the pedestrian (z=1.5m) experiences a maximum temperature reduction of 4.20°C/2.29°C. The Unit Area Cooling Index (UACI) quantitatively compares the effect of per unit tree canopy area on the distribution of cooling region. In PV, the TCR is smaller but the cooling region per canopy area is larger than NPV, which is opposite to the trend of cooling. The thermal comfort effects of tree transpiration are influenced by Ta,RH and TCCs, highlighting the need to study their combined thermal and aerodynamic effects to optimize urban tree plantings.

Keywords: Computational fluid dynamic (CFD) simulation, Tree canopy configurations (TCCs), Tree model, Tree transpiration, Cooling effect