Physical Modelling of Flow and Dispersion over Urban Areas

Abstract

Roughness and temperature of an urban area are commonly greater than for the surroundings. The resulting “roughness island” and “heat island” cause large three-dimensional perturbations of the approach flow as it passes over the urban area that can be investigated by physical modelling in boundary-layer wind tunnels. Similarity criteria are presented for modelling of thermally-stratified, boundary-layer approach flows and similarity of the urban-area roughness and thermal characteristics. A boundary-layer wind tunnel capable of satisfying the primary criteria is described. Effects on neutral and ground-based inversion approach flows by an idealized rectangular “roughness island” of uniform roughness and “heat island” with uniform heating rate have been explored using physical models (approximate scale of 1:1,300). Mean velocity, temperature and concentration (upwind ground-level line source) data are presented for the outer-city region above the urban canopy. Large-scale flow features observed over actual cities are found to be present in the modelled flow.