Effect of surface roughness on turbulence, ventilation and pollutant dispersion over hypothetical urban area

Abstract

Wind flows and pollutant dispersion over cities are strongly affected by urban morphology. Scientific evidence measuring the effect of surface roughness on transport processes is needed to effectuate air quality strategy. Based on idealised building models assembled by ribs and LEGO® bricks, a series of wind tunnel experiments are performed to investigate the effect of aerodynamic resistance on the ventilation ability and pollutant dispersion over rough surfaces. Water vapour is used as the tracer to examine the passive, inert pollutant dispersion from a line source in crossflows. Velocity and tracer concentrations are measured by hot-wire anemometers (HWAs) with X-wire probes and humidity sensors, respectively. The results show that the near-wall streamwise fluctuating velocity <u′u′>1/2 , vertical fluctuating velocity <w′w′>1/2 and momentum flux −<u′w′> are enhanced by the aerodynamic resistance induced by the surface-mounted roughness elements. The spatially averaged turbulence properties <u′u′>1/2 ,<w′w′>1/2 and −<u′w′> are found peaked in the measurement cases AR = 1/12 (rib-type elements) and h:2l-T (cube-type elements). Both turbulent air exchange rate ACH′ and vertical dispersion coefficient σz increase with increasing drag coefficient Cd, suggesting the prototypes of new parameterisations of ventilation and pollutant dispersion over urban areas.