Wind transport in an idealised urban canopy

At an intermediate stage in the dispersion of a plume of contaminant in an urban environment, inhomogeneity of the wind and turbulence is expected to remain an important factor, even though the precise details of the distribution of buildings may be redundant. We here examine a simple numerical model as potentially an efficient way to generate an adequately-realistic, non-uniform flow field (P3DUL, for Periodic 3-Dimensional Urban Layer), and we study the significance of the spatial-inhomogeneity of the (model) flow for plume dispersion, using a Lagrangian stochastic (LS) model to simulate the trajectories of marked fluid elements (tracer).

The mean wind (U) and turbulent kinetic energy (k) within a periodic urban array of buildings were calculated, by assigning a (fictitious) drag coefficient to the buildings, which were treated as porous (obviating need for a complex grid distribution). The momentum equations were closed using a simple K-theory model, that had proven useful for forest canopy flows. Three-dimensional (modelled) fields, U=U(x,y,z) and k=k(x,y,z), were then input to a fully 3-dimensional, well-mixed Lagrangian stochastic dispersion model, and two simplifications thereof. Simulations are compared with previous observations (of the turbulence and of the dispersion of trace heat) in the "Tombstone Canopy," an array of bluff obstacles in a wind tunnel boundary-layer flow.