Scaling of Flows Over Realistic Urban Geometries: A Large-Eddy Simulation Study

A large-eddy simulation (LES) method is used to investigate the characteristics and scaling of flows over realistic urban geometries. The LES method is first validated with wind-tunnel measurements for the flowover a staggered array of cubes and the grid dependence behaviour of the LES is tested. It is then applied to simulate the flows over four different realistic urban surfaces with sizes of about 1000 m x 500 m. After examining the overall flow patterns and turbulence characteristics, the time- and horizontally space-averaged profiles for the four cases are investigated. It is found that, in all cases, the vertical profiles of the mean streamwise velocity component can be characterized by an inflection point which can be identified as the height of the urban canopies. A new simple method is then proposed to predict this height for the realistic urban canopies based on the plan area fraction of the surfaces. By using this urban canopy height as the normalization length scale for the flows over realistic urban geometries, it is found that the vertical profiles of the dispersive stress, effective mixing length, and sectional drag coefficient show similar patterns as those of uniform height idealized urban canopies. This result would be useful for the development of urban flow parametrizations.