Tracer Flux Balance at an Urban Canyon Intersection

Abstract

Despite their importance for pollutant dispersion in urban areas, the special features of dispersion at street intersections are rarely taken into account by operational air quality models. Several previous studies have demonstrated the complex flow patterns that occur at street intersections, even with simple geometry. This study presents results from wind tunnel experiments on a reduced scale model of a complex but realistic urban intersection, located in central London. Tracer concentration measurements were used to derive threedimensional maps of the concentration field within the intersection. In combination with a previous study (Carpentieri et al., Boundary-Layer Meteorol. 133:277-296) where the velocity field was measured in the same model, a methodology for the calculation of the mean tracer flux balance at the intersection was developed and applied. The calculation highlighted several limitations of current state-of-the-art canyon dispersion models, arising mainly from the complex geometry of the intersection. Despite its limitations, the proposed methodology could be further developed in order to derive, assess and implement street intersection dispersion models for complex urban areas.

Influence of urban morphology on air flow over building arrays

“…The results have been extremely useful in identifying shortcomings of current modelling approaches (Carpentieri et al, 2012) and important phenomena likely to have a major impact on flow and dispersion (Carpentieri et al, 2009;Carpentieri and Robins, 2010). However, more systematic approaches are needed to make possible the quantification and identification of the relative importance of the various parameters involved.…”

“…The latter aspect (i.e. the effect of tall buildings) has been sporadically studied in the past, and previous experiments have already highlighted its importance (Carpentieri et al, 2009;Carpentieri and Robins, 2010;Heist et al, 2009;Brixey et al, 2009). However, no modelling approaches have been proposed to take account of associated flow and dispersion phenomena.…”

“…These properties depend strongly on the local geometry and existing models generally relate them to the mean building height (H b ) and the lambda parameters (λ p , and λ f , see above). Recent studies (Carpentieri et al 2009(Carpentieri et al , 2012aCarpentieri and Robins, 2010;Harms et al, 2011;Klein et al, 2011), however, have highlighted the complexity of the flow and dispersion fields in actual urban geometries (as opposed to idealised building arrangements). It is clear from such studies that more parameters (such as building height variability and building aspect ratio) should be taken into account for a more accurate prediction of flow and dispersion in actual urban canopies.…”

Wind tunnel measurements of pollutant turbulent fluxes in urban intersections

“…Street canyons are not infinitely long or isolated entities in cities, but they are often segmented and connected to other canyons at urban intersections. There have been only a few dispersion studies focussing on urban canyon intersections, where flow patterns and dispersion behaviour can be markedly different to those in idealised, isolated street canyons (Hoydysh and Dabberdt, 1994;Carpentieri et al, 2009;Carpentieri and Robins, 2010).…”

“…It involves the combined use of a two-component laser Doppler anemometer (LDA) and a fast response flame ionisation detector (FFID). The two instruments had already been used separately in order to estimate the mean pollutant fluxes in and around a street canyon intersection (Carpentieri et al, 2009;Carpentieri and Robins, 2010) -they were used together in the present work in order to be able to separately measure the turbulent tracer fluxes as well. Once developed, the technique was then applied to a small-scale model of a real urban area, in central London, UK.…”