Large-scale turbulent structures around various type of buildings

Large-scale turbulent structures within an urban canopy significantly affect the air exchange and thermal environment in urban areas by enhancing ventilation, dispersing pollutants, and facilitating heat transfer. However, their detailed turbulence characteristics remain poorly understood, partly because of the complex interactions between turbulent flows and urban geometries. In this study, proper orthogonal decomposition (POD) was applied to time-series data of streamwise and spanwise turbulent velocities obtained from large-eddy simulations to capture large-scale turbulence structures around an inline array of three different types of buildings: a cubic building, a vertical rectangular building, and a horizontal rectangular building. In all cases, in the first and second POD modes near the bottom surface, pairs of large-scale high-velocity and low-velocity structures regularly appear in the streamwise direction on both sides of the buildings. These structures are strongly linked to the spanwise velocity between buildings, indicating that air exchange and scalar transfer within the urban canopy are significantly affected by large-scale turbulent structures generated on both sides of the buildings. However, the characteristics of these structures vary with building geometry. Around cubic buildings, pairs of high- and low-velocity structures also appear in both the upper and lower regions on both sides of the buildings. In contrast, such vertical pairs are not observed for both vertical or horizontal rectangular buildings. Instead, large-scale structures extend further in the vertical direction, reaching the top of the buildings.