Spatially averaged flow within obstacle arrays

Knowledge of the flow velocity within an array of buildings is important in modelling the dispersion of urban air pollution. In this paper a spatially averaged model is derived for UC, the characteristic in-canopy velocity. The modelʹs predictions are compared with experimental results from various sources. The model predicts the in-canopy velocity for a broad range of building packing densities (0.01<λf<0.44). An alternative form of the model is proposed for sparse arrays (λf<0.2). A model is also derived for the exchange velocity UE between the in-canopy and above-canopy flows. This model for UE has the correct variation with packing density and gives predictions of the right order but has yet to be directly validated.