Transport mechanism of urban plume dispersion

Abstract

Winds (advection) and turbulence (diffusion) are the driving forces for urban plume dispersion whose partitioning is complicated by urban morphology and the conditions of atmospheric surface layer (ASL). Wind tunnel parametric tests are conducted to examine the plume behavior in the turbulent boundary layers (TBLs) over hypothetical buildings (roughness elements of ribs and bricks) with drag coefficient Cd (= 2uτ2/U∞2; where uτ and U∞ are, respectively, the friction velocity and the freestream velocity) varying from 4.7 × 10−3 to 10 × 10−3. Vehicular emission is modeled by atomizing water vapor via a ground-level line source in crossflows. The laboratory measurements show that the mean tracer concentrations ψ‾ exhibit the theoretical Gaussian form, supporting the classic plume dispersion framework. The horizontal advection tracer flux u‾ψ‾ dominates the transport processes compared with the vertical turbulent tracer flux w''ψ''‾. Increasing Cd leads to a decrease in u‾ψ‾ and an increase in w''ψ''‾ simultaneously, and vice versa. The changes in partitioning of transport processes in response to urban morphology imply that the conventional parameterization of dispersion coefficient σz (determined by atmospheric stability only) should be applied cautiously for urban setting. A new simple scheme of σz parameterization, which specifically accounts for the influence from urban roughness on the plume dispersion, is proposed.