Experimental simulation of air quality in street canyon under changes of building orientation and aspect ratio

Abstract

To assist validation of numerical simulations of urban pollution, air quality in a street canyon was investigated using a wind tunnel as a research tool under neutral atmospheric conditions. We used tracer gas techniques from a line source without buoyancy. Ethylene (C 2 H 4 ) was used as the tracer gas. The street canyon model was formed of six parallel building rows of the same length. The flow and dispersion field was analyzed and measured using a hot-wire anemometer with split fiber probe and fast flame ionization detector. The diffusion flow field in the boundary layer within the street canyon was examined at different locations, with varying building orientations (y ¼ 901, 112.51, 1351 and 157.51) and street canyon aspect ratios (W/H ¼ 1/2, 3/4 and 1) downwind of the leeward side of the street canyon model. Results show that velocity increases with aspect ratio, and with y4901. Pollutant concentration increases as aspect ratio decreases. This concentration decreases exponentially in the vertical direction, and decreases as y increases from 901. Measured pollutant concentration distributions indicate that variability of building orientation and aspect ratio in the street canyon are important for estimating air quality in the canyon. The data presented here can be used as a comprehensive database for validation of numerical models.

Asymptotic results for backwards two-particle dispersion in a turbulent flow

“…The reversal of the process described above -coalescence of forward perturbed trajectories -is known as backwards stochastic particle dispersion. Backwards particle dispersion is a key feature to understanding many physical problems ranging from hydrodynamic turbulence [4], passive scalar transport and dissipative anomalies [5][6][7], cloud formation [8], biological spread and enhanced growth [9][10][11][12], air pollution [13] stellar evolution [14,15], occurrence of solar flares [16] and galaxy formation [17,18].…”