Synoptic wind driven ventilation and far field radionuclides dispersion across urban block regions: Effects of street aspect ratios and building array skylines

Abstract

• Modern cities could be vulnerable to radionuclides and other airborne pollutants;. • Urban ventilation capacity could be reduced or enhanced by the urban skylines;. • Increase of inhomogeneity of adjacent buildings reduced the age of air in the canyons;. • Retention became longer in well-ventilated regions, and it shrank in poorly-ventilated regions;. • Built array morphology essentially affects the dispersion of radionuclides in urban blocks. In the present work, a parametric CFD research of an urban block model was conducted to investigate the effects of street aspect ratio and urban skyline on their ventilation and radionuclide dispersion within such urban blocks. Representative ventilation indicators, including velocity ratio ( V R ), age of air ( τ P ), retention time ( τ c ) and atmospheric dispersion factor ( A D F ), were introduced here to evaluate urban blocks ventilation and radionuclide dispersion. Our numerical results indicated that the street aspect ratio has a dominant impact on the air age of the main street canyons. The age of air in the canyon increased with the expansion of its street aspect ratio. When remarkable downwash and weak wind regions were observed around the higher buildings, the age of air on the windward side of a higher building became small, whereas it prolonged on the leeward side. Similarly, canyon retention time increased positively with the street aspect ratio. There was also a longer canyon retention time on the windward side of higher buildings. Reversely, short canyon retention time was found on the leeward side. Additionally, mechanical disturbance of buildings and the rugged underlying surface primarily influence the radionuclides dispersion in the urban region, whereas wind wake flows heavily affect the radionuclides dispersion in the downstream of urban blocks. This research has provided a theoretical reference for the ventilation and far-field pollutant dispersion in urban blocks located near mining or industrial areas.