Aerodynamics properties and turbulence structures of the urban boundary layer over a dense city

Abstract

Large-eddy simulations (LES) of urban boundary layer (UBL) flows over a real, dense city, Hong Kong, have been carried out in this study. Investigations of turbulence properties of UBL flows are conducted with statistical analysis. Because of the presence of large, heterogenous building clusters, the vertical zone of horizontal flow inhomogeneity (i.e., roughness sublayer, RSL) is elevated. The distinctive RSL turbulence dynamics are demonstrated that are different from their inertial sublayer (ISL) counterparts. Examination of RSL quadrant events shows that ejection (Q2) appears more often than sweep (Q4), but their contributions reverse. Further conditional sampling illustrates such behaviors result from the rare, large-scale structures. The strength of such motion scales is determined to be within 3-5 times of averaged momentum flux. For flows over densely built urban areas, two key factors of the local-scale dynamics are depicted by identifying their roles in momentum transport and the generation of turbulence structures. One is the giant upstream wake, whose footprints are recognized by the contours of two-point correlation of streamwise velocity Ruu. Another is building height variation. Both favor the production of turbulence kinetic energy (TKE) and the formation of intermittent, energetic Q4. However, the mechanism is different: the upstream wakes stimulate extreme streamwise accelerations; whilst building height heterogeneity intensifies vertical fluctuations, which also benefits the generation of large-scale Q2. The datasets of eddy diffusivity of momentum (KM) demonstrate a linearly increasing behavior within lower UCLs. Therefore, the effects from ground surface on the eddy organizations are parameterized by a linear function of KM, which implies a self-similarity alike a standard surface layer but with a smaller rate. On the other hand, the strong shear layers initiated at UCL top facilitate a mixing-layer-flow analogy that features eddies with sizes proportional to the shear length scale. Combining the effects from ground surfaces and shear at UCL top, a novel model is established to parameterize the mixing length (lm) and momentum flux for UCL. The effectiveness of the parameterization has been confirmed with the LES-calculated winds and momentum flux. Adopting an exponential model (exp-law) for the mean-wind-speed profiles of UCL, a rougher surface is characterized with a smaller attenuation coefficient, which results in stronger in-canopy winds. This is owing to the characteristic eddies with a longer shear length scale, which helps turbulence mixing that slows down the wind decay in canopies; and improves the efficiency of momentum transport through large-scale coherent structures, especially Q4. Investigations into the relationship between building height (distribution) and characteristic height for turbulence dynamics foster a conceptual model of UBL flows. It is suggested that building height variation is conductive for surface roughness, which enhances turbulence dynamics and wind speed, thus helping pollutant removal and fresh air entrainment for street canyons. These results could be applied in urban planning for better air quality and the development of multi-layer urban canopy models (UCMs) of real dense cities.