Fast fluid dynamics simulation of the effect of a single tree canopy on microclimates considering variations in solar position

Trees play a crucial role in regulating microclimates, and numerical simulation has been recognized as an effective tool for studying their regulatory mechanisms. However, conventional methods often require substantial computational time to simulate interactions between trees and microclimates due to the morphological complexity of trees and the nonlinear nature of thermal and moisture processes. Moreover, dynamic coupling mechanisms under diurnal variations in solar altitude and azimuth have not been thoroughly investigated. In this study, the wind, thermal, and moisture source terms of trees were incorporated into the Fast Fluid Dynamics (FFD) method to evaluate its computational accuracy and efficiency, as well as to investigate the interactions between a single tree canopy and microclimates under varying solar positions. The results show that FFD method improves computational speed by approximately 62 % compared with conventional methods. During the daytime, trees exert varying degrees of influence on microclimates, with regulatory effects strengthening as leaf area density (LAD) increases and stomatal resistance decreases. Additionally, dynamic changes in solar altitude and azimuth generate spatiotemporal variations in the microclimate regulation provided by the tree canopy. Under simulated conditions with an LAD of 10.0 m²/m³ and a minimum stomatal resistance of 50 s/m, the maximum cooling effect at 12:00 reaches 0.49 °C on the sunlit side, compared with 0.43 °C on the shaded side, accompanied by a relative humidity difference of 0.52 %. This study enhances the computational efficiency of simulating the impact of trees on microclimate and addresses a research gap regarding their effects under varying solar positions.