Laboratory Study of Passive-Scalar Dispersion over Hypothetical Urban Roughness

Behaviours of pollutant dispersion in the atmospheric boundary layers (ABLs) are strongly affected by surface roughness of urban areas. A wind tunnel approach was deployed in this study to investigate the characteristic plume dispersion of passive scalar over hypothetical urban roughness elements in the form of ribs (with various separations apart) in cross flows. An array of idealised street canyons was fabricated by identical square aluminum bars with different aspect (building-height-to-street-width) ratios, i.e. h/b = 1/2, 1/4, 1/8, and 1/10. Moisture was released from a ground-level source into the fully-developed turbulent boundary layers (TBLs) to simulate passive-scalar dispersion over rough surfaces. The velocity and humidity were measured simultaneously using hot-wire anemometry (HWA) and humidity sensors, respectively, on the vertical x-z plane. The profiles of wind and moisture content were compared among different surface configurations in attempt to elucidate the turbulent transport processes. Besides, the vertical dispersion coefficient (σ_z) was examined in details to illustrate the plume behaviours. Previous numerical modelling showed that plume dispersion strongly depended on the aerodynamic resistance. As such, the relationship between σ_z and friction factor f was tested based on the physical modelling results in order to verify the mathematical hypothesis. This study could advance our understanding of pollutant dispersion mechanism over urban areas in response to different surface roughness.