Statistical Analysis of the Wake after a Single Fractal Tree Model Based on Large-Eddy Simulation

Aerodynamic effect of vegetation is a critical component of urban microclimate. Its morphological characteristics were usually neglected in previous studies, which, however, have complex multi-scale structures. The turbulent transport mechanism in the wake after a single tree with multi-scale needs to be further examined. Fractal is a commonly adopted way to preserve sub-scales. In this study, the wake after a single fractal tree with two generations is investigated using the large-eddy simulation (LES). The spatial distribution of the time-averaged and instantaneous characteristics of winds in the wake is illustrated. Higher-order moments signify the spatial features in the wake that the rare, extreme events with strong upward acceleration and streamwise deceleration often occur at the canopy shear layer. Besides, quadrant analysis shows that the streamwise and spanwise motions dominate the dynamics in the far wake and recirculation region, respectively. Power spectrum density in the wake is calculated to modify the Strouhal number. The shedding frequency could be underestimated by at least 44% using the conventional simplified model, demonstrating the importance of including the multi-scale in the studies of urban climate.