PIV measurements of mean and instantaneous flow structures within and above an outdoor urban-like canopy layer

The flow field in an urban-like canopy has been investigated by applying Particle Image Velocimetry (PIV) to the atmospheric turbulence. The aim of this study is to reveal the spatial characteristics of the urban canopy flow with high spatial resolution and high Reynolds number. The PIV experiments were implemented at a comprehensive outdoor urban scale model (COSMO), in Japan. The COSMO site was constructed as a simplified model city to exclude the influences of human activity and heterogeneity of the surface geometry. Building models are 1.5m concrete cubes, and 512 blocks are regularly aligned in a square array. For the measurements of atmospheric turbulence, a PIV system was developed to capture two dimensional flow fields in a street canyon continuously for an hour at a sampling rate of 30Hz, thereby providing reliable turbulence statistics. The accuracy of turbulence statistics is favorable in comparison with a sonic anemometer. Although the inevitable data deficits in outdoor PIV measurements caused a certain degree of bias to the statistics, it was improved by introducing weighted average method, and the errors in mean statistics became less than 10%. Mean statistics were also compared with the results of wind tunnel experiments which was performed over similar roughness geometry to the COSMO site. Distributions of turbulent momentum flux below the outdoor and indoor canopy layers are comparable with each other, indicating durability of the scaling of Reynolds stress with friction velocity. Some different characteristics observed between outdoor and indoor flow fields were mainly caused by the larger fluctuations of the wind direction in the atmospheric environment. The focus of the analysis is on instantaneous flow structures. Flow structures inside street canyons are often pointed out as unsteady motions, and they are different from mean flow structures. Dispersion of scalars is also unsteady process, so the understanding of temporal change of flow fields is necessary. Shear layers at the canopy top were successfully visualized, and their flapping motions were found to be related to sweep motion above the canopy layer and upward flow from the street canyon. Among the various instantaneous flow structures, our attention is especially on a large scale upward motion, a kind of flushing phenomenon in canopy layers. Flushing is a contrastive flow aspect from the familiar cavity circulation in which scalars are often trapped. They were observed intermittently, and tracer particles were flushed vertically out from the street canyon during those events. Flushing phenomena were also observed in the wind tunnel experiments which has neither thermal stratification nor outer layer disturbance. It is suggested that flushing phenomena are correlated with the passing of large-scale low momentum regions above.