Large-eddy simulation for flow and dispersion in urban streets

Abstract

Large-eddy simulations (LES) with our recently developed inflow approach (Xie & Castro, 2008a) have been used for flow and dispersion within a genuine city areathe DAPPLE site, located at the intersection of Marylebone Rd and Gloucester Pl in Central London. Numerical results up to second-order statistics are reported for a computational domain of 1.2km (streamwise) x 0.8km (lateral) x 0.2km (in full scale), with a resolution down to approximately one meter in space and one second in time. They are in reasonable agreement with the experimental data. Such a comprehensive urban geometry is often, as here, composed of staggered, aligned, square arrays of blocks with non-uniform height and non-uniform base, street canyons and intersections. Both the integrative and local effect of flow and dispersion to these geometrical patterns were investigated. For example, it was found that the peaks of spatially averaged u rms , v rms , w rms and < u w > occurred neither at the mean height nor at the maximum height, but at the height of large and tall buildings. It was also found that the mean and fluctuating concentrations in the near-source field is highly dependent on the source location and the local geometry pattern, whereas in the far field (e.g. >0.1km) they are not. In summary, it is demonstrated that full-scale resolution of around one meter is sufficient to yield accurate prediction of the flow and mean dispersion characteristics and to provide reasonable estimation of concentration fluctuations.

Influence of avenue-trees on air quality at the urban neighborhood scale. Part II: Traffic pollutant concentrations at pedestrian level

“…Recently, research has been undertaken to study dispersion and ventilation in actual or generic building block arrays with tree-free streets (e.g. Boppana et al, 2010;Borrego et al, 2006Borrego et al, , 2003Buccolieri et al, 2010;Carpentieri et al, 2012;Di Sabatino et al, 2007;Garbero et al, 2010;Hamlyn et al, 2007;Hang et al, 2011;Panagiotou et al, 2013;Pascheke et al, 2008;Princevac et al, 2010;Ramponi et al, 2014;Soulhac et al, 2012Soulhac et al, , 2011Soulhac et al, , 2003Xie and Castro, 2009). It is evident that such study areas provide a more relevant description of the effects of trees on flow and pollutant dispersion than those with isolated street canyons or intersections.…”

Modelling Street-Scale Flow and Dispersion in Realistic Winds—Towards Coupling with Mesoscale Meteorological Models

“…So a polyhedral mesh with four grid sizes was used for the whole domain, identical to the medium mesh in Xie and Castro (2009), in which a numerical experiment on the sensitivity to grid resolution was conducted that showed that the medium mesh produced results in good agreement with those of a finer mesh. Again, an almost uniform grid was used in the DAPPLE site up to z/h m = 3 in order to maintain great accuracy in the region of most interest.…”

“…There is thus convincing evidence that LES is the most promising tool for computing unsteady, three-dimensional flows at high Re, and it is used in the present work. Indeed, LES with a proper wall treatment is arguably the only approach likely to produce reliable numerical data over a range of problems and yet be within the affordability of the current and near-future computer power for genuine urban geometries (Tseng et al, 2006;Xie and Castro, 2009). …”

Large-Eddy Simulation of Dispersion from Line Sources in a Turbulent Channel Flow

“…Xie and Castro, 2008). Before studying typical street-scale dispersion using LES (with results discussed in Xie and Castro, 2009;Boppana et al, 2010) we addressed the conceptually simpler case of scalar dispersion in a channel flow, largely because there are some existent laboratory experiments and direct numerical simulations (DNS) for such flows; it is this topic that is discussed in the present paper.…”