Numerical simulations of flow and pollution dispersion in urban atmospheric boundary layers

Flow fields and pollution dispersion characteristics in simple 2-D and 3-D canyons as well as a real canyon (Hong Kong Central) are studied using a computational fluid dynamics (CFD) model. The 2-D and 3-D simulations are used to support the development of a parameterization scheme for scalar transfer. The CFD model-calculated scalar fluxes for the 2-D cases are found to agree well with the estimates of the parameterization scheme and wind tunnel data. The flow behavior and pollution distribution in 3-D canyons are much more complex. Conceptually, the 3-D canyon flows can be divided into regimes of channel-flow along the aisles and of wake-flow in the canyons. For the wake-flow regime, the parameterization is useful subject to modifications and the parameterization becomes increasingly more valid as building density increases. The flow pattern for Hong Kong Central is extremely complex as a result of the irregular distribution of buildings and the interaction of turbulent wakes. In general, wind in channel-flow regimes is strong, which carries pollutants to downstream urban areas, and wind is weak in wake-flow regimes where pollutants accumulate. The vertical profiles of flow velocity, scalar flux and concentration in all simulations are found to vary strongly in space. However, when averaged horizontally in space, the profiles are well behaved, with flow speed increasing with height and concentration decreasing with height.