Modelling the wind environment in idealized and realistic urban areas

Abstract

Decades of urbanisation can lead to significant wind reduction in urban areas. At the King’s Park meteorological station in the heart of the Kowloon Peninsula Hong Kong, a wind speed reduction of 0.6 m/s per decade was observed from 1968 to 1995, and -0.16 m/s per decade from 1996 to 2017. We obtained data on the changing three-dimensional urban morphology of Kowloon during the period of 1964-2010, and conducted Computational Fluid Dynamics (CFD) simulations on the historical wind environment considering the prevailing winds. The wind speed and its loss were calculated within an elevation of 200 m. The results show that the overall mean wind speed in the studied urban areas gradually decreased due to the continuous urban development and elevation in building height. The total wind loss ratios at three representative locations increased from less than 10% to greater than 20% during the study period. The total wind loss ratio may increase to about 40% by 2050 if the current weakening trend continues. It reveals the importance and need for factoring in urban air ventilation into the design of urban morphology. Despite the overall wind reduction, local wind acceleration during extreme events can introduce wind hazards in cities. We investigated a real-world case of glass failure during Typhoon Mangkhut. Our results show that rather than the high-rise building, it is the adjacent lower building that is exposed to more dangerous wind environments during typhoons. This phenomenon is caused by specific building configurations and wind effects, mainly introduced by the combination of Venturi effect and downwash vortex. The spatially averaged velocity within an urban canopy may not show an ideal exponential profile in practice, as the underlying assumptions, i.e. a constant drag coefficient and mixing length scale, originate from vegetative canopies and might not be valid for urban canopies. This study uses vertical profiles of drag coefficient, mixing length and morphological parameters to estimate the wind profile within and above the urban canopy, based on the assumption of the balance between sectional drag force and the change of local shear stress. Empirical formulas for mixing length and drag coefficient are proposed through the results of CFD simulations over arrays of cubes. Unlike in conventional homogeneous canopies, the strongest shear may not occur at the mean building height, but at the inflection point of the velocity profile. The height of the inflection point or ‘effective urban canopy height’, which can be higher than the mean building height, is suggested to be used for a heterogeneous urban morphology. This new model is tested and performs well for an ideal homogeneous canopy, an ideal inhomogeneous canopy and the cases of Hong Kong. Finally, the similarities of 20 Chinese cities has been found and an empirical equation for the vertical profile of the sectional plan area density is proposed based on Amap building data. The surface wind speeds and aerodynamic parameters of those cities are summarised. The results indicate urban design should be determined case by case, as cities have both similarities and differences.