Parameterization of vertical dispersion coefficient over idealized rough surfaces in isothermal conditions

Abstract

Urban air quality is an important problem nowadays because of the close proximity of sources and receptors in densely built environment. Gaussian plume models have been commonly employed in the industry to estimate air pollution impact over open terrain for decades. However, they should be applied cautiously to urban environment in view of the complicated recirculating flows and turbulence-generation mechanism in the wakes around/over buildings. In particular, one of the key components in Gaussian plume models, dispersion coefficient σz, is usually determined empirically based on atmospheric stratification that might overlook the effect of rough urban surfaces in the bottom of atmospheric boundary layer, resulting in prediction uncertainty. In this paper, we report our recent study of the transport processes over idealized rough surfaces (repeated ribs in crossflows) to simulate the flows and transport processes after a ground-level pollutant source in crossflows over hypothetical urban areas. The effect of aerodynamic resistance (controlled by the rib separation b) on pollutant plume dispersion (measured by vertical dispersion coefficient σz) is critically examined. First of all, analytical solution shows that σz is proportional to x1/2 × δ1/2 × f1/4, where x is the downwind distance after the pollutant source, δ the turbulent boundary layer thickness, f (= 2uτ 2/U∞ 2) the friction factor, uτ the friction velocity and U∞ the free-stream wind speed. Afterward, a complementary approach, using both wind-tunnel measurements and large-eddy simulation results, is used to verify the newly developed theoretical hypothesis. Although mild discrepancies are observed among various solutions (due to unavoidable scaling effect), the aforementioned analytical proportionality is clearly depicted. The findings unveil the weakness of conventional practice using Gaussian plume models, proposing a new parameterization of dispersion coefficient for pollutant plume dispersion over urban areas. © 2018, The Author(s).