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

Chun-Ho Liu,Ziwei Mo,Zhangquan Wu

doi:10.1186/s40562-018-0123-x

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 (= 2uτ2/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.

Highlights

Air pollution poses major threat to premature mortality (Lelieveld et al 2015) but its levels over 80% of the cities in the world are unhealthy (WHO 2016)
While isothermal conditions are assumed, the turbulent boundary layer (TBL) are mainly affected by the bottom rough surfaces together with the mechanically generated turbulence
Though the wind-tunnel measurements and large-eddy simulation (LES) results are tested at different free-stream wind speeds, their output can be compared in a dimensionless manner because the Reynolds number is over the critical value

Summary

Introduction

Air pollution poses major threat to premature mortality (Lelieveld et al 2015) but its levels over 80% of the cities in the world are unhealthy (WHO 2016). The conventional Gaussian model (Roberts 1923) is most widely adopted to solve practical problems (Moreira et al 2006) in particular for regulatory enactment (Briant et al 2013), air toxic assessment (Scheffe et al 2016) and continental pollutant transport (Tsuang et al 2003). (2018) 5:24 near-wall turbulent transport processes right over urban canopy layers (Britter and Hanna 2003). The drag induced by (rough) urban surfaces increases the aerodynamic resistance (Raupach et al 1991; MacDonald et al 1998) that in turn modifies the pollutant transport processes aloft. The aerodynamic effect of roughness elements is often more influential to the transport processes in the vicinity over urban areas

Methods

Results

Discussion

Conclusion