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Poster 1 Mass flux balance at an urban intersection
Developments in Environmental Science, Volume 6 C. Borrego and E. Renner (Editors) Copyright r 2007 Elsevier Ltd. All rights reserved. ISSN: 1474-8177/DOI:10.1016/S1474-8177(07)06801-5
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Poster 1 Mass flux balance at an urban intersection Sandro Baldi, Matteo Carpentieri and Alan G. Robins Abstract The understanding of the behaviour of pollutants released in urban sites is of paramount importance for a number of reasons, mainly related to human health. Furthermore, the particular present international political situation adds further concerns, as the deliberate discharge of toxic material in populated areas is a serious threat. Wind tunnel experiments were performed in order to study flow and pollutant dispersion in a real urban environment. The work is part of a larger EPSRC funded project (DAPPLE, Dispersion of Air Pollution & Penetration into the Local Environment) involving six British Universities. The study concerned the 350 m-diameter circular area shown above, centred on the intersection between Marylebone Road and Gloucester Place in central London and modelled at 1:200 scale in a 20 35 1.5 m wind tunnel, (Fig. 1). A 2D Laser Doppler Anemometer (LDA) was used to measure mean and turbulent velocities across the site in all three directions, which provided a full three-dimensional velocity mapping of the flow on a 31 31 6 grid over a 600 600 150 mm region. The concentration of pollutant across the area was measured with the use of a ‘‘Fast’’ Flame Ionisation Detector (FFID). The LDA and the FFID devices were then coupled in order to produce simultaneous measurements of velocity and concentration, which permitted the estimation of mean and turbulent pollutant fluxes, thus giving an important insight into the understanding of turbulent diffusion. The mean flow reveals a very complex behaviour across the whole area. Recirculation vortices can be observed in many locations but they are not always well defined as they interact with the along-the-street component of the flow. This is rather important since the wind is directed 511 from Marylebone road and therefore the x and y components of wind velocity
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are rather similar in magnitude. Generally the flow in Gloucester Place appears to be mainly two-dimensional with an almost zero vertical component, both south and north of the intersection. The pattern is rather different as soon as we reach the top of the buildings as wakes and vortices interact quite strongly there. The asymmetric configuration of the building geometry plays an important role in defining the mean and turbulent flow in the upper part of the street canyon. The mean flow along Marylebone Road shows a much more complicated behaviour. Upwind of the intersection (at negative x) the flow in the lower part of the canyon is generally redirected upwards in the direction of the intersection and then down again partly in Gloucester Place (north side) and partly in Marylebone Road (east side). The mean flow in Marylebone Road downwind of the intersection is fully three-dimensional with vortices forming on the building on the lower-right corner of the intersection mixing with the down-flow coming from the upper levels of west Marylebone. Nevertheless, in this region the recirculation vortices are clearly visible and the flow assumes a more ‘‘classical’’ canyon-style pattern. Particular features of the site, mainly towers and tall buildings, affect the flow very strongly, causing strong recirculation areas and wakes. The analysis of the Reynolds stresses shows that the boundary
Figure 1. A schematic view of the central region of the study area, showing the intersection of Marylebone Road and Gloucester Place. The vectors show the direction of the undisturbed flow above roof level.
Mass Flux Balance at an Urban Intersection
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layer might be affected by the buildings up to heights in excess of three times the tallest building of the intersection. Measurements were then performed employing both LDA and FFID in order to obtain mass fluxes estimations; both mean and turbulent fluxes were measured. Despite the fact that only one wind direction was used, results were quite varied. This was thought to be due to the strong effect exerted by the geometry of the intersection and the surrounding area. Generally, horizontal turbulent mass fluxes were found to be small with respect to the mean mass flux driven by the advection mechanism. Vertical turbulent fluxes results emphasised the increased turbulent exchange at roof level, which confirmed the importance of this region in the general exchange mechanism between the canopy and external flow. A complete and detailed database of the velocity field, the concentrations of pollutant released from point sources in the area, the geometry and approach has been assembled. Furthermore, turbulence and fluxes analysis showed that the area is characterised by a number of local turbulent structures, primarily located in the wake of tall buildings, where mixing and mass fluxes appeared to be enhanced. The examination of the results obtained so far suggests that future experiments should be focussed on the measurement of local mean and turbulent fluxes in order to highlight how small regions in the measurement area contribute to and differ from the total mass exchange and mixing mechanism (i.e., the classical street canyon process).
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Poster 1 Mass flux balance at an urban intersection Sandro Baldi, Matteo Carpentieri and Alan G. Robins Abstract The understanding of the behaviour of pollutants released in urban sites is of paramount importance for a number of reasons, mainly related to human health. Furthermore, the particular present international political situation adds further concerns, as the deliberate discharge of toxic material in populated areas is a serious threat. Wind tunnel experiments were performed in order to study flow and pollutant dispersion in a real urban environment. The work is part of a larger EPSRC funded project (DAPPLE, Dispersion of Air Pollution & Penetration into the Local Environment) involving six British Universities. The study concerned the 350 m-diameter circular area shown above, centred on the intersection between Marylebone Road and Gloucester Place in central London and modelled at 1:200 scale in a 20 35 1.5 m wind tunnel, (Fig. 1). A 2D Laser Doppler Anemometer (LDA) was used to measure mean and turbulent velocities across the site in all three directions, which provided a full three-dimensional velocity mapping of the flow on a 31 31 6 grid over a 600 600 150 mm region. The concentration of pollutant across the area was measured with the use of a ‘‘Fast’’ Flame Ionisation Detector (FFID). The LDA and the FFID devices were then coupled in order to produce simultaneous measurements of velocity and concentration, which permitted the estimation of mean and turbulent pollutant fluxes, thus giving an important insight into the understanding of turbulent diffusion. The mean flow reveals a very complex behaviour across the whole area. Recirculation vortices can be observed in many locations but they are not always well defined as they interact with the along-the-street component of the flow. This is rather important since the wind is directed 511 from Marylebone road and therefore the x and y components of wind velocity
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Sandro Baldi et al.
are rather similar in magnitude. Generally the flow in Gloucester Place appears to be mainly two-dimensional with an almost zero vertical component, both south and north of the intersection. The pattern is rather different as soon as we reach the top of the buildings as wakes and vortices interact quite strongly there. The asymmetric configuration of the building geometry plays an important role in defining the mean and turbulent flow in the upper part of the street canyon. The mean flow along Marylebone Road shows a much more complicated behaviour. Upwind of the intersection (at negative x) the flow in the lower part of the canyon is generally redirected upwards in the direction of the intersection and then down again partly in Gloucester Place (north side) and partly in Marylebone Road (east side). The mean flow in Marylebone Road downwind of the intersection is fully three-dimensional with vortices forming on the building on the lower-right corner of the intersection mixing with the down-flow coming from the upper levels of west Marylebone. Nevertheless, in this region the recirculation vortices are clearly visible and the flow assumes a more ‘‘classical’’ canyon-style pattern. Particular features of the site, mainly towers and tall buildings, affect the flow very strongly, causing strong recirculation areas and wakes. The analysis of the Reynolds stresses shows that the boundary
Figure 1. A schematic view of the central region of the study area, showing the intersection of Marylebone Road and Gloucester Place. The vectors show the direction of the undisturbed flow above roof level.
Mass Flux Balance at an Urban Intersection
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layer might be affected by the buildings up to heights in excess of three times the tallest building of the intersection. Measurements were then performed employing both LDA and FFID in order to obtain mass fluxes estimations; both mean and turbulent fluxes were measured. Despite the fact that only one wind direction was used, results were quite varied. This was thought to be due to the strong effect exerted by the geometry of the intersection and the surrounding area. Generally, horizontal turbulent mass fluxes were found to be small with respect to the mean mass flux driven by the advection mechanism. Vertical turbulent fluxes results emphasised the increased turbulent exchange at roof level, which confirmed the importance of this region in the general exchange mechanism between the canopy and external flow. A complete and detailed database of the velocity field, the concentrations of pollutant released from point sources in the area, the geometry and approach has been assembled. Furthermore, turbulence and fluxes analysis showed that the area is characterised by a number of local turbulent structures, primarily located in the wake of tall buildings, where mixing and mass fluxes appeared to be enhanced. The examination of the results obtained so far suggests that future experiments should be focussed on the measurement of local mean and turbulent fluxes in order to highlight how small regions in the measurement area contribute to and differ from the total mass exchange and mixing mechanism (i.e., the classical street canyon process).